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Experimental and Theoretical Investigation of the High-Pressure Behavior of Concrete

AFRL-RW-EG-TP-2008-7419EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THEHIGH-PRESSURE BEHAVIOR OF CONCRETEMartin J. Schmidt (AFRL/RWAC)Mark L. Green (AFRL/RWMW)Air Force Research LaboratoryEglin AFB, FL 32542Oana Cazacu (UF/Reef)Department of Mechanicaland Aerospace Engineering University of Florida REEFShalimar, FL 32579SEPTEMBER 2008JOURNAL ARTICLE© 2008 John Wiley & Sons, Ltd. [doi: 10.1002/nag.700]This article has been published online in the International Journal for Numerical and Analytical Methods in Geomechanics, 6 March 2008, and is copyrighted.One or more of the authors is a U.S. Government employee working within the scope of their position; therefore, the U.S. Government is joint owner of the work, and has the right to copy, distribute, and use the work by, or on behalf of, the U.S. Government. All other rights are reserved by the copyright owner.This paper is published in interest of the scientific and technical exchange.Publication of this report does not constitute approval or disapproval of the ideas or findings.AIR FORCE RESEARCH LABORATORY, MUNITIONS DIRECTORATE ⏹ Air Force Material Command ⏹ United States Air Force ⏹ Eglin Air Force Base5(3257 '2&80(17$7,21 3$*(A Ã6 rq PH7ÃI Ã & # ''ÃÃS@QPSUÃ96U@Ã 99 HH ```` ! ÃÃS@QPSUÃU`Q@Ã# ÃÃUDUG@Ã6I9ÃTV7UDUG@$h ÃÃ8PIUS68UÃIVH7@S % ÃÃ6VUCPS T & ÃÃQ@SAPSHDIBÃPSB6IDa6UDPIÃI6H@ T Ã6I9Ã699S@TT @T( ÃÃTQPITPSDIB HPIDUPSDIBÃ6B@I8`ÃI6H@ T Ã6I9Ã699S@TT @T ' ÃQ@SAPSHDIBÃPSB6IDa6UDPI ÃÃÃÃS@QPSUÃIVH7@S ÃTQPITPS HPIDUPS TÃ68SPI`H T " ÃTVQQG@H@IU6S`ÃIPU@T! 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ÃÃSr qr Ã u yqÃirÃh h rÃ uh Ã v u h qv tÃh Ã ur Ã v v Ã sÃyh Ã Ã r Ã uhyyÃir iwrp Ã Ãh Ã r hy Ãs Ãshvyv tÃ Ãp y Ã v uÃhÃp yyrp v Ã sÃv s h v ÃvsÃv Ãq r Ã Ãqv yh ÃhÃp r y Ã hyvqÃPH7Ãp yÃ ir QG@6T@Ã9PÃIPUÃS@UVSIÃ`PVSÃÃAPSHÃUPÃUC@Ã67PW@Ã699S@TT ÃÃ" ÃÃ96U@TÃ8PW@S@9Ã A Ã ÃU$i ÃÃBS6IUÃIVH7@S$p ÃÃQSPBS6HÃ@G@H@IUÃIVH7@S$q ÃÃQSPE@8UÃIVH7@S$r ÃÃU6TFÃIVH7@S$s ÃÃXPSFÃVIDUÃIVH7@SÃTQPITPS HPIDUPS TÃS@QPSUÃÃÃÃÃÃÃIVH7@S T % ÃT@8VSDU`Ã8G6TTDAD86UDPIÃPA)(i ÃU@G@QCPI@ÃIVH7@SÃ D py qrÃh rhÃp qr XX-09-2008Journal ArticleEXPERIMENTAL AND THEORETICAL INVESTIGATION OF THEHIGH-PRESSURE BEHAVIOR OF CONCRETE N/AN/AN/AN/A N/AN/A1. Martin J. Schmidt (AFRL/RWAC)1. Mark L. Green (AFRL/RWMW)2. Oana Cazacu (UF/Reef)1. Air Force Research Laboratory 2. Department of MechanicalMunitions Directorate and Aerospace EngineeringEglin AFB, FL 32542-6810 University of Florida REEFShalimar, FL 32579AFRL-RW-EG-TP-2008-7419Air Force Research LaboratoryMunitions DirectorateAFRL/RWACEglin AFB, FL 32542-6810AFRL-RW-EG SAME AS BLOCK 8DISTRIBUTION A: Approved for public release; distribution unlimited.Approval Confirmation AAC/PA # 04-11-07-243; dated 11 April 2007.See 'cover page' for pertinent metadata information.The results of an experimental study aimed at characterizing the behavior of concrete for high confining pressures (up to 500MPa) are reported. The main characteristics of the response under deviatoric conditions are quasi-linearity in the elastic regime, stress-pathdependency, and gradual change from compressibility to dilatancy under increasing deviatoric stress. By performing the cyclic triaxial compression tests with several load–creep–unload and reload cycles, the time influence on the overall behavior was detected. Further, whether the main features of the observed behavior can be described within the framework of elastic/viscoplasticity theory wasinvestigated. For this purpose, Cristescu’s (Rock Rheology. Kluwer Academic Publishers: The Netherlands, 1989) approach was used. It was shown that the proposed elastic/viscoplastic model captures the main features of concrete behavior at high pressures.Copyright © 2008 John Wiley & Sons, Ltd.concrete; high confinement; compressibility; viscoplasticityUNCLASSIFIED UNCLASSIFIED UNCLASSIFIED SAR 25Martin SchmidtINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int.J.Numer.Anal.Meth.Geomech.(2008)Published online in Wiley InterScience().DOI:10.1002/nag.700Experimental and theoretical investigation of the high-pressurebehavior of concreteMartin J.Schmidt1,∗,†,Oana Cazacu2and Mark L.Green11Air Force Research Laboratory,Eglin AFB,FL32542,U.S.A.2Department of Mechanical and Aerospace Engineering,University of Florida REEF,Shalimar,FL32579,U.S.A.SUMMARYThe results of an experimental study aimed at characterizing the behavior of concrete for high conﬁning pressures(up to500MPa)are reported.The main characteristics of the response under deviatoric conditions are quasi-linearity in the elastic regime,stress-path dependency,and gradual change from compressibility to dilatancy under increasing deviatoric stress.By performing the cyclic triaxial compression tests with several load–creep–unload and reload cycles,the time inﬂuence on the overall behavior was detected. Further,whether the main features of the observed behavior can be described within the framework of elastic/viscoplasticity theory was investigated.For this purpose,Cristescu’s(Rock Rheology.Kluwer Academic Publishers:The Netherlands,1989)approach was used.It was shown that the proposed elastic/viscoplastic model captures the main features of concrete behavior at high pressures.Copyright 2008John Wiley&Sons,Ltd.Received3August2007;Revised30November2007;Accepted14January2008KEY WORDS:concrete;high conﬁnement;compressibility;viscoplasticityDedicated to Professor N.D.Cristescu in recognition of his major contributions to geomechanics1.INTRODUCTIONA rather signiﬁcant literature exists on the quasi-static mechanical response of cementitious mate-rials and geomaterials.However,most of the experimental results are obtained using triaxial cells and concern the response under moderate conﬁning pressures,i.e.up to approximately100MPa (see,for example,[1–4],etc.).True triaxial compression test results reported are for the same range of pressures(see,for example,[4,5]).Bazant et al.[6]have developed an oedometric-type test that allows characterization of the response at high pressures.A cylindrical specimen(height44.4mm,Correspondence to:Martin J.Schmidt,Air Force Research Laboratory,Eglin AFB,FL32542,U.S.A.†E-mail:martin.schmidt@Copyright2008John Wiley&Sons,Ltd.M.J.SCHMIDT,O.CAZACU AND M.L.GREENdiameter18.8mm)is placed in a very rigid hollow plate,which prevents lateral deformation,and is subjected to compression parallel to its vertical axis.The major limitations are related to the size of the specimens and accurate evaluation of frictional effects.Bourlion et al.[7]have extended this testing method to allow testing of concrete with aggregate sizes up to16mm at levels of mean pressure of about600MPa.These authors have also reported true hydrostatic test results on small mortar size specimens.There is a large body of literature on modeling the behavior of cementitious materials.For moderate levels of pressure,the non-linearity of the response is mainly due to damage by micro-and meso-crack’s growth.Major developments have occurred over the past decades in modeling the behavior at continuum level.Continuum damage models of various degrees of complexity have been proposed(for comprehensive reviews of the main contributions,see[8,9]).For higher levels of pressure(mean stress or conﬁnement),the behavior is more ductile and plasticity models(e.g.[10,11])as well as coupled damage-plasticity models([12–16],etc.)are more appropriate.To model time effects on the non-linear behavior of cementitious materials,a very large number of models developed for inviscid behavior have been extended in the rate-dependent range using the overstress concept(see,[17]).Thus,the expressions of the constitutive func-tions(i.e.yield function,plastic potential)used in these rate-sensitive models are the same as the ones used to describe inviscid behavior.In contrast,the approach introduced by Cristescu (see,for instance,[4])is to determine the speciﬁc expressions of the yield function and viscoplastic potential for a given material directly from creep data.This allows describing both compressibility and dilatancy by a unique continuous yield function and viscoplastic potential, respectively.In this paper,we present the results of an experimental investigation into the effects of high conﬁnement on the deformation and strength of a concrete material.By performing cyclic triaxial compression(CTC)tests with several load–creep–unload and reload cycles,the time inﬂuence on the overall behavior was detected.Based on these data,an elastic/viscoplastic model that captures compressibility,dilatancy,and strain rate effects has been developed.The structure of this model and a procedure for determining the expressions of the constitutive functions and material parameters based on a minimal set of data are given in Section3.We conclude by presenting comparison between model simulations and data.2.EXPERIMENTAL STUDYThe aim of this experimental study is to extend the existing database on the deformation and failure of concrete by investigating the behavior over a range of pressures characteristic of impact events.Of particular interest was the inﬂuence of very high conﬁning pressure(up to500MPa)on the material’s deformation and strength.Thus,the experimental program included(a)quasi-static hydrostatic compression tests from which the increase in the compressibility of the material and an estimate of the compaction properties over the range pressures encountered in impact events can be inferred,(b)quasi-static CTC tests under lateral conﬁning pressure, 3,ranging from50 to500MPa,based on which the effect of loading history on the response and the importance of short-term time effects on the behavior can be determined.A total of21tests were conducted at the U.S.Army Corps of Engineer Engineering Research and Development Center,Waterways Experiment Station(WES)in Vicksburg,Mississippi.Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagCHARACTERIZING THE BEHA VIOR OF CONCRETE2.1.Material studiedThe mixture proportions and resulting basic properties of the concrete studied are given in Table I. The concrete is a mixture of Portland cement,chert aggregates(ﬁne and coarse),ﬂy ash,water,and water-reducing admixture.The uniaxial compressive and tension strengths at28days were45.6 and3.13MPa,respectively.The test results indicate that under uniaxial compression,the material exhibits typical brittle behavior.2.2.Specimen preparation and quasi-static test instrumentationPrior to each test,the prepared specimen was weighed,carefully measured,and ultrasonic measure-ments(P-and S-wave velocities)were taken.For all tests,the specimens had a nominal height of 110mm and a diameter of50mm.Because of the inherent heterogeneity of the material and the high-pressure range in which it was tested,instrumentation with strain gages was found unreli-able.Instead,to measure the displacement between the top and the base caps(i.e.determine the axial deformation of the specimen),two linear variable differential transformers(LVDTs)mounted vertically on the instrumentation stand and positioned180◦apart were used.The radial deﬂection was measured using two LVDTs,which were connected to two small steel footings mounted diametrically(180◦apart)at the mid-height of the specimen(see Figure1).A gypsum paste was used to patch all voids.Subsequent to patching of the voids,the specimen was encased in a latex membrane,and this membrane was then surrounded by a layer of putty-like material.The putty served the function of self-sealing any potential punctures of the inner membrane.A second latex membrane was then applied over the putty and the membrane was coated with a sealant to prevent its degradation by the petroleum-based hydraulicﬂuid.The axial load was provided by an8.9MN loading machine(Secodyne)coupled with600-MPa-capacity pressure cell(see the schematic diagram in Figure2).The loading rate/proﬁle was computer programmable,allowing for precise control of the experiments under load,displacement, or strain control.2.3.Experimental resultsA triaxial compression test is generally conducted in two phases:a hydrostatic compression phase and a deviatoric phase.During the hydrostatic phase,the specimen is subjected to isotropic compression,while measurements of the axial and radial deformations are made.The deviatoric Table I.Ingredients and mixture proportions for WES5000concrete.Item Mixture proportions, saturated surface–dryType I Portland cement264.0kg/m3Flyash55.8kg/m39.5mm local unprocessed chert coarse aggregate1037.6kg/m3Local unprocessed chertﬁne aggregate840.7kg/m3Water145.9kg/m3Water-reducing admixture‘300N’0.65l/m3High-range water reducing admixture‘Rheobuild716’1.6l/m3Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagM.J.SCHMIDT,O.CAZACU AND M.L.GREENFigure1.Spring-arm lateral LVDT mounted on test specimen.phase of the test is conducted after the desired conﬁning pressure has been reached during the hydrostatic phase.While holding the desired lateral conﬁning pressure constant,the axial load is increased until the specimen fails.As an example,the pressure vs volumetric data obtained from the hydrostatic phase of a CTC test under200MPa conﬁning pressure are presented in Figure3.In order to evaluate the bulk modulus,an unloading–reloading cycle was performed at100MPa.Note the signiﬁcant hysteresis displayed,which clearly makes the determination of the bulk modulus highly imprecise.To determine the elastic properties with accuracy,it is necessary to ensure a good separation between viscous effects and unloading.One way of accomplishing this is to hold the stress constant before each unloading/reloading cycle such as to allow the material to creep.The duration of the creep stage(time interval in which the load is kept constant)is usually determined by monitoring the rate of change of the strains.When the strain rate approaches zero,i.e.the material reaches by creep a quasi-stable state,unloading is conducted.More details concerning this experimental procedure and its application to a variety of geologic materials can be found in Cristescu and Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagCHARACTERIZING THE BEHA VIOR OF CONCRETEFigure2.Section through triaxial pressure cell.Figure3.Mean stress–volumetric strain data from a hydrostatic cyclic test on concrete. Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagM.J.SCHMIDT,O.CAZACU AND M.L.GREENHunsche[4].This step-wise loading procedure was used for the hydrostatic phase of each CTC test conducted.As an example,the results of a hydrostatic test up to500MPa are shown in Figure4. The close agreement between the axial and radial gage measurements indicates that the material is isotropic.The test consisted of seven loading–creep–unloading–reloading cycles.For each cycle, prior to unloading,the pressure was held constant for30min.Then,partial unloading and further reloading were performed.In Figure5,the volumetric strain rate vs time data corresponding to theFigure4.Mean stress–volumetric strain data from a hydrostatic test up to500MPa conducted followinga step-wise loading/creep/partial unloading procedure.Figure5.V olumetric strain rate vs time from the second cycle(unload at100MPa)of ahydrostatic test up to500MPa.Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagCHARACTERIZING THE BEHA VIOR OF CONCRETEsecond cycle(unloading at100MPa)are presented.It can be seen that within the duration of thecreep stage,the rate of change of the volumetric strain with time approached zero.In Figure6,the pressure–strain curve corresponding to the second creep–loading–unloading–reloading cycleis shown.Note that very little hysteresis is present,and the slope of the quasi-linear part of theunloading curve coincides with the slope of the quasi-linear part of the reloading curve.The bulkmodulus,K,corresponding to several pressure levels was evaluated from the slopes of the quasi-linear part of the unloading curves of pressure vs volumetric strain.The bulk modulus was foundto increase with the applied pressure from K=23.24GPa(second cycle)to K=28.76GPa(lastcycle).The reduction in porosity at the end of the test(i.e.at0.5GPa)is approximately2.5%.Figure7shows the principal stress–strain data from the deviatoric phase of a CTC test conductedat 3=375MPa conﬁning pressure following a step-wise loading procedure.Note that the material exhibits irreversible time-dependent behavior.The stress vs volumetric strain curve also shows ahighly non-linear volume response,with the onset of dilatancy being very close to failure.Thesame trends have been observed in all the tests conducted.Figure8shows a comparison of thestress–strain data from the deviatoric phase of CTC tests at conﬁning pressures of200,300,and450MPa,whereas in Figure9presents the strength values determined in the same tests.It is clearlyseen that higher conﬁnement produces higher yield stress,increased hardening,and increasedstrength.From the test results,we can conclude that for high pressures,the material displays irreversibletime-dependent properties.On the other hand,the analysis of the variation of Young’s modulus withpressure(see Figure10)shows that for speciﬁc levels of radial conﬁnement,E is decreasingwith the mean stress,which is indicative of damage.Modeling of damage and damage couplingswith plasticity and viscous effects is beyond the scope of this paper.We will model only themain features of the behavior,i.e.time effects on the plastic behavior within the framework ofviscoplasticity.Figure6.Mean stress–volumetric strain data from the second cycle(unload at100MPa)of ahydrostatic test up to500MPa.Copyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagM.J.SCHMIDT,O.CAZACU AND M.L.GREENFigure7.Stress–strain data from the deviatoric phase of a cyclic triaxial compression test at aconﬁning pressure of375MPa.Figure8.Stress–strain data from the deviatoric phase of cyclic triaxial compression tests at conﬁningpressures of200,300,and450MPa.3.ELASTIC–VISCOPLASTIC MODEL DEVELOPMENTThe most widely used approach to model coupling between plastic and viscous effects on the behavior of geological or cementitious materials is that proposed by Perzyna[17](see also[18]). The basic assumption is that the viscous properties of materials become manifest only after theCopyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagFigure9.Ultimate strength as a function of conﬁning pressure.parison between theoretical and experimental variations of Young’smodulus E with the mean stress.passage to the plastic state.The expressions of the constitutive functions(yield function,viscoplastic potential)are the same as the ones used to describe the elastic/plastic,time-independent behavior. In this paper,we will use the elastic/viscoplastic modeling framework proposed by Cristescu (see,for instance,[4]),i.e.the mathematical expressions of the constitutive functions are a priori unknown and will be determined from the experimental data(creep test results).We begin by brieﬂy reviewing the general elastic/viscoplastic framework and then present the simplifying assumptionsCopyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagmade in order to obtain a reasonably simple mathematical model for the given material.It is assumed that the material is homogeneous and isotropic.The reference conﬁguration is the actual conﬁguration.The displacements and material rotations are assumed to be small.Thus,the strain rate˙e can be decomposed additively into an elastic part,˙e E,and an irreversible part,˙e I,i.e.˙e=˙e E+˙e I(1) The elastic component of the strain rate is given by˙e E=˙r2G+13K−12G˙p I(2)where the dot stands for the derivative with respect to time,G and K are the shear and bulk moduli, respectively,I is the second-order identity tensor,and p is the mean stress(i.e.p=tr(r)/3).No assumption concerning the existence of a viscoplastic potential is made.Thus,the irreversible strain rate is considered to be given by an overstress-type law of the form:˙e I=k T1−W(t)H(r)(r)(3)where t is the actual time and W(t)is the irreversible stress work per unit volume.The only restriction imposed on the tensor-valued N(r),which deﬁnes the orientation of the viscoplastic strain rate,is to be isotropic(see[19]).In(3),the symbol ,known as the Macaulay bracket, denotes x =(x+|x|)/2and k T is a viscosity coefﬁcient.Thus,there is viscoplasticﬂow if and only if H(r)>W(t).Only transient creep is modeled.From(3)it follows that the deformation due to transient creep stops after a certainﬁnite time interval and stabilization takes place,the equation for the stabilization boundary for creep beingH(r)=W(t)(4) Note that this choice of hardening variable is appropriate for the description of the behavior of concrete because it captures both the shear and the volumetric inelastic effects.Indeed,W(t)can be decomposed into two terms:W(t)=t0r(t):˙e I(t)d t=W H(t)+W D(t)tp(t)˙ I v(t)d t+tr (t):˙e (t)d t(5)where W H(t)is the energy related to irreversible volume change that is stored during compaction or released during dilatancy(volumetric expansion)of the material,and W D(t)is always positive and represents the energy input needed for change in shape.In(5),˙ I v(t)is the irreversible volumetric strain rate and r (t)is the stress deviator.Theﬁrst step in the development of the elastic–viscoplastic model for the material studied is the description of the elastic behavior.For each CTC test conducted,Young’s modulus,E,and the bulk modulus,K,were evaluated from the slopes of the quasi-linear parts of the unloading–reloading cycles performed at the end of each of the creep steps.We also estimated the corresponding Poisson ratio, .As it is the case with most rocks,soils,and cementitious materials,the elastic moduli are stress dependent.Thus,the stress–strain relationship(2)is hypoelastic in the sense of Truesdell [20].The necessary and sufﬁcient conditions for an isotropic material with stress-dependent elastic moduli to have a conservative response in the elastic regime were established by Loret[21].It wasCopyright2008John Wiley&Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagshown that the choice of the laws of variation of the elastic moduli with the stress state cannot be arbitrary.The laws of variation should be expressible in terms of the stress invariants:the meanstress p ,and the second invariant of the stress deviator q = 32tr (r )2.An analysis of the variationin Young’s modulus with pressure indicates a global increase in E with increasing p .However,for certain levels of radial conﬁnement,a decrease in E ,which is indicative of damage,is observed.Modeling of damage and damage couplings with plasticity and viscous effects is beyond the scope of this paper.We model only the increase in E with pressure by considering the following law of variation:E (p )=E ∞−b exp (−p /c )(6)In (6),E ∞is the asymptotic limit towards which E tends as the pressure becomes very large,(p →∞),i.e.the value of the Young modulus corresponding to a fully compacted state.Good overall agreement with all the available data is obtained for E ∞=61GPa ,b =E ∞−E |p =0=18.92GPa ,and c =b (d E /d p |p =0)=0.33MPa (see Figure 10).We consider =0.175,a value that corresponds to the mean of the experimental values from all tests (see Figure 11),and calculate K from the elasticity relationship:K =E3(1−2 )(7)Several factors entered into assuming a constant value of the Poisson ratio.First,for the sake of simplicity,it was seen advisable to limit the number of coefﬁcients involved in the model.Also,due to the overall higher level of conﬁdence in the axial measurements,it was found best to approximate the experimental variation of E with the mean stress rather than to approximate the experimental evolution of the bulk modulus.Furthermore,a value of 0.175for the Poisson ratio isFigure 11.Experimental variation of Poisson’s ratio with the mean stress.Copyright 2008John Wiley &Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nagwell within the range of values typically observed for concrete (see [22]).Note that Equation (7)with E given by (6)approximates well the experimental variation of the bulk modulus K with the mean stress (see Figure 12).The material being isotropic,the yield function,H ,depends on r through its invariants.The experimental data available were obtained from CTC tests in which only two principal stresses can be varied and measured independently,i.e. 1and 2= 3.The effect of the third invariant of stress cannot be inferred without data for which all three principal stressescan be varied independently.That is why we assume that H (r )=H (p ,q ),where q = 32tr (r )2is the second invariant of the stress deviator r .To determine the speciﬁc expression of H (p ,q )based on CTC results,we adopt the procedure proposed by Cristescu [3](see also [4]).Thus,we assume that H (p ,q )can be expressed as H (p ,q )=H H (p )+H D (p ,q )(8)such thatH D (p ,0)=0(9)where H H (p )=W H and H D (p ,q )=W D ,W H and W D being the volumetric and deviatoric parts of the irreversible stress work at creep stabilization.Thus,in order to determine H H (p ),we calculated the irreversible stress work corresponding to creep stabilization according to the following equation:W H (t H )= t H 0p (t )˙ I v (t )d t (10)using data from the hydrostatic phase of CTC tests at 150,300,450,and 500MPa conﬁnement,respectively.In (10),˙ I v is the irreversible volumetric strain and t H corresponds to the end of the hydrostatic phase of a CTC test.Next,we plotted the obtained values of W H (t )as a function of p .Note that W H increases monotonically with the mean pressure.However,for very high levelsFigure parison between theoretical and experimental variations of the bulk modulus K with the mean stress.Copyright 2008John Wiley &Sons,Ltd.Int.J.Numer.Anal.Meth.Geomech.(2008)DOI:10.1002/nag。

