Studies on temperature induced creep in high strength Q460 steel

ORIGINALARTICLEStudies on temperature induced creep in high strength Q460steelWeiyong Wang .Shouhai Yan .Jiepeng LiuReceived:11May 2015/Accepted:16August 2016/Published online:22August 2016ÓRILEM 2016Abstract Significant levels of creep deformation in structural steel members exposed to high temperatures have been widely observed in experiments;however inclusion of creep effects in numerical simulations has been limited due to lack of reliable experimental data and corresponding constitutive creep models for structural steel,especially for high strength steel.This paper presents an experimental investigation into temperature induced creep in high strength Q460steel which is commonly used in structural member.A set of creep tests is carried out at various stress levels in 300–900°C temperature range.Data from these creep tests indicate that temperature has significant influence on the level of creep deformations in Q460steel,especially when the temperature in steel exceeds 400°C.The creep strain at a given temperature increases with stress level and the creep strain at a given stress level increases with temperature.Results from creep tests are utilized to develop relations for expressing creep as a function of temperature and stress.These relations,which are based on creep power law and unified theory on plasticity and creepphenomenon,can be utilized in modeling the response of steel structures made of Q460steel exposed to fire.Keywords High strength Q460steel ÁCreep response ÁHigh temperature ÁCreep model ÁFire resistance1IntroductionIt is well known that steel structures exhibit relatively lower fire resistance due to faster loss of strength and stiffness properties of steel with temperature and also due to rapid rise in temperatures resulting from high thermal conductivity and low specific heat of steel.The fire resistance of a steel structural member depends mainly on thermal and mechanical properties of steel.Thermal properties that influence fire response of steel structures include thermal conduc-tivity,specific heat,and thermal expansion.Mechan-ical properties that include strength,elastic modulus,and creep have significant influence on fire response of steel structures.In recent years,high-strength steel has found wide applications in modern construction due to the many advantages it offers over conventional mild steel,such as high strength and strength to weight ratio [1].When exposed to elevated temperatures,steel structures undergo increasing permanent deformations with time even when the applied stress level is belowW.Wang (&)ÁS.Yan ÁJ.LiuSchool of Civil Engineering,Chongqing University,Chongqing 400045,China e-mail:wywang@W.Wang ÁJ.LiuKey Laboratory of New Technology for Construction of Cities in Mountain Area (Ministry of Education),Chongqing University,Chongqing 400045,ChinaMaterials and Structures (2017)50:68DOI 10.1617/s11527-016-0941-2that of yield stress and this time-dependent deforma-tion is referred to as creep.Thus,creep can be defined as an increase in strain in a solid material under constant stress over a period of time.Significance of creep deformations underfire conditions has been highlighted by various research-ers.Morovat et al.[2]examined the effect of creep on the buckling strength of steel columns using Harmathy creep model[3]and Fields and Fields creep model[4]. It was shown that the buckling load capacity of columns not only depends on load level and temper-ature range,but also on time duration for which the load was applied.In a series of recent studies,Kodur et al.[5,6]studied the influence of high temperature creep on mid-span deflections and axial forces in steel beams by utilizing ANSYS software package.It was concluded that the exclusion of creep leads to lower mid-span deflections,an overestimation offire induced compressive axial forces and an underesti-mation offire induced tensile axial forces in restrained steel beams.Further,accounting for high temperature creep in the numerical model leads to good predictions of deflections and restraint forces,and compares well with measured data from tests on the beams,again highlighting the importance of creep.In addition, recent studies have clearly shown that the extent of creep is influenced by the chemical composition of steel and this varies for different grades of steel[7], that is,different types of steel exhibit different creep response.Currently creep,in most structuralfire engineering applications,is modeled either using Harmathy creep model[3],or AYSYS creep models[5,8],or Fields and Fields creep model[4].Though a few other models have been proposed in literatures[9–11],they are rarely used.Due to lack of creep test data,the commonly employed creep models were developed based on creep tests on Australian A149steel or ASTM-A36structural steel.Also,the creep models, used in structuralfire engineering applications,have been proposed on an ad-hoc basis and are not usually presented in a consistent mechanics based framework. As the creep models are highly phenomenological, without a consistent description it is difficult to evaluate and extend the application of existing creep models to different structural steels.Also,much of the available creep data in literature are for carbon steels. Therefore,for better understanding of creep effects in high strength Q460steel,which is widely used in structural engineering applications,relevant experi-mental creep data and the corresponding constitutive creep models,are needed.High strength Q460structural steel is widely used in construction applications in China,there are two challenges in incorporating creep strain in high temperature applications.Thefirst challenge is the lack of available experimental data on high temper-ature creep for this type of steel and the second being the lack of the corresponding constitutive creep model that can simulate the high temperature creep effects as observed in experiments.To overcome this knowledge gap,a comprehensive study on the high temperature creep in high strength Q460steel was undertaken.As part of this study,a set of creep tests were carried out on Q460steel coupons at various stress levels in300–900°C ing these high temperature creep data,two creep models based on the creep power law and unified theory on plasticity and creep strains,are proposed for Q460 steel.2Typical creep–time curveCreep is the inelastic response in a material when subjected to elevated temperatures.Temperature has an important effect on deformation phenomena.When a material is subjected to stress due to loading,it undergoes some level of strain.In contrast,creep strain is induced in the material as time dependent mechanical response.The standard tensile strength test on materials,with appropriate modifications,can be applied to investigate the creep response of materials at high temperatures.There are different approaches to undertake stress–strain tests at elevated temperatures[12].The less complex approach to measure creep is through tension tests at constant load and temperature.The resulting strain is measured as a function of time.This is often referred to as creep test and the resulting strain–time curves include the effects of creep.If the creep test extends over a sufficiently long period of time,a stage may be reached at which the specimen weakens rapidly and eventually ruptures. Tests aimed at investigating thisfinal stage of the creep phenomenon are referred to as creep rupture tests.Typical strain–time curves obtained from creep tests at constant loading,applied over sufficiently long68Page2of14Materials and Structures(2017)50:68periods of time,often exhibit three characteristic stages as shown in Fig.1.In stage1:referred to as primary or transient creep;following setting-in of instantaneous elastic strain,e0,the material deforms rapidly but at a decreasing rate.The duration of this stage is typically relatively short in relation to the total creep curve.In stage2:referred to as secondary or steady-state creep,the creep strain rate reaches a minimum value and remains approximately constant over a relatively longer period of time.In stage3: referred to as tertiary creep,the creep strain rate accelerates rapidly and attains ruptures when the material is unable to withstand the applied loading. 3Materials mechanical propertiesHigh strength steels provide better performance in term of strength,weld ability and corrosion/weather resistance.One such widely used high strength steel in building applications in China and Far East region is Q460steel[13],which is equivalent to ASTM A575 Gr65[14]steel in North America.Before undertaking creep tests,tensile strength tests were carried out to evaluate stress–strain response of Q460steel at ambient conditions.The shape and size of tensile strength coupons were prepared in accor-dance with GB/T228.1-2010[15].These tests were conducted using MTS-CWT5305test machine with 300kN loading capacity.The stress–strain response of Q460steel at room temperature is evaluated using data from tensile test.A total of10coupon tests(MS-1to MS10)were carried out and the thickness of steel sheet in specimen was20mm.The displacements recorded at various load levels,during tension test were used to generate