High stress

High stress140023安帅Stress is a common experience. We may feel stress when we are very busy, have important deadlines to meet, or have too little time to finish all of our tasks. Often people experience stress because of problems at work or in social relationships, such as a poor evaluation by a supervisor or an argument with a friend. Some people may be particularly vulnerable to stress in situations involving the threat of failure or personal humiliation. Major life events, such as the death of a loved one, can cause severe stress.Stress can have both positive and negative effects. Stress is a normal, adaptive reaction to threat. It signals danger and prepares us to take defensive action. Fear of things that pose realistic threats motivates us to deal with them or avoid them. Stress also motivates us to achieve and fuels creativity. Although stress may hinder performance on difficult tasks, moderate stress seems to improve motivation and performance on less complex tasks. In personal relationships, stress often leads to less cooperation and more aggression. If not managed appropriately, stress can lead to serious problems.The circumstances that cause stress are called stressors. Stressors vary in severity and duration. For example, the responsibility of caring for a sick parent may be an ongoing source of major stress, whereas getting stuck in a traffic jam may cause mild, short-term stress. Some events, such as the death of a loved one, are stressful for everyone. But in other situations, individuals may respond differently to the same event—what is a stressor for one person may not be stressful for another. Stressors can be classified into three general categories: catastrophic events, major life changes, and daily hassles. In addition, simply thinking about unpleasant past events or anticipating unpleasant future events can cause stress for many people.A person who is stressed typically has anxious thoughts and difficulty concentrating or remembering. Stress can also change outward behaviors. Physicians increasingly acknowledge that stress is a contributing factor in a wide variety of health problems. These problems include cardiovascular disorders such ashypertension (high blood pressure) and coronary heart disease (coronary atherosclerosis, or narrowing of the heart’s arteries). Stress also appears to be a risk factor in cancer, chronic pain problems, and many other health disorders.There is substantial evidence that stress suppresses the activity of the immune system, leaving an organism more susceptible to infectious diseases.。

A Thought of Wrapping Space Shuttle External Tank with Ceramic Fiber Fishnet Stockings

a r X i v :p h y s i c s /0603114v 1 [p h y s i c s .g e n -p h ] 14 M a r 2006A Thought of Wrapping Space Shuttle External Tank with Ceramic Fiber Fishnet Stockings Sun Hong Rhie ABSTRACT The new camera system of the shuttle Discovery on STS-114that blasted oﬀat 10:39am,Tuesday,July 26,2005,after 906days of grounding since the Columbia accident,has produced high resolution data of foam sheddings.The 0.9lbs piece from the Protuberance Air Load (PAL)ramp on the LH2tank is believed to be comparable in its potential adversities to the ∼1.67lbs BX-250foam from the −Y bipod ramp that demised shuttle Columbia in 2003.The two known incidences indicate that protuberant foams,possibly in conjunction with the liquid hydrogen temperature,oﬀer lame targets of the aerodynamic forces.Seven other relatively large divots in the STS-114external tank foam insulation have been reported,and foam shedding remains to be a challenge to be resolved before the next space shuttle launch.The relatively large divots from the newly streamlined foam around the -Y bipod area suggests a potential necessity for a new line of resolution.We suggest an option to wrap the insulated external fuel tank with a grid of high temperature resistant ceramic ﬁbers (ceramic ﬁber ﬁshnet stockings ).As-suming ﬁducial acreage of 20000ft 2,one inch square cell single ﬁber grid will weigh only 60g with ﬁber cost $66.Even with 1500-ﬁber-equivalent strength,one inch square cell grid will add only 200lbs and “miniscule”$100,000.Subject headings:space shuttle,ceramic ﬁbers 1.Foam Losses on STS-114While HST customers were crossing their ﬁngers,the STS-114camera system exhibited a spectacular scene of a top-hat shape 0.9lbs insulation foam ﬂying oﬀthe external tank.The public release of the foam loss locations by NASA shown in Fig.1read as follows.1.LH2PAL ramp:36.3′′×11′′section near stattion Xt-1281released at 127.1sec MET;2.8.4′′×7.3′′divot on the forward outboard portion of the -Y bipod closeout at 148.1sec MET;3.Divot(∼5.6′′L×3.5′′W×2.9′′D)in LH2tank ice/frost ramp at Xt-1262at154.8secMET(exposed Conathane adhesive at center of divot);4.Divot(∼7.3′′L×1.9′′W×2.5”D)in LH2tank ice/frost ramp at Xt-1525(exposedConathane adhesive);5.Shallow divot(∼4.0′′L×2.6′′W×0.3′′D)in LH2tank ice/frost ramp at Xt-1841;6.Divot(∼4′′dia)∼3ft below the-Y bipodﬁtting(Xt-1163)at135.8sec MET;7.Shallow divot(∼8′′−10′′dia)in LH2tank acreage between LO2feedline and pressline(Xt-1839);8.2divots(∼3′′dia)on upper intertank/LH2tankﬂange closeout on-Y axis(releasetime unknown).2.“Rationale”for an Alternative ResolutionThe press release by NASA immediately following the dislodge of the large0.9lbs piece of insulation foam from PAL ramp indicates NASA’s continuing eﬀorts to prevent defects in foam bubbles and to tame supersonic vortices and turbulences as it reads in the last paragraph:“...An enhanced spray process is in work for future tanks,as well as continued work in developing redesign options including elimination of the ramps;reducing the ramps’sizes by two thirds;or building a trailing edge‘fence’on the back side of the cable tray, which would act like a nozzle throat and prevent unsteadyﬂow in that area.”The divots in items2and8in section1are from the left side(-Y)of the bipod area. This is exactly the same area where the∼1.67lbs piece of foam broke oﬀfrom the ramp and doomed shuttle Columbia in2003.The ramps have been removed,and the surface of the foam in the bipod area has been streamlined since.But,the air managed to dump enough energy to break oﬀlarge chunks from the streamlined foam.The fourth volume of the Columbia Accident Investigation Board(CAIB)Report1exhibits a result of hydrody-namic simulations that shows streamline vortices around the-Y bipod area.We have no clue,especially not being an expert either in supersonic hydrodynamics or in external tank conﬁgurations,whether the vortices around the obstacles of the struts that hold the nose of the shuttle can be tamed without undesirable,or unpredictable and untested,side eﬀects.However,NASA’s investments of2.5years and∼1billion dollars since Coumbia accident point to a possibility that the same line of eﬀorts cited in theﬁrst paragraph of this section might have already served its capacity.An alternative is suggested to strengthen the insulated external fuel tank withﬁsh-net stockings made of high temperature resistant ceramicﬁbers that are immediately available. Oxideﬁber3M Nextel720manufactured by3M Technologies is considered.3.No Foam Loss as Speciﬁed in the Book3.2.1.2.17External Tank Debris Limits:No debris shall emanate from the critical zone of the External Tank on the launch pad or during ascent except for such material which may result from normal thermal protection system recession due to ascent heating2.It is the design requirements in“Flight and Ground System Speciﬁcation-Book1,Re-quirements”as quoted in CAIB report volume1chapter6,p.122.The report continues,“The assumption that only tiny pieces of debris would strike the Orbiter was also built into original design requirements,which speciﬁed that the Thermal Protection System(the tiles and RCC panels)would be built to withstand impacts with a kinetic energy less than0.006 foot-pounds.”The corresponding maximum mass and size of the allowed foam bebris is0.652g and ∼1in3where the latter is commonly referred to as“the size of a marshmallow”in news media. The large piece foam debris from the PAL ramp on STS-114is∼620times the maximum allowed.Nextel720α-Al2O3ﬁber properties shown inﬁgures2,3,and4(electronically cut out of3MTTN)indicate that the ceramicﬁber technology is suﬃciently mature to meet the design speciﬁcation of“no foam loss”.1.In civilized units(cgs or mks),the impact energy limit0.006foot-pounds is about8.13×104ergs.Some of the convesion formulae for pressure(or strain)are listed.1P a(scal)≡1Newton/m2=1kg m−1sec−2=1.4503774×10−4psi(1)1atm=101325P a=1.01325bar=14.6959psi(2)2.In order to convert the impact energy limit into the maximum allowed mass of a foamdebris,we need to know the areodynamic velocity due to the ascending motion of thespacecraft and wind,air density,streamline,etc,as a function of time and the position in the external tank.A recent review,found in the web site of Lockheed Martin,“STS-114Flight Readiness Review:External Tank(ET-121),”Appendix D,P.5,June29-30, 2005,indicates that the mass of a foam debris should not exceed∼0.023oz=0.652g.The density of the foam is∼2.4lbs/ft3,henced the volume of the inﬁmum mass foam debris is∼16.96cm3≈1in3.We take0.652g and1in3as the currently accepted mass and volume limits.3.Table7.2.1.8.1.2.6-1of CAIB Report titled“Summary of Acreage Stress Analysis Pa-rameters”listsﬂight requirement(foam)cell pressures in various parts of the foam insulation on the external tank:Oxygen tank:23.6,19.2,15.9psi depending on the parts;Intertank:19.7psi;Hydrogen tank:16.3psi,and bond tension test requirement:30.0psi.Hence we take30psi as the tensile strength requirement of the ceramicﬁbergrid structure.3.1.High Temperature Resistant Ceramic Fiber3M Nextel720High tensile strengthﬁbers are based in carbon,SiC,and Al2O33.Space shuttle RCC panels–now a household name since the Columbia accident–are made of carbonﬁber composite matrix with SiC protection coating and glassy sealant4,5.During the reentry of the shuttle,the sealant melts forming an air tight thinﬁlm.A report6indicates that oxide ﬁber tows(or roving:loose bundle of untwistedﬁbers)perform better than SiC-based tows in the condition under consideration where it is highly oxidating and the usage time is short (q<30000).The interactions within the bundle make the tow weaker or stronger in terms of creep or rupture in comparison to the average strength of the singleﬁber.At1200◦C, singleﬁber SiC outperforms SiC tow;At1400◦C,SiC tow outperforms singleﬁber SiC.We consider an oxideﬁber3M Nextel720.3M Nextel720ﬁber7is a continuousﬁber with the composition of85%Al2O3and 15%SiO2,melting temperature1800◦C,density3.40g/cm3,theﬁlament tensile strength2100MP a(300ksi)at25.4mm guage,the diamter∼12µm,and the crystal phaseα-Al2O3+mullite. Nextel oxideﬁbers are various mixtures of Al2O3,SiO2,and B2O3.Fiber610is made of pure Al2O3(>99%)and its crystal phase isα-Al2O3.Figure2shows the stable strength ofﬁber720tow in high temperatures.(3M Nextel oxideﬁber tows come inﬁber counts of400or750.)Figures3and4show the singleﬁber tensile strengths of theﬁbers610and720in room temperature and high temperatures where the high temperature heatings lasted about90seconds.(Note that the maximum aerodynamic friction and heating occurs around80seconds after the launch.)Theﬁber610is stronger at room temperature,butﬁber720is preferred because of the stability of the strength in high parision ofﬁgures2and4shows that theﬁber720tow performs better than singleﬁber at higher temperatures(>1150◦C)consistently with the report by DY.The concern of foam shedding is on the area where the aerodynamic stress is high,andthe design requirement8of the temperature tolerance due to aerodynamic heating is1200◦C.Within the temperature range,ﬁber720shows excellent strength behavior and also very slight diﬀerence in the strengths between a tow and an average singleﬁber.The temperature tolerance requirements for the areas with heating from solid rocket booster and from main engine are very high with1650◦C and3300◦C.Foam shedding or ablation from those areas (aft dome and lower part of the acreage)is not a concern in regard to the potential damageto the thermal protection systems(TPS),and reinforcementﬁbers should not be used to avoid generating unnecessary potential hazard of looseﬁber fragments or melts.3.2.Added Mass Estimation1.The total acreage of the external tank foam insulation is∼24000ft2.(The total massof the foam is∼4800lbs.)The area that needsﬁber reinforcement is likely to be lessthan20000ft2,and20000ft2will be considered theﬁducial acreage.Most of the foamloss seems to come from intertank area where liquid oxygen tank and liquid hydrogentank are joined(CAIB report).2.Consider a singleﬁber one inch square cell grid.The number of cells in the grid is2,880,000,the total length of theﬁber in the grid is5,760,000in,and the total massis60g.(According to3MTTN,400ﬁber count tow has1500denier(denier=numberof grams in9000m or10000yd of a product).)Assuming absolute no failure of thesingleﬁber grid,added weight of60g ofﬁbers will keep the foams from being released.It is most likely not practicable,but the negligibly small mass of the singleﬁber grid is a good news.The cost of the singleﬁber grid will be$66.It is only getting better.3.The tensile strength measurement is done with a short piece ofﬁber of length25.4mm,and I did notﬁnd failure probability of longﬁbers in3MTTN.The total strength equivalent of400or750ﬁbers of the commercial products may be more than fail-safe for the lengths of the girth and height of the appication area of the external tank.One can consider woven yarns or very narrow fabric to replace the singleﬁber strand for the grid.The ceramicﬁber net may be embedded near the surface of the foam or wrap the surface.A1500ﬁber-equivalent strength grid will add mass∼200lbs with the price tag of$100,000.The analysis is preliminary and the mass estimation is made over a broad range because of the lacking information of the properties of longﬁbers,exact behavior of the insulation foam under stresses,and so the engineering formulation of the grid.For example,the foam will have surface waves generated by the aerodynamicﬂows,and theﬁber(yarn)would need to have certain elasticity whether theﬁber grid is embedded or on the surface.9The proper elasticity may be engineered through the bundle structure such as braiding or weaving. Inelasticﬁber can act as a“diamond string saw”to an adverse eﬀect.Even if it shaves oﬀonly benignly small pieces,it should be best to avoid generating additional foamﬂakes.In summary,a ceramic-ﬁber-grid reinforcement can eliminate foam-shedding problem. The projected mass and price of the grid are small.There are some details to be speciﬁed to engineer a practical ceramic-ﬁber grid that will secure insulation foam of the external tank –economically.“No foam loss”makes theﬂight safer and reduces turn-around repair costs. We call for cooperations from the interested parties.AcknowledgmentsWe appreciate discussions with J.A.DiCarlo on creep properties of tows and introduction to3M Technologies and with3M Technologies personnel on Nextelﬁbers and the costs,and the library of University of Notre Dame for the access of reference books.Fig. 1.—Foam loss locations on STS-114external fuel tank publicly released by NASA: or http://www.spaceﬂ.Fig.2.—3M Nextel720tow shows relativley high stability in high temperatures.Fiber610 shows sudden drop in strength at1200◦C.(Theﬁgure is from3MTTN.)Fig.3.—Room temperature singleﬁber tensile strengths ofﬁber610andﬁber720.(The ﬁgure is from3MTTN.)Fig. 4.—Fiber720shows a relatively stable strength at high temperature heating for ∼90sec.The maximum aerodynamic heating of the external tank occurs at∼80sec after the launch.(Theﬁgure is from3MTTN.)。

很高的强度承受载荷英语

很高的强度承受载荷英语High-Intensity Load-Bearing CapacityIn engineering and construction, the ability to withstand high-intensity loads is crucial for ensuring the safety and longevity of structures. This capability is fundamental in various fields, from building bridges and skyscrapers to designing vehicles and machinery. In this essay, I will discuss the importance ofhigh-intensity load-bearing capacity, the factors influencing it, and its implications for modern engineering.Firstly, the importance of high-intensity load-bearing capacity cannot be overstated. Structures and systems that can handle significant loads are essential for maintaining stability and safety. For instance, bridges must endure the weight of vehicles, pedestrians, and environmental forces such as wind and earthquakes. Similarly, skyscrapers must withstand the combined load of their own weight, occupants, and external pressures. Ensuring that these structures can handlehigh-intensity loads is critical to preventing failures that could lead to catastrophic consequences.Several factors influence the load-bearing capacity of a structure. Material selection is one of the most important considerations. High-strength materials, such as reinforced concrete, steel, and advanced composites, provide the necessary support and durability. These materials are engineered to endure extreme conditions and are often tested to ensure they meet rigorous standards. Additionally, the design and construction methods used play a significant role. Engineers must consider factors such as load distribution, structural reinforcement, and safety margins to achieve the desired load-bearing capacity.The implications of high-intensity load-bearing capacity extend beyond safety and structural integrity. For example, in the aerospace industry, aircraft must be designed to handle the forces encountered during flight, including takeoff, turbulence, and landing. In the automotive sector, vehicles must be able towithstand the stresses of acceleration, braking, and collision. In each case, high load-bearing capacity contributes to performance, reliability, and overall safety.Moreover, advancements in materials science and engineering have led to the development of innovative solutions for enhancing load-bearing capacity. Techniques such as computational modeling and stress analysis allow engineers to design structures that can handle unprecedented loads. Additionally, the use of smart materials and adaptive systems can improve a structure's ability to respond to changing conditions, further enhancing its load-bearing capabilities.In conclusion, high-intensity load-bearing capacity is a fundamental aspect of modern engineering and construction. It ensures the safety, stability, and performance of structures and systems across various industries. By focusing on material selection, design, and technological advancements, engineers can create robust solutions that meet the demands of today'scomplex and challenging environments.。