stress–strain response of Q460steel and these curves are plotted in Fig.2.It can be seen in Fig.2that the general trend of stress–strain curves at room temperature are linear-elastic up to yield in steel,at a lower bound stress of 492MPa,followed by strain hardening response. Once the ultimate stress is reached at a stress of 621MPa,steel undergoes necking and rupture.The averaged elastic modulus and ultimate strain obtained from the strain–stress curves are202GPa and23.5%, respectively.4Test on temperature induced creepA comprehensive test program was designed to undertake creep tests on Q460steel.The creep tests were carried out at nine temperatures in the range of 300–900°C;namely,300,400,450,500,550,600, 700,800and900°C and at various stress levels ranging from13to509MPa.4.1Test set-upFor undertaking high temperature creep tests,high temperature creep test equipment was specially designed and fabricated.A schematic of test set-up for high temperature creep test shown in Fig.3a, comprises of a tensile testing equipment,an electric furnace,a strain measurement device and a dataMaterials and Structures(2017)50:68Page3of1468acquisition system.The tensile testing equipment is MTS loading system with a load capacity of 50kN.In this equipment,the axial deformation in the test coupon at elevated temperature can be measured through two displacement transducers (±10mm LVDTs),with 0.001mm sensitivity,that are mounted outside the furnace through two connecting rods placed inside the furnace (Fig.3b).The electric furnace comprises of cylindrical chamber with a maximum heating length of 150mm.In creep tests,the specimen is mounted between two cylinder grips with threads inside.The programmable furnace enables generation of target temperatures up to 1100°C at a specified rate of temperature rise and keeps the target temperature inside the furnace constant for the duration of creep test.Three built-in thermocouples are mounted inside the furnace to measure temperatures at upper,middle and lower zones,and the average readings of these three thermocouples are taken as the furnace temperature (as shown in Fig.3c).The furnace and LVDTs are connected to a data acquisition system and a computer wherein temperature,tension load and displacements during the creep test are recorded.4.2Test specimensFor high temperature creep tests,a number of round steel coupons were cut from Q460steel sheet of 20mm in thickness.A total of 45creep test specimens were fabricated.The test coupons measured 187mm in total length and the gauge length for creep measurement is 100mm.The diameter of cross section of the tapered section is 10mm.Theconfiguration of a typical test coupon used in creep test is shown in Fig.4.The stress levels (load ratio)and target temperatures at which these tests were carried out are tabulated in Table 1,in which the yield strength (1.0%strain)of Q460at elevated tempera-tures was determined by Ref.[16].The load ratio is defined as the ratio of applied stress on the specimen to the yield strength at a given temperature.When stress level in steel is too high,the coupon experiences rupture in a very short time.Conversely,a coupon exhibits inconspicuous creep response under low stress level.The stress levels during creep tests were varied to observe the creep effect on mechanical response at various temperatures.4.3Test procedureMechanical properties of steel,including the creep,at a specific temperature can be evaluated through transient or steady-state tests.Although transient-state tests are more realistic in simulating real fire exposure conditions,steady state tests are more commonly employed in evaluating mechanical properties due to ease of undertaking these tests.In the current study,to enable comparison with previously developed test data,steady state test method was utilized for under-taking high temperature creep tests.Creep tests were conducted at different temperatures and stress levels using the test set-up described above.For these creep tests,steel coupons were installed into the grips of the MTS machine and then three thermocouples were mounted at top,bottom and mid-length of the gauge length of specimen as shown in Fig.3c.These thermocouples are to monitoractualFig.3Creep test set-up and instrumentations on test specimen.a Creep test set-up,b LVDTs and c Thermocouples68Page 4of 14Materials and Structures (2017)50:68temperature in the furnace during the creep test.Then,an electric furnace was set-up around the specimen with special care being taken to ensure that the coupon is placed at the centre of the furnace.Following this,the furnace is turned-on to be heated to a specified