Fracture testing of welded single edge notch tensile specimens

Fracture testing of welded single edge notch tensilespecimensChristopher Bayley ⇑,Neil AucoinDRDC-Atlantic,Dockyard Laboratory Paciﬁc,CFB Esquimalt,Victoria BC,PO Box 17,000Stn Forces,Canada V9A 7N2a r t i c l e i n f o Article history:Received 3July 2012Received in revised form 2February 2013Accepted 11February 2013Keywords:Heat affected zone Crack tip opening displacementExperimental strain analysisFinite element analysisMarine structuresa b s t r a c tThe fracture behavior of welded single edge notched tension specimens was examinedusing constraints and loading conditions which approximated those of a ﬂawed ship struc-tural panel.The specimens were notched and fatigue pre-cracked in the microstructuralregion associated with a brittle coarse grained heat affected zone and dynamically frac-tured at the minimum design temperature.In all cases,the fracture event was precededby some degree of plasticity.With the aid of validated ﬁnite element method companionmodels,a crack mouth to crack tip opening displacement transfer function was generatedallowing the fracture toughness at failure to be determined.Ó2013Published by Elsevier Ltd.1.IntroductionShip classiﬁcation societies have always recognized that fracture toughness is a key material property.This is reﬂected in their steel grading schemes which consider not only strength,but also fracture properties.Lloyds Register for Shipping [1],for example,designates a letter grade from A to F which corresponds with the test temperature and associated minimum Charpy V-Notch impact energy.The grade selection for commercial ships is then empirically related to its operational envi-ronment,type of cargo and fore-aft location.While this grading system is an important quality assurance mechanism,it is difﬁcult to compare these test results with the loading conditions and constraints encountered by the material in service [2].The use of higher strength steels in commercial ship construction is uncommon,however,there has always been interest in the development and use of higher strength steels for Naval construction due to its increased toughness,particularly un-der high strain rate loading events [3].While this fostered the development of speciﬁc Naval grades of steel,a 1994US Department of Defense directive required its Navy to wean itself off these grades in favor of commercial standards [4].HSLA-65[5]is such a replacement,for its strength was developed to lie in-between that of conventional high strength struc-tural steels and high yield alloyed steels.These strength requirements balance the desire for thinner structures with the need for minimum thickness to resist buckling.The replacement of military speciﬁc grades of steel with commercial grade HSLA-65hinged,to some degree,on the use of pre-existing welding consumables and associated processes.Through numerous welding studies,American Welding Society (AWS)70series consumables were deemed satisfactory,despite concerns over its fracture toughness and matching weld strength.Some of these concerns have been duplicated with a Flux Cored Arc Welding (FCAW)study of 15.8mm thick HSLA-65plates,which was intended to examine the fracture toughness of the coarse grained heat affected zone [6–9].While these studies consistently showed a signiﬁcant reduction in the fracture toughness,the structural signiﬁcance of this reduced toughness was unknown.This is because fracture toughness tests use a highly constrained conﬁguration which develops a highly tri-axial stress state at the crack tip.While these conﬁgura-tions provide a lower bound,they are inherently conservative.0013-7944/$-see front matter Ó2013Published by Elsevier Ltd./10.1016/j.engfracmech.2013.02.020⇑Corresponding author.Tel.:+12503634784;fax:+12503632856.E-mail addresses:Christopher.bayley@drdc-rddc.gc.ca (C.Bayley),Neil.aucoin@drdc-rddc.gc.ca (N.Aucoin).258 C.Bayley,N.Aucoin/Engineering Fracture Mechanics102(2013)257–270Nomenclaturea/W normalized crack lengthCTOD(d)crack tip opening displacement(calculated value of fracture toughness)CMOD crack mouth opening displacement(physically measured displacement)HSLA high strength low alloyK elastic fracture toughnessJ elastic–plastic fracture toughnessJ c fracture instability toughnessE elastic modulusr0yield strengthM dimensionless constant relating J to CTODWeld Series A MIL-E-24403-1D(71-T1HYN Consumable)Weld Series C MIL-E-24403-2C(101TM Consumable)The toughness requirements for ship materials have been based on a historical association of a small scale sample being able to resist brittle failure.This is despite the method of loading and sample geometry being different from those of an actual ship structure.To examine whether a previously known brittle region of a proposed hull structural material was signiﬁcant,eight representative large scale samples are panion Finite Element Method(FEM)simulations of these samples were used to infer the fracture toughness.2.Sample conﬁgurationFrom two pre-existing batches of welds,eight specimens were machined(Fig.1a)which have a central transverse weld joining two pieces of15.8mm HSLA-65base plate[7–8].All of the specimens were welded using the Flux Core Arc Welding procedure with either a Mil Spec71-T1HYN weld consumable,denoted as series A,or a Mil Spec101TM[10]weld consum-able,denoted as series C.These consumables were selected in order to either match or overmatch the strength of the parent material,respectively.However,all-weld metal tensile testing subsequently determined that the as-weld strengths were similar[7],compromising the initial intention of examining the inﬂuence that weld metal strength has on the fracture characteristics.As shown in Fig.1b,a through-thickness notch was machined into one side of the specimen with the notch tip located in the straight fronted fusion zone of the weld.Along with showing the notch locations,Fig.1b also establishes the weld orientation nomenclature used in this experiment.This nomenclature is relative to the original edge preparation with a45°bevel (Angle Side)or straight edge (Square Side),while the upper and lower surfaces of the weld are relative to the root pass,such that the root pass is located at the bottom.With reference to Fig.1b,measurements of weld height refer to the vertical direction,while weld width refers to the horizontal direction.A laser scanner consisting of a laser distance sensor mounted on linear bearings and connected to a string potentiometer was used to measure the two-dimensional proﬁles of the plates.The upper surfaces of the welds share a similar rounded rectangular shape,with their maximum heights varying from 2.4to 4.3mm,while the lower surface of the welds are less varied due to the use of a ceramic backing strip to support the root welding pass.The measured weld proﬁles are summa-rized in Table 1,where weld heights are the distances from their respective base plate surfaces and were acquired in order to include realistic weld shapes into the companion numerical models.Of equal importance to the weld proﬁle,was the extent of the angular misalignment and offset,as this is believed to inﬂuence the fracture behavior.In all cases,the angular mis-alignment resulted in the upper surface becoming visibly concave,caused by the un-balanced weld preparation and insuf-ﬁcient constraints.The weld misalignments,summarized in Table 1,were determined from scans covering the complete length of each sample.The fracture behavior of structures is inﬂuenced by a combination of geometry,material,temperature and loading con-ditions.In the context of Naval operations,the worst case environmental conditions are associated with winter operations in which air temperatures can be signiﬁcantly cooler than the water.Hence the target testing temperature coincides with that speciﬁed for hull structural steels i.e.À40°C [11].For ship structures,strain rates associated with hull girder bending under wave action is approximately 10À4s À1with elevated strain rates associated with wave induced slamming (2Â10À3s À1at midships and 2Â10À2s À1at the forward deck)while the strain rates associated with local hull plating during collisions is 2.0s À1[12–14].The stress intensity rates associated with these strain rates,with the assumption of a 50mm long crack for hull girder bending under wave action corresponds to 5MPa p m s À1,while for wave slamming at the midships deck is 102MPa p m s À1,103MPa p m s À1,at the forward deck,while locally the stress intensity rates at local hull plating is around 3Â104MPa p m s À1[13].Geometry and crack location inﬂuence the fracture behavior too,as illustrated by the difference in the crack driving force associated with three common conﬁgurations:the Double Edge Notch Tension (DENT),the Centre Cracked Tension (CCT)and the single edge notched tension (SENT)specimens.The crack driving force for these three con-ﬁgurations,expressed as either the elastic–plastic fracture toughness (J ),or equivalently the crack tip opening displacement (CTOD),is plotted along with the range of experimentally acquired fracture toughness and limit loads in Fig.2.All of the conﬁgurations utilize the HSLA-65base metal properties and a normalized crack length of a /W =0.125,and maintains the Table 1Specimen weld proﬁles and misalignment.Angularmisalignment (°)Offset (mm)Lower weld height (mm)Lower weld width (mm)Upper weld height (mm)Upper weld width (mm)A10.750 2.510.5 2.419A20.930 3.210 3.619A310.6 2.79.5 3.520A40.90 2.19.5 2.919C11.1312.5103.320C20.750.3 1.47.5 3.920C30.850.6 2.17 4.320C4 1.120.5 3.5 4.321C.Bayley,N.Aucoin /Engineering Fracture Mechanics 102(2013)257–270259260 C.Bayley,N.Aucoin/Engineering Fracture Mechanics102(2013)257–270same basic specimen widths and remaining ligament length as those used in this experiment.The expected failure morphol-ogy can be interpreted from the intersection of the curve with either of the J c values or the limit load with increasing applied force,whichever comesﬁrst.For example,when the curve enters the range of experimental J c values,crack initiation and propagation is the expected outcome,while loads above the load limit will result in ductile necking.For the case of the CCT and DENT specimen,failure associated with exceeding the limit load of the specimen(i.e.a ductile necking)is expected rather than crack initiation and propagation.On the other hand,crack initiation and propagation is expected for the SENT conﬁguration prior to the sample exceeding its limit load,and thus the SENT conﬁguration is a suitable vehicle for fracture toughness studies.In addition to producing the desired fracture morphology,the tensile loading of the SENT conﬁguration is representative of actual ship board conditions,as it allows the weld proﬁle to be preserved,which Sumpter[15]indicates produces an effec-tive fracture mechanics specimen suitable for examining the fracture safety of ship hull materials,and Ruggieri[16–17]use for representative evaluation of marine steel risers in off-shore applications.3.Experimental fracture testing3.1.Experimental setupThe eight SENT samples were tested in tension in a1MN servo-hydraulic test frame.The samples were positioned in be-tween a pair of clevises and attached with metal straps as shown in Fig.3.While the clevises allow in-plane rotation about a pin located a distance from the ends of the sample,the relativelyﬂexible strap and bolt arrangement impart some out-of-plane constraint,but do not preclude out-of-plane motion.Prior to fracturing the samples,each sample was fatigue pre-Experimental setup of a SENT specimen in between eight metal straps and afrom the end of the notch.The rationale for positioning the strain gauges on both the upper and lower surfaces of the sample was to monitor the extent of out-of-plane bending associated with the straightening of the distorted specimens.The CMOD gauge was positioned in the mid-thickness of the machined recess (bird’s mouth)in order to locally monitor the onset of crack initiation and propagation.Together with the strain gauges,the transducers also provide valuable validation data for the companion ﬁnite element models.In addition to the specimen mounted gauges,full Wheatstone bridge strain gauge circuits were attached onto the gripping metal straps.These strap gauges were intended to monitor the out-of-plane and in-plane bending moments of the sample,but due to the inadequate stiffness of the bolted connections joining the straps to the sample and clevis,the data was deemed inadequate for validation purposes.All of the strain gauges were conditioned through separate channels of a Vishay 2310signal conditioner which output an analog signal to the MTS data acquisition system.These were recorded simultaneously with the time,load,actuator displacement and CMOD.3.3.Fracture testingApart from the ﬁrst sample (C3),the specimens were tested to failure under displacement control with an initial rate of 26.3–36mm s À1which,as seen in Fig.4,resulted in a 700kN load in less than 0.15s.This displacement rate (and the sta-bility of the displacement rate)was limited by the pressure and ﬂow characteristics of the hydraulic power supply used.Be-yond 700kN,the near constant displacement rate combined with the plastic deformation of the samples caused a decrease in loading rate,comparable to the yielding behavior of a standard tensile test.All of the samples were loaded to failure and resulted in either brittle cleavage,ductile micro-void coalescence or mixed fracture morphologies.While three failure mor-phologies were observed,the loading rates associated with the ductile failures (samples A1,A4and C1)were signiﬁcantly reduced after the onset of plastic deformation,indicating that necking had occurred.As a result of the test being performed using displacement control,the loading rates at failure for these plastically deforming specimens were low,and in these the failure initiated as a neck within the bulk of the base metal rather than crack propagation from the notch.Thus,for these two reasons,they are neither representative of the types of failure nor the loading rates which are being examined in this pro-gram.In comparison,the cleavage fracture initiated at the notch and propagated either along the weld fusion boundary or into the parent material.In the case of mixed-morphology fractures,the crack initiated at the notch initially with a brittle cleavage morphology,but subsequently changed to a ductile morphology and propagated in the base metal.Photographs of two fracture surfaces for brittle failures are shown in Fig.5a and c for samples A3and C4,respectively,while Fig.5b and d are side views opposite the original notch showing that the ﬁnal crack length had propagated either through the base metal (A3)or along the weld fusion boundary (C4).For these two cases,the fractures appear completely ﬂat with no signiﬁcant shear lips.Other fracture surfaces in which the fracture propagated away from the fusion line into the base material developed more pronounced shear lips,while those identiﬁed in Table 2as ductile developed signiﬁcant shear lips extending across the thickness of the samples.Table 2summarizes the failure conditions for all eight specimens,including the fracture surface morphology,the fatigue pre-crack length and the CMOD and peak loads,note the fracture data for C3was not recorded.The linear elastic loading rate (_K)was estimated from the CTOD rate (_d )and subsequently converted to the linear elastic fracture rate through:_K ¼ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃm _dE r 0q where m is the material dependent dimensionless constant used in relating J to CTOD,E the elastic modulus,and r 0the yield strength.The value of the material dependent dimensionless constant was estimated from ASTM E1820[19]assumingaC.Bayley,N.Aucoin /Engineering Fracture Mechanics 102(2013)257–270261compact tension geometry,measured initial crack lengths and mechanical properties.The equation yields m =1.82which falls in between the range associated with plane stress and plane strain [20].The CTOD rate was based on the CMOD rate and a transfer function.Examination of the specimen strain gauge and CMOD data indicates that all failures occur following some amount of plas-tic deformation prior to ﬁnal fracture.The separation of the specimen strain into the through-thickness membrane and bending components provides a more intuitive understanding of the inﬂuence of the angular misalignment and offset.Plot-ted in Fig.6are the strain responses for sample C4which initially had an angular misalignment of 1.10°with the upper sur-face being on the concave side.Examining Fig.6a,the strains on the upper surface are initially greater than the lower surface,and coincide with the initial straightening of the bent sample.The lower surface records compressive strain before the axial tension overcomes the initial bending stresses acting on the plate.The associated membrane and bending strain components for the same sample are replotted in Fig.6b,which reveals that the membrane strain component increases linearly until just before failure.The reduction in the bending strain reﬂects the straightening of the sample as it responds to the axial tensile load.While it would have been desirable to use the strain gauges to compare the total strains at failure,foil bonded strain gauges are limited to approximately 3000microstrain before they either disbond,or fail themselves.Both of these types of gauge failures were witnessed in the samples and therefore they cannot be used to assess the strains at failure.The loads required to straighten the sample are extensive,and well above the maximum load of 300kN used in the fa-tigue pre-cracking phase.The implication is that during the pre-cracking phase,the specimens were subjected to both through-thickness membrane and bending components which were responsible for the angled fatigue cracks.This through-thickness variation in the original crack length is clearly seen in Fig.5a and c with the longest portion of the crack located on the upper surface of the welded plate which would have seen the highest stresses due to bending as the specimen (a) A3 Brittle – Upper surface on top of image (b) A3-Brittle (Side viewof (c) C4 Brittle – Upper surface on top of imageTable 2Experimental summary.Avg.fatiguepre-crack length(max deviation)(mm)Crack path End point Max load (kN)Elastic strain rate (s À1)Time at fracture (s)_K MPa m s À1CMOD at end point-(mm)A110.01(3.44)Base Pop-In 7530.0220.5402NA A28.11(5.44)Base Cleavage 7610.0190.17412 1.06A37.57(2.73)Fusion line/base Cleavage 7520.0220.18377 1.28A49.50(2.75)Base Ductile 7650.018NA 392Transducer error C18.37(1.78)Base Pop-In 7490.0220.39376NA C29.64(2.58)Base Cleavage 7240.0220.133700.21C37.71(1.64)Base Cleavage NA 0.022NA 399Transducer error C48.86(2.09)Fusion line Cleavage 745.60.0210.1604000.66262 C.Bayley,N.Aucoin /Engineering Fracture Mechanics 102(2013)257–270C.Bayley,N.Aucoin/Engineering Fracture Mechanics102(2013)257–270263straightened.After the specimen straightened this through-thickness stress was signiﬁcantly reduced,as evident by sharp decrease in the bending strains.The implication however of the signiﬁcant load required to straighten the sample was the non-linear crack.During subsequent fracture testing,this non-linear nature suppressed fracture initiation as the average crack length is shorter,and in particular,the mid-thickness region of the crack subjected to the greatest hydrostatic stress component was the shortest.While the eight samples exhibited signiﬁcantly different failure characteristics,the local sensor data was largely insen-sitive to the failure type.For the sample strain gauges,both brittle and ductile failure resulted in strains above3000micro-strain and peak loads in the range of724–765kN.The most discernable characteristic was the time at fracture,with brittle fractures occurring in less than0.17s and ductile fracture exceeding0.39s.The specimen data is therefore unable to inde-pendently resolve whether the samples were either elastically or plastically deforming at failure,and also cannot be used independently to determine the fracture toughness.4.Numerical analysisNumerical analyses provided a virtual counterpart to each experimental sample.When validated,it provides details of the sample stresses and strains,along with a transfer function which relates the CMOD to the CTOD,and hence can be used to estimate the fracture mechanics response.While the models endeavored to be numerical replicas,their realism was con-strained to include only the most relevant criteria.The limitations of the numerical model are highlighted and compared with their experimental attributes in Table3.Simulations were conducted using both LS-DYNA(LSTC,California)[21]and WARP3D(University of Illinois,Urbana-Champagne)[22].As a fracture speciﬁc program,WARP3D was employed in order to employ the domain integral approach to estimate the elastic–plastic fracture response.However,WARP3D has little support for the mechanisms necessary to de-ﬁne the complex boundary conditions.In comparison,LS-DYNA is a commercial code with a wealth of these mechanisms, but lacks the ability to deﬁne a domain integral.Previous efforts to develop such a post-processing routine were successful within the elastic range,but have yet to be extended to included plasticity[23–24].While an estimation of the elastic–plastic fracture mechanics was initially focused on the domain integral approach,direct measurement of the CTOD was subse-quently adopted as an interim measurement.While the measurement procedures for CTOD measurement are simple,they are less sensitive and signiﬁcantly more mesh dependent than the domain integral.A signiﬁcant aspect of the modeling effort revolved around capturing the essence of the external boundary conditions.As seen in Fig.3,the clevises used to attach the specimen within the load train involved a signiﬁcant number of joints and con-nections.Initial modeling attempts within LS-DYNA included the rotational boundary conditions associated with the main pin of each clevis along with a number of case studies which attempted to capture the inﬂuence that the bolts and straps had on the out-of-plane displacement[25].Through these trials,the clevises were modeled as rigid bodies which provided a sim-ple geometric representation since the center of mass and rotation are coincident.The bolts used to join the straps onto the specimen and clevis are represented as revolute joints which,by deﬁnition,only allow rotation about the axis of the bolt, while the straps were modeled as linear-elastic shells.The representation of theseﬁxtures is shown in Fig.7a.In contrast, the WARP3D models did not include a representation of the straps,bolts or clevis,but applied a surface traction to the upper and lower faces of the specimen.While the surface traction permits in-plane rotation of the sample associated with the asymmetric crack opening,the center of this rotation is not coincident with the clevis pin,and hence does not capture the true experimental boundary conditions.Further details of the FEM mesh used for the analyses are shown in Fig.7b and c.Each sample was constructed from 28,672quadrilateral elements and 33,462nodes.The crack tip can be identiﬁed as the focus of the radiating mesh,and.as suggested by Anderson [18],the crack tip is deﬁned as a ﬁnite radius (0.05mm)rather than as a collapsed quadrilateral element.The dark elements in Fig.7identify those elements associated with weld metal properties while the lighter ele-ments represent the base metal.For all of the models presented herein,the crack front was considered straight.The weld and base metal are represented by a piece-wise plasticity model with a yield function deﬁned by an equivalent stress,and an isotropic hardening rate based on experimentally collected stress–strain data.While of interest,incorporating the mechanical properties of the weld heat affected zone were not considered for these models.In order to represent the weld geometrical characteristic of each specimen listed in Table 1,individual models were con-structed which included an idealization of the weld upper and lower proﬁles along with the angular misalignment.On the upper surface,the weld proﬁle was idealized as a rounded rectangular while the lower proﬁle assumed an arc of a curve.In all cases the weld on the upper surface was 22mm wide and tapered to 6mm wide on the lower surface.End views of two cases (A1and C4)are illustrated in Fig.8which were created using an algorithm that displaced the upper and lower surface nodes into the desired position.A mesh sensitivity study was performed at the outset of the project which identiﬁed that the number of through-thick-ness elements was integral in capturing the straightening of the plates.Eight through-thickness elements was determined as an effective compromise between numerical accuracy and computational efﬁciency.The mesh sensitivity study did not ex-tend to the estimation of the fracture initiation parameter.The validation of the LS-DYNA and WARP3D models,along with their associated methods of deﬁning the global boundary conditions,is based on comparison of the physical specimen strain gauges and CMOD with their virtual counterparts.Owing to the similarity in the sensor data,only a single specimen (A1)is included.The comparison of the membrane and bending strain components is plotted in Fig.9a and overlaid on top of the experimental data,while Fig.9b compares the CMOD.In both ﬁgures,the importance of capturing the experimental boundary conditions is clear.By neglecting to include the exper-imental rotation point of the clevis pin,the WARP3D models predict a far greater CMOD than observed experimentally.This manifests itself as an increased membrane and bending strain component.On the other hand,by modeling the ﬁxtures,theTable 3Comparison of experimental and numerical attributes.ExperimentalNumerical Material propertiesIncludes microstructural heterogeneity with rate and temperature dependent properties Includes only weld and base metal properties determined at quasi-static rates and at room temperature.Weld geometryIrregular shape along the length of the weld.Upper and lower weld reinforcement idealized as a constant convex surfaces MisalignmentAngular and offset Angular only CrackIrregular,dependent on the through-thickness position Gradient through-thickness Compliance Includes complete load train Limited to the specimen andstraps264 C.Bayley,N.Aucoin /Engineering Fracture Mechanics 102(2013)257–270C.Bayley,N.Aucoin/Engineering Fracture Mechanics102(2013)257–270265LS-DYNA simulations capture not only the CMOD behavior,but also the membrane strain component.Fully capturing the bending strain component continues to plague this validation,and may in part be due to omitting the offset associated with the welding distortion.Further models which incorporate this offset are required in order to examine this possibility.5.DiscussionWith a validated numerical model,it is possible to gain a better understanding of the stress and strain state of the sample. This includes conﬁrmation of whether the samples are plastically deforming prior to failure and also allows the determina-tion of a transfer function between the physically measurable CMOD and the crack driving force,CTOD.Experimentally,the membrane strain components deviate from linearity at approximately600kN,above which the membrane strains increase exponentially.Contour plots of the Von-Mises effective stress at the point where the FEM model departs linearity are plotted in Fig.10for both the upper and lower surfaces of the plate.The squares highlight the location of the strain gauges on both surfaces from which it can be veriﬁed that the departure from linearity in the strain data corre-sponds to the material yielding.As seen experimentally,the upper surface of the plate yields at lower applied loads than the lower surface.This is a direct result of the added bending strain associated with the straightening of the plates.The through-thickness view along the fusion boundary illustrates that the region ahead of the crack tip is clearly in the plastic range,assuring that an elastic–plastic fracture analysis is indeed required.The crack driving force can be equivalently determined from either the domain integral,or the CTOD.As LS-DYNA does not offer built-in fracture mechanics analyses,the crack driving force was determined by measuring the crack tip opening displacement.The CTOD was measured from lines extending at±45°from the crack tip with the intersection of a2nd order polynomialﬁt to the eight nodes along the crack face.In order to consistently capture their intersection,the nodal coordi-nates closest to the crack tip were not included in the polynomialﬁt(see Fig.11).This was required as the crack front be-comes increasingly distorted with applied load,and the inclusion of the nodes closest to the crack tip results in an under prediction of the CTOD.Also obvious from Fig.11,is the asymmetry of the crack opening resulting from the combination of increased weld metal strength and the increased weld cross-sectional area,which caused the majority of the mouth open-ing to occur in the base material.。

描述抗压能力英语作文高中

描述抗压能力英语作文高中Title: Enhancing Resilience: An Essential Trait for High School Students。

Resilience, an indispensable quality in navigatinglife's challenges, stands as a cornerstone of success. Especially in the tumultuous journey of high school, where academic pressures, social dynamics, and personal growth intersect, cultivating resilience becomes paramount. Here, we delve into the significance of resilience, strategiesfor its development, and its profound impact on high school students.Firstly, resilience serves as a shield against the storms of life, enabling individuals to bounce back from setbacks stronger than before. In the context of high school, academic hurdles such as difficult exams or disappointing grades often test students' resolve. However, resilient individuals view setbacks as temporary setbacks rather than permanent failures. They harness setbacks asopportunities for growth, utilizing feedback to refinetheir strategies and enhance their performance.Moreover, resilience empowers students to maintain a positive mindset amidst adversity. High school, with its myriad of challenges, can sometimes feel overwhelming. From managing heavy workloads to navigating complex social dynamics, students encounter numerous stressors daily. Yet, resilient individuals possess the mental fortitude to stay optimistic, focusing on solutions rather than dwelling on problems. They adopt a proactive approach, seeking support from peers, teachers, and family members when needed, thereby building a robust support network.Furthermore, resilience fosters adaptability, a crucial skill in an ever-changing world. High school acts as a microcosm of society, exposing students to diverse experiences and perspectives. Resilient individuals embrace change, recognizing it as an inevitable aspect of life. They adapt to new environments, forge connections with unfamiliar faces, and embrace opportunities for personal growth. By doing so, they cultivate a growth mindset,viewing challenges as opportunities for self-improvement rather than insurmountable barriers.Additionally, resilience cultivates perseverance, enabling students to persist in the face of adversity. High school presents a myriad of obstacles, from demanding coursework to extracurricular commitments. Yet, resilient individuals remain steadfast in their pursuits, refusing to succumb to despair in the face of setbacks. They understand that success often requires perseverance and grit, and they are willing to put in the effort required to achieve their goals.To cultivate resilience among high school students, various strategies can be employed. Firstly, fostering a supportive school environment is crucial. Schools can implement programs aimed at promoting mental well-being, such as counseling services and peer support groups. Additionally, educators can incorporate resilience-building activities into the curriculum, such as mindfulness exercises and reflective journaling.Moreover, teaching students coping mechanisms is essential. By equipping students with practical strategies for managing stress and adversity, such as deep breathing exercises and time management techniques, educators empower them to navigate challenges effectively. Furthermore, encouraging a growth mindset can enhance resilience. By praising effort rather than innate ability, educatorsinstill in students the belief that their abilities are not fixed but can be developed through dedication and perseverance.In conclusion, resilience stands as a fundamental trait for high school students, enabling them to navigate the challenges of adolescence with grace and determination. By fostering resilience, educators empower students to overcome setbacks, maintain a positive mindset, adapt to change, and persevere in the face of adversity. Through a combination of supportive environments, practical strategies, and a growth mindset, high schools cancultivate resilience among their students, equipping them with the tools they need to thrive in school and beyond.。