target temperature at a heating rate of 10°C/min.Once the target temperature is attained,the steel coupon was maintained at this target temperature for 30min,to attain a steady-state condition,and then the coupon was maintained at the target temperature for the entire test duration.Then a pre-defined load was applied at a rate of 10kN/min.The maximum load applied in the creep tests is 40kN,and hence the loading phase lasts no more than 5min.After the applied loading reaches the target load,the creep displacements were measured using the two high sensitive LVDTs.The total length of the coupon that is exposed to high temperature during a creep test is 150mm,and of this only 100mm in the middle portion is taken to be the gauge length to calculate corresponding creep strain (D L /L ).Using this proce-dure,creep tests were conducted at different stress levels and at different temperatures.When the creep deformation reaches the maximum allowable dis-placement of LVDTs,namely 10mm,the LVDTs were adjusted by hand quickly and the data were continuously recorded.In this way,the creepdeformation was obtained within a wide displacement range,till the maximum movement of hydraulic jack.5Test results of temperature induced creep Data generated from above creep tests can be utilized to develop creep response of Q460steel as a function of temperature and stress level.5.1Creep–time curves at various stress levels The measured creep strains at various stress levels and at different temperatures,from 300to 900°C,are plotted in Fig.5.Note that the creep strain versus time curves presented here does not include the instanta-neous strain and only contains the resulting creep strain.Zero time on the X axis corresponds to the point at which full loading was applied on the coupon.In Fig.5,symbol ‘‘È’’represents the initiation (bound-ary point)of secondary creep and tertiary creep,and symbol ‘‘ ’’represents the occurrence of fracture at the termination of tertiary creep.Generally,at a given temperature,the creep strain at a higher stress level is larger than that at a lower stress level.Increased creep strain at higher stress level can be attributed to dislocation movement due to diffusion of atoms thatTable 1Selectedtemperature and stress level for creep testsTemperature(°C)Yield strength (MPa)Stress levels (MPa)Load levels 300575457,483,495,5090.79–0.89400518406,432,457,4760.78–0.92450532369,382,394,400,4060.69–0.76500430242,254,267,2800.56–0.65550374165,178,204,2100.44–0.5660036789,102,114,127,1780.24–0.4970018225.5,38,51,640.14–0.358008913,19,25.5,32,380.15–0.43900–13,19,25.5,32–Materials and Structures (2017)50:68Page 5of 146868Page6of14Materials and Structures(2017)50:68occur at higher temperatures and under high stress levels.The general trend of creep response in lower temperatures(300–400°C),shown in Fig.5a,b,can be grouped into two stages,namely primary(stage I) and secondary(stage II).The increase in secondary creep strain with time during stage II is dependent both on temperature and stress levels.At low stress levels, there is little secondary creep generated at300and 400°C.However,at temperatures of400°C and higher stress level of476MPa,the tertiary creep occurs.During the moderate temperature range of 450–550°C(Fig.5c–e),secondary creep and tertiary creep increased at a rapid rate and most specimens experienced rupture.The maximum creep strain is relatively low before rupture when the stress levels are very high.However,at a lower stress level,the maximum creep strain becomes much bigger before rupture.The increasing of maximum creep strain with lower stress level can be attributed to better develop-ment of plasticity in steel with longer time duration, and the time lasting of creep in steel is longer with the lower stress levels.For example,at temperature of 500°C,the maximum creep strain at stress of 267MPa is much higher than that at stress of 280MPa even though both specimens experienced rupture.The same trend can be found at temperatures of450and550°C.When the temperature is very high(600–900°C), the trend of creep response can be also generally grouped into two stages(Fig.5f–i),namely secondary (stage II)and tertiary(stage III).The primary creep becomes very short and the deformation fully devel-oped in very few minutes.The quick rise in creep strain with time in stage II is due to thermal softening. Secondary creep is considered to be highly important underfire exposure conditions because it dominates the creep response,occur in a constant rate,and over short period of time.In stage III,the tertiary creep increases at a faster rate due to reduced cross section of the specimen resulting from necking phenomenon which results in higher stresses for the same level of applied load.With the increase of temperature,the ductility of steel increases and maximum creep values become higher.The same phenomenon can also be seen in higher temperature tension test[16].Due to the limitation of movement of hydraulic jack,the rupture of some specimens during the temperature range of 600–900°C were not obtained.However,the limited data has little effect on the creep response of steel due to the tertiary creep is not important as the secondary creep in structural analysis.5.2Creep–time curves at various temperaturesThe effect of temperature on creep response of Q460 steel is illustrated in Fig.6.It can be seen from Fig.6a–c,for temperatures range of300–700°C,creep at a certain stress level increased significantly with tem-perature,even the elevation of temperature is only 50°C.For example,at stress level of406MPa,the creep at temperature of450°C is much bigger than that at temperature of400°C.Therefore,at a given stress level,the creep deformation increases with tempera-ture.This is due to diffusion of atoms within material grains resulting from temperature rise.When temper-atures are in the range of up to800°C,which represents50%of steel melting temperature,grain boundary diffusion tends to be the dominant mecha-nism that results in accelerated creep strain[17]. However,at temperature range of800–900°C,the general trend observed above is not applicable,as is shown in Fig.6d.The creep strain at900°C is similar with that at800°C under the same stress level,no matter what stress levels are.The trends in the figure indicate that creep deformation increases sub-stantially with increasing temperature up to800°C and then keep constant with the increase of temperature.5.3Rupture of specimensThe failure modes in test specimens at various temperatures and stress levels are shown in Fig.7. These illustrations,at each temperature,correspond to the test coupons with stress levels ranging from low to high,as tabulated in Table1.As can be seen from Fig.7,at temperature of900°C,the failure of test specimens exhibited a non-significant necking phe-nomenon but a very large ductile response was observed.However,during the temperatures range of 450–550°C,brittle fracture was observed for most specimens and the ductility varies greatly with differ-ent stress level.In order to compare the creep strain prior to rupture,all the specimens with occurrence of rupture are listed in Table2.From the table,the creep strains at rupture are within the range of9–25%in different stress levels and different temperatures.Materials and Structures(2017)50:68Page7of1468Fig.7Failure modes in creep test coupons at various temperatures,a T =300°C,b T =400°C,c T =450°C,d T =500°C,e T =550°C,f T =600°C,g T =700°C,h T =800°C,i T =900°C68Page 8of 14Materials and Structures (2017)50:686Proposed creep modelSome empirical models[3,4]to predict creep strain of carbon steel at elevated temperature are available in the literature.However,due to the facts that creep response in steel is highly material dependent,current models are not applicable to Q460steel.Test data generated from the above creep test can be utilized to develop creep models as a function of temperature and stress levels for Q460steel.As can be seen from the test data,little creep strain was observed at the temperature of300°C.Therefore,only the test data for temperature above400°C were selected in the creep models.Such models can be useful to evaluate fire response of Q460steel structures.6.1Fields andfields modelOne of the widely used creep models in structuralfire engineering applications is the one proposed by Fields and Fields[4].This model incorporates a power law and represents general expression for creep strain,e cr, in the form of a Norton–Bailey equation,which is given ase cr¼at b r cð1Þwhere e cr is the creep strain in steel;t is time(in min) and r is stress(in ksi);a,b and c are temperature-dependent material properties.Since Fields and Fields creep model is very simple in its form,this model can easily be incorporated in numerical analysis offire resistance of steel structures. Utilizing Eq.(1)as the basis for temperature induced creep,Fields and Fields[4]derived expressions for‘‘a’’,‘‘b’’and‘‘c’’based on test data on ASTM A36steel.a¼10À6:10À0:0053T350 C T\500 C10À13:25þ0:00851T500 C T650 Cð2Þb¼À1:1þ0:0035T350 C T650 Cð3Þc¼2:1þ0:0064T350 C T650 Cð4ÞCowan and Khandelwal[11]calibrated the above equations for creep of Australian A149structural steel and Japanese SS41steel(both similar to ASTM A36 steel),where the creep data was obtained for temper-atures between350and650°C and for a time period of4h.Assuming that stress and time are independent, Eq.