钢板组合梁桥混凝土温度效应研究

0引言近年来，钢板组合梁桥作为广泛应用于桥梁工程的结构形式，其施工工艺及性能研究备受关注。

段亚军等[1]综述了钢板组合梁桥的施工工艺，着重介绍了顶推法、吊装法以及桥面板的预制和现浇施工等关键环节。

周辉的研究[2]通过数值模拟深入探讨了钢-混组合梁界面滑移效应与掀起效应之间的相互影响。

张彦玲等[3]通过试验研究钢-混凝土组合梁的性能，发现横隔板数目对切向滑移影响相对较小。

齐书瑜的研究[4]通过弯扭性能模型试验发现，曲线组合梁在负弯矩与扭矩作用下，切向和径向滑移随跨径比减小而减小。

张兴虎等[5]提出了一种新型钢-混凝土组合梁设计方案，通过试验表明其相对于传统组合梁具有更高的抗剪承载力和变形能力。

焦驰宇等[6]通过有限元软件研究了梁格法在曲线箱梁桥的适用条件，并提出了单梁法和梁格法在应力计算中的对比。

满建琳[7]对4跨钢-混组合曲线连续箱梁桥进行了全桥模拟分析，关注了支座反力、桥面板位移和结构应力等关键参数。

在这一背景下，本文旨在深入研究钢板组合梁桥的温度效应，通过参数分析探讨其在不同工况下的性能变化，为钢板组合梁桥的设计和实际工程应用提供科学的理论支持。

1工程背景项目位于阜阳市，路线全长6.273公里，一级公路建设标准，公路段长度5.45公里，市政段长度0.85公里，引桥为预应力组合箱梁，预制梁板共计176片，引桥下部结构桥台采用肋板式桥台、桩基础，桥墩采用桩柱式桥墩、桩基础。

2有限元建模说明采用有限元分析软件Abaqus ，通过空间模型简化建立全桥有限元分析模型。

简化的过程就是将实际的小半径钢板组合梁桥通过力学抽象，进而简化为能用于矩阵分析的空间力学模型。

模型简化必须使力学模型尽可能符合实际原型结构。

模型中选用的单元类型必须能够模拟实际构件的受力特征，同时采用的单元参数要符合实际构件的参数，边界约束状况必须和实际结构的约束状态相符。

在建模过程中，钢板组合梁的钢箱梁采用壳单元进行模拟，混凝土桥面板均采用实体单元模拟，支座采按照实际工程进行布置。

一级负荷中特别重要的负荷英语

一级负荷中特别重要的负荷英语Critical Loads within Tier-1 Requirements.The concept of tier-1 loads in electrical engineering and power system design is fundamental in ensuring the reliability and continuity of power supply, especially in critical infrastructure and vital services. Within this tier, the identification of "particularly important loads" (PILs) is crucial as they represent those facilities or processes that, if power is interrupted, would result in significant safety risks, economic losses, or societal impact.Defining Particularly Important Loads (PILs)。

PILs are those loads that, due to their nature or the services they provide, are considered essential to thewell-being of society or the operation of critical infrastructure. These loads are typically found in hospitals, emergency services facilities, data centers, andother facilities where power interruption could lead tolife-threatening situations or significant operational downtime.Characteristics of PILs.1. Criticality: PILs are those loads that, if lost, would result in significant harm or damage. This could be to human life, the environment, or critical infrastructure.2. Non-negotiable: Unlike other loads that can be temporarily shut down or have their demand reduced, PILs must remain operational at all times.3. High Priority: In the event of a power outage or emergency, PILs are prioritized for restoration, ensuring their continuous operation.Examples of PILs.Healthcare Facilities: Hospitals, ambulances, and other healthcare facilities rely on a constant power supplyfor life-saving equipment and services.Emergency Services: Fire stations, police stations, and other emergency response centers need uninterrupted power to maintain their operational readiness.Data Centers: Facilities housing critical data and information systems for banks, governments, and other key organizations are PILs as they support essential services and functions.Transportation Hubs: Airports, railway stations, and other major transportation nodes are considered PILs due to their impact on societal mobility and the economy.Importance of PIL Identification.Identifying PILs within a power system is crucial for several reasons:Planning: It allows for better planning and design of power systems, ensuring that PILs are supplied withredundant and reliable power sources.Prioritization: In the event of a power outage, knowing which loads are PILs helps in prioritizing their restoration, minimizing downtime and potential damage.Safety: In facilities like hospitals, identifying PILs ensures that life-saving equipment remains operational during emergencies.Conclusion.Tier-1 loads, especially PILs, are the backbone of any reliable power system. Their identification, prioritization, and protection are paramount in ensuring the smooth operation of critical infrastructure and services, safeguarding human life, and minimizing economic losses. As power systems and technologies continue to evolve, so must our approach to identifying and managing these vital loads.。

极限强度英语

极限强度英语The ultimate strength of a material refers to the maximum stress it can withstand before it fails. It is an important factor in determining the safety and reliabilityof engineering structures and components. The ultimate strength is usually determined through tensile testing, where a specimen of the material is subjected to increasing amounts of tensile force until it fractures. The stress at which this fracture occurs is the ultimate tensile strength.There are several factors that can affect the ultimate strength of a material. These include the material's composition, microstructure, and processing history. For example, the presence of impurities or defects within the material can act as stress concentrators, reducing its ultimate strength. Similarly, the way in which the material is processed, such as through cold working or heat treatment, can also have a significant impact on itsultimate strength.In engineering design, it is important to consider the ultimate strength of materials to ensure that they cansafely withstand the expected loads and forces. Thisinvolves selecting materials with appropriate ultimate strength values and designing structures and components with adequate safety factors. Failure to account for the ultimate strength of materials can lead to catastrophic failures and compromise the safety of the designed system.In addition to ultimate strength, it is also importantto consider other mechanical properties of materials, such as yield strength, ductility, and toughness. These properties collectively influence the behavior of materials under different loading conditions and are essential for ensuring the overall performance and reliability of engineered systems.Ultimately, understanding the ultimate strength of materials is crucial for ensuring the safety andreliability of engineering structures and components. By considering this important mechanical property, engineers can make informed decisions in material selection and design, ultimately leading to safer and more efficient systems.材料的极限强度是指在材料破裂之前能够承受的最大应力。

极端温度作业的管理要求

极端温度作业的管理要求This procedure describes the minimum requirements for the management of work in extreme hot or cold environments which may create a significant risk to health and cause harm.本规程描述了关于在极端高温或低温环境中作业（可能造成严重健康风险并且导致伤害）的最低管理要求。

T本文适用于化工企业对在极端高温或低温环境中作业承担管理责任的场所。

本文件的目标读者包括对在极端高温或低温环境中作业承担责任的管理人员、主管人员、作业许可证签发人员和安健环质工作人员。

Workplace conditions with the potential to cause a person's normal deep body core temperature of 37°C (98.6°F) to increase by 1°C.Extreme hot environments can lead to heat stress illnesses, such as heat rash, heat syncope (fainting), heat cramps, heat exhaustion or heat stroke.Extreme hot environments are characterised by factors such as especially high temperatures and/or humidity, radiant heat or low air velocity.可能导致个人正常深度核心体温37°C（98.6°F）增加1°C的作业场所条件极端高温环境可能导致热应力疾病，例如痱疮、热晕厥（昏厥）、热痉挛、热衰竭或中暑。

routine练习题

routine练习题一、词汇练习1. 选择正确的单词填空：1. I usually _______ to work bus.2. She _______ her homework every evening.A. doesB. doC. does not doD. doesn't do3. They _______ a movie last night.A. watchB. watchesC. watchedD. watching2. 选择正确的词组：1. I _______ (go, going) to the gym this morning.2. He _______ (be, is) late for school again.3. She _______ (do, does) her homework every day.3. 选择正确的形容词：1. This is a _______ (good, bad) book.2. She is a _______ (smart, silly) girl.3. The weather is very _______ (hot, cold) today.二、语法练习1. 选择正确的时态：1. I _______ (go, went) to the park yesterday.2. She _______ (be, was) happy when she received the gift.3. They _______ (do, did) their homework last night.2. 选择正确的语态：1. The teacher _______ (teach, is taught) Mr. Wang.2. The book _______ (write, is written) a famous author.3. The letter _______ (send, is sent) to her last week.3. 选择正确的连词：1. I _______ (go, am going) to the movies, _______ (because, because of) I have free time.2. She _______ (like, likes) coffee, _______ (but, but) she doesn't like tea.3. I _______ (finish, finished) my homework, _______ (so, so) I can go out now.三、阅读理解1. 阅读短文，回答问题：1. What is the main idea of the passage?2. Who is the main character in the story?3. What happens at the end of the passage?2. 阅读文章，判断正误：1. The story is about a boy who goes to the park every weekend.2. The boy meets his friends at the park and they play games together.3. The boy goes home after playing games with his friends.3. 阅读文章，找出关键信息：1. What is the author's favorite color?2. Why does the author like this color?3. What does the author think about other colors?四、写作练习1. 介绍动物的名字和种类。

The Stress Effect完整版(教师参考)

The Stress Effect你有抑郁症吗RHONDA ROWLAND, CNN MEDICAL CORRESPONDENTHeavy traffic, job pressure, working couples with kids, cell phones, e- mails, pagers and Palm Pilots（掌中宝）. Do you wonder what all that stress is doing to you?DR. JEFFREY KELSEY, PSYCHIATRISTI have to take this real quick.ROWLANDOK.罗兰德好的。

KELSEYI have five more minutes of being on call today.凯尔塞因为今天我还有5分钟就下班了。

ROWLANDPsychiatrist, Dr. Jeffery Kelsey, sees it every day in his practice. Stress can set the stage for depression.罗兰德杰弗里·凯尔塞医生是一位精神病学家，这就是他每天都会面对的情况。

压力能够导致抑郁。

KELSEYWell, we think that the vulnerability of getting depression is about a third from genetics, what we inherit, and is about two thirds from those experiences that we have.凯尔塞好的，我们认为忧郁症三分之一来自遗传，是我们继承下来的，另外三分之二是由我们的经历引发的。

ROWLANDStress you can't control appears to do the most harm, accidents, death, job loss, the events of 9/11, even the turbulence of Wall Street. With Wall Street, a lot of pressure now, especially for CEOs' performance and to deliver. What kind of impact does that have?罗兰德那些无法控制的因素带来的压力危害性最大，如意外事故、死亡、失业、"911"事件，甚至华尔街的动荡。

2023-2024学年全国全部人教版高考专题英语高考真卷习题及解析

2023-2024学年全国全部人教版高考专题英语高考真卷1.阅读理解第1题.As I walked into the primary school, I looked down at my new black Mary Jane shoes—I wanted to be anywhere in the world but Fountain Inn Elementary School.I stood at Mrs Blackstone's door. Before I tried the great escape, the wooden door suddenly opened to the singsong voice of Mrs Blackstone, my new fifth grade teacher."We've been waiting on you!" she said in an unusually high voice. With a smile my mom walked away and I was left standing in the center stage in front of a group of students.Over the next few days, Mrs Blackstone made it her job to find me a new best friend and help me join a choir（合唱队）. During history lessons, as I didn't want to speak in front of the class she would not only call on me but have me stand up to address my classmates. She laughed at my nervousness when I wouldn't get in line to sharpen my pencil for fear of being embarrassed. Her silliness made me smile and finally feel much more relaxed in the new environment.One spring afternoon, Mrs Blackstone asked me to stay after class."I'd like to speak with you, Amanda."My mind raced. Had I made a bad grade? Had I hurt someone's feelings?"Are you enjoying your new school?" She sat comfortably behind her desk."I want to tell you something, between you and me, not to be shared with anyone else.""Okay." My throat all of a sudden felt very dry.With her eyebrows raised she said, "I see something special in you. The way you communicate with others, the kindness you show—I think you have great potential（潜能）to be something very important."I listened attentively to every word."I've been a teacher for over twenty-five years; I can see it. But you must believe in yourself. That's very important. Remember that and you'll go far."She then hugged me and said, "See you tomorrow."Her singsong voice was music to my ears that day. I walked out of her class, not staring at my Mary Jane shoes, but looking straight ahead with my head high. My teacher saw something special in me!（1）What can we know about the author from the first two paragraphs?A: He went to school unwillingly.B: He liked the new school very much.C: He was afraid of his new teacher.D: He liked his teacher's singing.（2）How did the author feel when he was asked to leave behind one afternoon?A: Embarrassed.B: Excited.C: Surprised.D: Nervous.（3）Why did Mrs Blackstone have a talk with the author?A: To tell him about his grades.B: To encourage him to be confident.C: To help him realize his mistakes.D: To make him feel cared and loved.（4）Which of the following can be the best title for the text?A: A Teacher for Every ChildB: An Unforgettable LessonC: Magical WordsD: Special Treatment【答案】ADBC【解答】（1）A 细节理解题。

高血压英语课件

Moderate exercise
Moderate exercise is beneficial for controlling blood pressure and avoiding physical discomfort caused by excessive exercise.
Weight control
Extreme heat or cold can temporarily raise blood pressure levels
Mental stress
Chronic stress
Constant exposure to severe situations or emotional trauma can lead to persistent elevation in blood pressure
Drug therapy
要点一
Antihypertensive drugs
These drugs are used to lower blood pressure by relaxing the blood vessels, reducing the heart rate, or removing excess fluid from the body Commonly used antihypertensive drugs include angiotensin converting enzyme inhibitors, beta blockers, and diuretics
Dizziness or falling
High blood pressure can lead to insufficient blood flow to the brain, causing dizziness or falling falls

机械强度英语

机械强度英语When it comes to the backbone of engineering and construction, mechanical strength is the unsung hero that holds it all together. It's the invisible force that ensures the stability of bridges, the resilience of skyscrapers, and the durability of our vehicles. Mechanical strength, in essence, is the ability of a material to withstand the forces acting upon it without breaking or deforming. It's a critical property that engineers must consider when designing anything from the smallest component to the largest structure.Materials like steel, concrete, and even plastics have mechanical strength that can be quantified through various tests, such as tensile, compression, and fatigue tests. Each material has its own set of properties that define its strength, elasticity, and toughness. For instance, steel is known for its exceptional tensile strength, making it ideal for heavy-duty applications, while certain polymers might offer more flexibility and resistance to fatigue, perfect for parts that undergo repeated stress.Understanding mechanical strength is not just about knowing the maximum force a material can handle; it's also about how it behaves under different conditions. Factors such as temperature, moisture, and exposure to chemicals can all affect a material's strength. That's why thorough testing and a deep understanding of material science are crucial infields where safety and reliability are paramount.Innovations in material science are continuously pushing the boundaries of what's possible. New materials with enhanced mechanical strength are being developed to meet the demands of modern technology and industry. For example, advanced composites are now being used in aerospace to reduce weight without compromising on safety, thanks to their high strength-to-weight ratio.In conclusion, mechanical strength is the silent guardian of our built environment, a fundamental aspect of material science that underpins the functionality and longevity of all that we build. It's a testament to human ingenuity and our relentless pursuit of progress in the face of physical challenges.。

听歌写作业可以干什么英语

Listening to music while doing homework can serve multiple purposes and have various effects on the individual.Here are some of the benefits and potential drawbacks:1.Enhancing Mood:Music can improve your mood,making the task of doing homework more enjoyable and less of a chore.2.Reducing Stress:It can help to reduce stress and anxiety,which can be particularly helpful during periods of heavy workload or exam preparation.3.Improving Focus:For some people,certain types of music,especially instrumental or classical,can enhance focus and concentration on the task at hand.4.Providing a DistractionFree Environment:Background music can help to drown out other distracting noises,creating a more consistent and focused environment.5.Increasing Productivity:Some studies suggest that music can increase productivity, especially when the tasks are repetitive or do not require deep cognitive processing.6.Facilitating Creativity:Music can stimulate the creative process,which might be beneficial for homework that involves creative thinking or problemsolving.7.Creating a Routine:Associating certain types of music with homework time can help to create a routine,signaling to your brain that its time to focus on the task.8.Personal Preference:Ultimately,the effect of music on homework efficiency can be highly individual.Some people find that music helps them to work better,while others may find it distracting.However,there are also potential downsides:1.Distraction:For some,any type of music with lyrics or a strong beat can be distracting and make it harder to concentrate on the homework.2.Impairing Memory:Studies have shown that background music can impair the ability to remember information,especially if the music is complex or unfamiliar.3.Interrupting Thought Processes:Music can interrupt the natural flow of thoughts, which might be disruptive when trying to solve complex problems or understand difficult concepts.4.Creating a Dependence:Relying on music to get work done can create a dependence, making it difficult to concentrate without it in the future.5.Inappropriate Volume:Listening to music at a high volume can be harmful to hearing over time and can also be disruptive to others around you.6.Preference for Silence:Some individuals work best in complete silence,finding that any sound,even music,interferes with their ability to concentrate.In conclusion,whether or not to listen to music while doing homework is a personal choice that depends on how it affects your individual concentration and productivity.Its important to experiment with different types of music and volumes to find what works best for you.。