(1)can be rewritten in rate form as:e0cr¼At B r Cð5Þwhere A=a9b,B=b-1and C=c.In the rate form,it can be observed that this formulation(Eq.5)is a time hardening model.Also, tertiary creep is not considered in this model.In order to simulate high temperature creep response in Q460steel using the form of Fields and Fields creep model,a nonlinear regressionfitting tool was employed and the values of coefficients a,b and c were evaluated at different temperatures,using the creep test data in Q460steel.For example,whenfitting the test data at300°C,four creep–time cures are selected and pre-defined function of Fields and Fields creep model is employed.Then,values range of a, b and c are set up.Afterfinishfitting,thefitting values are accepted if the adjusted R-square is higher than 0.95.Otherwise,other ranges are set-up again until the adjusted R-square reaches0.95.Thesefitting values of a,b,c and adjust R-square are tabulated in Table3.Table2Ultimate creep in specimens with fractureTemperature(°C)Stress(MPa)Ultimate creep(%)Temperature(°C)Stress(MPa)Ultimate creep(%)45038224.47006413.2 55020424.250028013.1 55017823.755016512.2 50026721.160012711.4 45039419.860017811.2 55025416.445040610.1 55021013.44504008.9Materials and Structures(2017)50:68Page9of1468Similar to the expressions in Eqs.(2–4),a nonlinear regression fitting tool was employed and the values of coefficients a,b and c were fitted as a function of temperature,using the creep test data in Q460steel.The expressions of a,b and c for Q460steel are given in Eqs.(6–8).Figure 8shows a comparison of high temperature creep strains computed using Eqs.(1,6–8)with those of measured test data for different stress levels at various temperatures.As can be seen from the figure,the proposed creep model based on fitting to creep data shows good agreement with the test data for Q460steel.To further study the influence of steel types on resulting coefficients,the parameters a,b and c ob-tained in this paper for Q460steel are compared with Eqs.(2)–(4),proposed by Fields and Fields [4]for conventional carbon A36steel,and this is shown in Fig.9.As can be seen in Fig.9,the coefficients obtained from fitting Q460steel creep test data arequite different with those for A36steel and this further infers that different types of steel exhibit varying creep response.This phenomenon can be attributed to the different content of chemical compositions in two steels and different condition of heat treatment.The main chemical compositions of the two steels aretabulated in Table 4.As can be seen from the table,the value of manganese in Q460steel is much higher than that in A36steel.In addition,the heat treatment for Q460steel includes hot rolling and normalizing,however,for A36steel,only hot rolling is conducted.Therefore,coefficients generated for A149steel or A36steel cannot be directly utilized to predict the creep behavior of other steels,such as Q460steel.It can also be seen from the Fig.9that the parameter b for both A36steel and Q460steel are quite close.However,for parameters a and c ,the trends are quite different.For Q460steel,at 450°C,parameter a reaches the minimum value and parameter c reaches the maximum value.6.2ANSYS modelANSYS is a versatile finite element analysis program that provides 13creep models for evaluating response of steel structures in fire.ANSYS also provides a nonlinear regression tool option for fitting a creep model based on specific data generated from creep experiment.The creep models in ANSYS account for the primary and secondary stages of creep that occurs in steel structures under fire conditions.Tertiary creep is usually not accounted for since it implies impendingTable 3Coefficients of a ,b and c in Fields and Fields creep model for Q460steel Temperature a b c Adj R 2400 6.48E -220.4911.250.986450 1.00E -250.6213.940.992500 1.55E -210.8012.370.9605509.38E -120.89 6.750.971600 4.20E -090.95 5.530.970700 2.52E -05 1.05 3.500.982800 5.00E -030.98 2.250.9919004.00E -020.661.980.994Log a ¼À56:797þ0:2265T À3:562Â10À4T 2300 C T 450 C 2286:225À13:65927T þ2:65644Â10À2T 2À1:6964Â10À5T 3450 C \T 600 C À69:01þ0:155T À1:040Â10À4T 2þ1:420Â10À8T 3600 C \T 900 C8<:ð6Þb ¼À2:71þ0:0128T À1:20Â10À5T 2300 C T 450 C À14:96þ0:08T À1:36Â10À4T 2þ7:81Â10À8T 3450 C \T 600 C 1:18À7:07Â10À3T þ1:9Â10À5T 2À1:31Â10À8T 3600 C \T 900 C 8<:ð7Þc ¼39:25À0:18T þ2:75Â10À4T 2300 C T 450 C À1557:61þ9:21T À1:78Â10À2T 2þ1:132Â10À5T 3450 C \T 600 C 22:52À2:07Â10À2T À3:34Â10À5T 2þ3:45Â10À8T 3600 C \T 900 C8<:ð8Þ68Page 10of 14Materials and Structures (2017)50:68。