专八英语阅读

英语专业八级考试TEM-8阅读理解练习册（1）(英语专业2012级)UNIT 1Text AEvery minute of every day, what ecologist生态学家James Carlton calls a global ―conveyor belt‖, redistributes ocean organisms生物.It’s planetwide biological disruption生物的破坏that scientists have barely begun to understand.Dr. Carlton —an oceanographer at Williams College in Williamstown,Mass.—explains that, at any given moment, ―There are several thousand marine species traveling… in the ballast water of ships.‖ These creatures move from coastal waters where they fit into the local web of life to places where some of them could tear that web apart. This is the larger dimension of the infamous无耻的，邪恶的invasion of fish-destroying, pipe-clogging zebra mussels有斑马纹的贻贝.Such voracious贪婪的invaders at least make their presence known. What concerns Carlton and his fellow marine ecologists is the lack of knowledge about the hundreds of alien invaders that quietly enter coastal waters around the world every day. Many of them probably just die out. Some benignly亲切地，仁慈地—or even beneficially — join the local scene. But some will make trouble.In one sense, this is an old story. Organisms have ridden ships for centuries. They have clung to hulls and come along with cargo. What’s new is the scale and speed of the migrations made possible by the massive volume of ship-ballast water压载水— taken in to provide ship stability—continuously moving around the world…Ships load up with ballast water and its inhabitants in coastal waters of one port and dump the ballast in another port that may be thousands of kilometers away. A single load can run to hundreds of gallons. Some larger ships take on as much as 40 million gallons. The creatures that come along tend to be in their larva free-floating stage. When discharged排出in alien waters they can mature into crabs, jellyfish水母, slugs鼻涕虫，蛞蝓, and many other forms.Since the problem involves coastal species, simply banning ballast dumps in coastal waters would, in theory, solve it. Coastal organisms in ballast water that is flushed into midocean would not survive. Such a ban has worked for North American Inland Waterway. But it would be hard to enforce it worldwide. Heating ballast water or straining it should also halt the species spread. But before any such worldwide regulations were imposed, scientists would need a clearer view of what is going on.The continuous shuffling洗牌of marine organisms has changed the biology of the sea on a global scale. It can have devastating effects as in the case of the American comb jellyfish that recently invaded the Black Sea. It has destroyed that sea’s anchovy鳀鱼fishery by eating anchovy eggs. It may soon spread to western and northern European waters.The maritime nations that created the biological ―conveyor belt‖ should support a coordinated international effort to find out what is going on and what should be done about it. (456 words)1.According to Dr. Carlton, ocean organism‟s are_______.A.being moved to new environmentsB.destroying the planetC.succumbing to the zebra musselD.developing alien characteristics2.Oceanographers海洋学家are concerned because_________.A.their knowledge of this phenomenon is limitedB.they believe the oceans are dyingC.they fear an invasion from outer-spaceD.they have identified thousands of alien webs3.According to marine ecologists, transplanted marinespecies____________.A.may upset the ecosystems of coastal watersB.are all compatible with one anotherC.can only survive in their home watersD.sometimes disrupt shipping lanes4.The identified cause of the problem is_______.A.the rapidity with which larvae matureB. a common practice of the shipping industryC. a centuries old speciesD.the world wide movement of ocean currents5.The article suggests that a solution to the problem__________.A.is unlikely to be identifiedB.must precede further researchC.is hypothetically假设地，假想地easyD.will limit global shippingText BNew …Endangered‟ List Targets Many US RiversIt is hard to think of a major natural resource or pollution issue in North America today that does not affect rivers.Farm chemical runoff残渣, industrial waste, urban storm sewers, sewage treatment, mining, logging, grazing放牧,military bases, residential and business development, hydropower水力发电,loss of wetlands. The list goes on.Legislation like the Clean Water Act and Wild and Scenic Rivers Act have provided some protection, but threats continue.The Environmental Protection Agency (EPA) reported yesterday that an assessment of 642,000 miles of rivers and streams showed 34 percent in less than good condition. In a major study of the Clean Water Act, the Natural Resources Defense Council last fall reported that poison runoff impairs损害more than 125,000 miles of rivers.More recently, the NRDC and Izaak Walton League warned that pollution and loss of wetlands—made worse by last year’s flooding—is degrading恶化the Mississippi River ecosystem.On Tuesday, the conservation group保护组织American Rivers issued its annual list of 10 ―endangered‖ and 20 ―threatened‖ rivers in 32 states, the District of Colombia, and Canada.At the top of the list is the Clarks Fork of the Yellowstone River, whereCanadian mining firms plan to build a 74-acre英亩reservoir水库，蓄水池as part of a gold mine less than three miles from Yellowstone National Park. The reservoir would hold the runoff from the sulfuric acid 硫酸used to extract gold from crushed rock.―In the event this tailings pond failed, the impact to th e greater Yellowstone ecosystem would be cataclysmic大变动的，灾难性的and the damage irreversible不可逆转的.‖ Sen. Max Baucus of Montana, chairman of the Environment and Public Works Committee, wrote to Noranda Minerals Inc., an owner of the ― New World Mine‖.Last fall, an EPA official expressed concern about the mine and its potential impact, especially the plastic-lined storage reservoir. ― I am unaware of any studies evaluating how a tailings pond尾矿池，残渣池could be maintained to ensure its structural integrity forev er,‖ said Stephen Hoffman, chief of the EPA’s Mining Waste Section. ―It is my opinion that underwater disposal of tailings at New World may present a potentially significant threat to human health and the environment.‖The results of an environmental-impact statement, now being drafted by the Forest Service and Montana Department of State Lands, could determine the mine’s future…In its recent proposal to reauthorize the Clean Water Act, the Clinton administration noted ―dramatically improved water quality since 1972,‖ when the act was passed. But it also reported that 30 percent of riverscontinue to be degraded, mainly by silt泥沙and nutrients from farm and urban runoff, combined sewer overflows, and municipal sewage城市污水. Bottom sediments沉积物are contaminated污染in more than 1,000 waterways, the administration reported in releasing its proposal in January. Between 60 and 80 percent of riparian corridors (riverbank lands) have been degraded.As with endangered species and their habitats in forests and deserts, the complexity of ecosystems is seen in rivers and the effects of development----beyond the obvious threats of industrial pollution, municipal waste, and in-stream diversions改道to slake消除the thirst of new communities in dry regions like the Southwes t…While there are many political hurdles障碍ahead, reauthorization of the Clean Water Act this year holds promise for US rivers. Rep. Norm Mineta of California, who chairs the House Committee overseeing the bill, calls it ―probably the most important env ironmental legislation this Congress will enact.‖ (553 words)6.According to the passage, the Clean Water Act______.A.has been ineffectiveB.will definitely be renewedC.has never been evaluatedD.was enacted some 30 years ago7.“Endangered” rivers are _________.A.catalogued annuallyB.less polluted than ―threatened rivers‖C.caused by floodingD.adjacent to large cities8.The “cataclysmic” event referred to in paragraph eight would be__________.A. fortuitous偶然的，意外的B. adventitious外加的，偶然的C. catastrophicD. precarious不稳定的，危险的9. The owners of the New World Mine appear to be______.A. ecologically aware of the impact of miningB. determined to construct a safe tailings pondC. indifferent to the concerns voiced by the EPAD. willing to relocate operations10. The passage conveys the impression that_______.A. Canadians are disinterested in natural resourcesB. private and public environmental groups aboundC. river banks are erodingD. the majority of US rivers are in poor conditionText CA classic series of experiments to determine the effects ofoverpopulation on communities of rats was reported in February of 1962 in an article in Scientific American. The experiments were conducted by a psychologist, John B. Calhoun and his associates. In each of these experiments, an equal number of male and female adult rats were placed in an enclosure and given an adequate supply of food, water, and other necessities. The rat populations were allowed to increase. Calhoun knew from experience approximately how many rats could live in the enclosures without experiencing stress due to overcrowding. He allowed the population to increase to approximately twice this number. Then he stabilized the population by removing offspring that were not dependent on their mothers. He and his associates then carefully observed and recorded behavior in these overpopulated communities. At the end of their experiments, Calhoun and his associates were able to conclude that overcrowding causes a breakdown in the normal social relationships among rats, a kind of social disease. The rats in the experiments did not follow the same patterns of behavior as rats would in a community without overcrowding.The females in the rat population were the most seriously affected by the high population density: They showed deviant异常的maternal behavior; they did not behave as mother rats normally do. In fact, many of the pups幼兽，幼崽, as rat babies are called, died as a result of poor maternal care. For example, mothers sometimes abandoned their pups,and, without their mothers' care, the pups died. Under normal conditions, a mother rat would not leave her pups alone to die. However, the experiments verified that in overpopulated communities, mother rats do not behave normally. Their behavior may be considered pathologically 病理上，病理学地diseased.The dominant males in the rat population were the least affected by overpopulation. Each of these strong males claimed an area of the enclosure as his own. Therefore, these individuals did not experience the overcrowding in the same way as the other rats did. The fact that the dominant males had adequate space in which to live may explain why they were not as seriously affected by overpopulation as the other rats. However, dominant males did behave pathologically at times. Their antisocial behavior consisted of attacks on weaker male,female, and immature rats. This deviant behavior showed that even though the dominant males had enough living space, they too were affected by the general overcrowding in the enclosure.Non-dominant males in the experimental rat communities also exhibited deviant social behavior. Some withdrew completely; they moved very little and ate and drank at times when the other rats were sleeping in order to avoid contact with them. Other non-dominant males were hyperactive; they were much more active than is normal, chasing other rats and fighting each other. This segment of the rat population, likeall the other parts, was affected by the overpopulation.The behavior of the non-dominant males and of the other components of the rat population has parallels in human behavior. People in densely populated areas exhibit deviant behavior similar to that of the rats in Calhoun's experiments. In large urban areas such as New York City, London, Mexican City, and Cairo, there are abandoned children. There are cruel, powerful individuals, both men and women. There are also people who withdraw and people who become hyperactive. The quantity of other forms of social pathology such as murder, rape, and robbery also frequently occur in densely populated human communities. Is the principal cause of these disorders overpopulation? Calhoun’s experiments suggest that it might be. In any case, social scientists and city planners have been influenced by the results of this series of experiments.11. Paragraph l is organized according to__________.A. reasonsB. descriptionC. examplesD. definition12．Calhoun stabilized the rat population_________.A. when it was double the number that could live in the enclosure without stressB. by removing young ratsC. at a constant number of adult rats in the enclosureD. all of the above are correct13.W hich of the following inferences CANNOT be made from theinformation inPara. 1?A. Calhoun's experiment is still considered important today.B. Overpopulation causes pathological behavior in rat populations.C. Stress does not occur in rat communities unless there is overcrowding.D. Calhoun had experimented with rats before.14. Which of the following behavior didn‟t happen in this experiment?A. All the male rats exhibited pathological behavior.B. Mother rats abandoned their pups.C. Female rats showed deviant maternal behavior.D. Mother rats left their rat babies alone.15. The main idea of the paragraph three is that __________.A. dominant males had adequate living spaceB. dominant males were not as seriously affected by overcrowding as the otherratsC. dominant males attacked weaker ratsD. the strongest males are always able to adapt to bad conditionsText DThe first mention of slavery in the statutes法令，法规of the English colonies of North America does not occur until after 1660—some forty years after the importation of the first Black people. Lest we think that existed in fact before it did in law, Oscar and Mary Handlin assure us, that the status of B lack people down to the 1660’s was that of servants. A critique批判of the Handlins’ interpretation of why legal slavery did not appear until the 1660’s suggests that assumptions about the relation between slavery and racial prejudice should be reexamined, and that explanation for the different treatment of Black slaves in North and South America should be expanded.The Handlins explain the appearance of legal slavery by arguing that, during the 1660’s, the position of white servants was improving relative to that of black servants. Thus, the Handlins contend, Black and White servants, heretofore treated alike, each attained a different status. There are, however, important objections to this argument. First, the Handlins cannot adequately demonstrate that t he White servant’s position was improving, during and after the 1660’s; several acts of the Maryland and Virginia legislatures indicate otherwise. Another flaw in the Handlins’ interpretation is their assumption that prior to the establishment of legal slavery there was no discrimination against Black people. It is true that before the 1660’s Black people were rarely called slaves. But this shouldnot overshadow evidence from the 1630’s on that points to racial discrimination without using the term slavery. Such discrimination sometimes stopped short of lifetime servitude or inherited status—the two attributes of true slavery—yet in other cases it included both. The Handlins’ argument excludes the real possibility that Black people in the English colonies were never treated as the equals of White people.The possibility has important ramifications后果，影响.If from the outset Black people were discriminated against, then legal slavery should be viewed as a reflection and an extension of racial prejudice rather than, as many historians including the Handlins have argued, the cause of prejudice. In addition, the existence of discrimination before the advent of legal slavery offers a further explanation for the harsher treatment of Black slaves in North than in South America. Freyre and Tannenbaum have rightly argued that the lack of certain traditions in North America—such as a Roman conception of slavery and a Roman Catholic emphasis on equality— explains why the treatment of Black slaves was more severe there than in the Spanish and Portuguese colonies of South America. But this cannot be the whole explanation since it is merely negative, based only on a lack of something. A more compelling令人信服的explanation is that the early and sometimes extreme racial discrimination in the English colonies helped determine the particular nature of the slavery that followed. (462 words)16. Which of the following is the most logical inference to be drawn from the passage about the effects of “several acts of the Maryland and Virginia legislatures” (Para.2) passed during and after the 1660‟s?A. The acts negatively affected the pre-1660’s position of Black as wellas of White servants.B. The acts had the effect of impairing rather than improving theposition of White servants relative to what it had been before the 1660’s.C. The acts had a different effect on the position of white servants thandid many of the acts passed during this time by the legislatures of other colonies.D. The acts, at the very least, caused the position of White servants toremain no better than it had been before the 1660’s.17. With which of the following statements regarding the status ofBlack people in the English colonies of North America before the 1660‟s would the author be LEAST likely to agree?A. Although black people were not legally considered to be slaves,they were often called slaves.B. Although subject to some discrimination, black people had a higherlegal status than they did after the 1660’s.C. Although sometimes subject to lifetime servitude, black peoplewere not legally considered to be slaves.D. Although often not treated the same as White people, black people,like many white people, possessed the legal status of servants.18. According to the passage, the Handlins have argued which of thefollowing about the relationship between racial prejudice and the institution of legal slavery in the English colonies of North America?A. Racial prejudice and the institution of slavery arose simultaneously.B. Racial prejudice most often the form of the imposition of inheritedstatus, one of the attributes of slavery.C. The source of racial prejudice was the institution of slavery.D. Because of the influence of the Roman Catholic Church, racialprejudice sometimes did not result in slavery.19. The passage suggests that the existence of a Roman conception ofslavery in Spanish and Portuguese colonies had the effect of _________.A. extending rather than causing racial prejudice in these coloniesB. hastening the legalization of slavery in these colonies.C. mitigating some of the conditions of slavery for black people in these coloniesD. delaying the introduction of slavery into the English colonies20. The author considers the explanation put forward by Freyre andTannenbaum for the treatment accorded B lack slaves in the English colonies of North America to be _____________.A. ambitious but misguidedB. valid有根据的but limitedC. popular but suspectD. anachronistic过时的，时代错误的and controversialUNIT 2Text AThe sea lay like an unbroken mirror all around the pine-girt, lonely shores of Orr’s Island. Tall, kingly spruce s wore their regal王室的crowns of cones high in air, sparkling with diamonds of clear exuded gum流出的树胶; vast old hemlocks铁杉of primeval原始的growth stood darkling in their forest shadows, their branches hung with long hoary moss久远的青苔;while feathery larches羽毛般的落叶松,turned to brilliant gold by autumn frosts, lighted up the darker shadows of the evergreens. It was one of those hazy朦胧的, calm, dissolving days of Indian summer, when everything is so quiet that the fainest kiss of the wave on the beach can be heard, and white clouds seem to faint into the blue of the sky, and soft swathing一长条bands of violet vapor make all earth look dreamy, and give to the sharp, clear-cut outlines of the northern landscape all those mysteries of light and shade which impart such tenderness to Italian scenery.The funeral was over,--- the tread鞋底的花纹/ 踏of many feet, bearing the heavy burden of two broken lives, had been to the lonely graveyard, and had come back again,--- each footstep lighter and more unconstrained不受拘束的as each one went his way from the great old tragedy of Death to the common cheerful of Life.The solemn black clock stood swaying with its eternal ―tick-tock, tick-tock,‖ in the kitchen of the brown house on Orr’s Island. There was there that sense of a stillness that can be felt,---such as settles down on a dwelling住处when any of its inmates have passed through its doors for the last time, to go whence they shall not return. The best room was shut up and darkened, with only so much light as could fall through a little heart-shaped hole in the window-shutter,---for except on solemn visits, or prayer-meetings or weddings, or funerals, that room formed no part of the daily family scenery.The kitchen was clean and ample, hearth灶台, and oven on one side, and rows of old-fashioned splint-bottomed chairs against the wall. A table scoured to snowy whiteness, and a little work-stand whereon lay the Bible, the Missionary Herald, and the Weekly Christian Mirror, before named, formed the principal furniture. One feature, however, must not be forgotten, ---a great sea-chest水手用的储物箱,which had been the companion of Zephaniah through all the countries of the earth. Old, and battered破旧的，磨损的, and unsightly难看的it looked, yet report said that there was good store within which men for the most part respect more than anything else; and, indeed it proved often when a deed of grace was to be done--- when a woman was suddenly made a widow in a coast gale大风，狂风, or a fishing-smack小渔船was run down in the fogs off the banks, leaving in some neighboring cottage a family of orphans,---in all such cases, the opening of this sea-chest was an event of good omen 预兆to the bereaved丧亲者;for Zephaniah had a large heart and a large hand, and was apt有…的倾向to take it out full of silver dollars when once it went in. So the ark of the covenant约柜could not have been looked on with more reverence崇敬than the neighbours usually showed to Captain Pennel’s sea-chest.1. The author describes Orr‟s Island in a(n)______way.A.emotionally appealing, imaginativeB.rational, logically preciseC.factually detailed, objectiveD.vague, uncertain2.According to the passage, the “best room”_____.A.has its many windows boarded upB.has had the furniture removedC.is used only on formal and ceremonious occasionsD.is the busiest room in the house3.From the description of the kitchen we can infer that thehouse belongs to people who_____.A.never have guestsB.like modern appliancesC.are probably religiousD.dislike housework4.The passage implies that_______.A.few people attended the funeralB.fishing is a secure vocationC.the island is densely populatedD.the house belonged to the deceased5.From the description of Zephaniah we can see thathe_________.A.was physically a very big manB.preferred the lonely life of a sailorC.always stayed at homeD.was frugal and saved a lotText BBasic to any understanding of Canada in the 20 years after the Second World War is the country' s impressive population growth. For every three Canadians in 1945, there were over five in 1966. In September 1966 Canada's population passed the 20 million mark. Most of this surging growth came from natural increase. The depression of the 1930s and the war had held back marriages, and the catching-up process began after 1945. The baby boom continued through the decade of the 1950s, producing a population increase of nearly fifteen percent in the five years from 1951 to 1956. This rate of increase had been exceeded only once before in Canada's history, in the decade before 1911 when the prairies were being settled. Undoubtedly, the good economic conditions of the 1950s supported a growth in the population, but the expansion also derived from a trend toward earlier marriages and an increase in the average size of families; In 1957 the Canadian birth rate stood at 28 per thousand, one of the highest in the world. After the peak year of 1957, thebirth rate in Canada began to decline. It continued falling until in 1966 it stood at the lowest level in 25 years. Partly this decline reflected the low level of births during the depression and the war, but it was also caused by changes in Canadian society. Young people were staying at school longer, more women were working; young married couples were buying automobiles or houses before starting families; rising living standards were cutting down the size of families. It appeared that Canada was once more falling in step with the trend toward smaller families that had occurred all through theWestern world since the time of the Industrial Revolution. Although the growth in Canada’s population had slowed down by 1966 (the cent), another increase in the first half of the 1960s was only nine percent), another large population wave was coming over the horizon. It would be composed of the children of the children who were born during the period of the high birth rate prior to 1957.6. What does the passage mainly discuss?A. Educational changes in Canadian society.B. Canada during the Second World War.C. Population trends in postwar Canada.D. Standards of living in Canada.7. According to the passage, when did Canada's baby boom begin?A. In the decade after 1911.B. After 1945.C. During the depression of the 1930s.D. In 1966.8. The author suggests that in Canada during the 1950s____________.A. the urban population decreased rapidlyB. fewer people marriedC. economic conditions were poorD. the birth rate was very high9. When was the birth rate in Canada at its lowest postwar level?A. 1966.B. 1957.C. 1956.D. 1951.10. The author mentions all of the following as causes of declines inpopulation growth after 1957 EXCEPT_________________.A. people being better educatedB. people getting married earlierC. better standards of livingD. couples buying houses11.I t can be inferred from the passage that before the IndustrialRevolution_______________.A. families were largerB. population statistics were unreliableC. the population grew steadilyD. economic conditions were badText CI was just a boy when my father brought me to Harlem for the first time, almost 50 years ago. We stayed at the hotel Theresa, a grand brick structure at 125th Street and Seventh avenue. Once, in the hotel restaurant, my father pointed out Joe Louis. He even got Mr. Brown, the hotel manager, to introduce me to him, a bit punchy强力的but still champ焦急as fast as I was concerned.Much has changed since then. Business and real estate are booming. Some say a new renaissance is under way. Others decry责难what they see as outside forces running roughshod肆意践踏over the old Harlem. New York meant Harlem to me, and as a young man I visited it whenever I could. But many of my old haunts are gone. The Theresa shut down in 1966. National chains that once ignored Harlem now anticipate yuppie money and want pieces of this prime Manhattan real estate. So here I am on a hot August afternoon, sitting in a Starbucks that two years ago opened a block away from the Theresa, snatching抓取，攫取at memories between sips of high-priced coffee. I am about to open up a piece of the old Harlem---the New York Amsterdam News---when a tourist。

Pathophysiology Part 3 (10) Stress应激

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Disbennifit effects:
Negative effects may be occur after prolonged increase of GC: ① Suppressing immune and inflammatory response and thus susceptible to infection; ② Slowing down individual growth and development caused by inhibition of growth hormone and insulin-like growth factor (IGF-1). ③ Inhibition of sexual gland axis (GnRH and LH) may lead to decreased sexual function and irregular menstruation. ④ Inhibition of thyroid gland caused by inhibition of TRH and TSH secretion. ⑤ Abnormality of behavior (depress, suicide tendency)
Physical stress Peripheral sensor Locus ceruleus of pons (adrenal nurone) ↓ Sympathetic ↓ Adrenal medulla ↓ EP, NE (catecholamine)
EP: epinephrine NE: norepinephrine
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The locus ceruleus- sympatheticSocial-psychological stress Physical stress adrenomedullary system