双语教学(第二章)

＜450HBS; ＜650HBW;
HB
2P
D( D D d )
2 2
Brinell hardness tester
布氏硬度计
Rockwell test
Asphero-conical diamond cone of 120o angle and a sphercal apex of radius 0.2mm is used to make the indentation and the depth of the indentation, t is used as a criterion to calculate the hardness number. The Rockwell hardness number(R)is given by R=100-500t Depending on the load used for indentation there are a number of scales A,B,C,etc. available in Rockwell tests.

L1 L0 100% L0
Reduction in cross-section area: In quantitative terms it may be expressed as the ratio of reduction in cross-section area in the fractured specimen to the original cross-section


F0 F1 100% F1
2.1.2 Hardness
Hardness is the resistance offered by a material to indentation. The most commonly used tests are: Brinell hardness test A sphere(diameter 10± 0.01mm)made of steel or tungsten carbide is indented with a gradually applied load at right angles to the specimen surface and the indentation diameter made on the specimen measured.The Brinell hardness number,HB is given by

(3)Ductility
It is the measure of the plastic deformation a material can undergo under tensile force without fracture. Elongation:In quantitative terms it is normally measured as the ratio of elongation of the material at fracture during tensile test to the original length,expressed as a percentage.
Chapter 2 Mechanical properties of engineering materials
Main points： *Static properties *Impact properties Fatigue properties Properties at high temperature Fracture toughness
F
Impact tester and specimen
Section 2.3 Fatigue properties
If
the stress is decreased to a certain level, the part can under alternating load for ever. The stress is considered as fatigue strength. It can be denoted with σr

Rockwell hardness tester
洛氏硬度测试示意图 洛 氏 硬 度 计
h1-h0
Section 2.2 Impact properties
Definition:
Toughness signifies the amount of energy absorbed by a material at the time of fracture under impact load.
(2)Strength
The resistance of deformation and fracture offered by a material on application of external force is called strength. Depending on the type of load applied, the strength could be tensile, compressive,bending or shear. Yield strength(σS ): The resistance of small plastic deformation offered by a material on application of external force is called yield strength. The smallest stress which starts plastically deforming. Tensile strength(σb ): The resistance of fracture offered by a material on application of external force is called tensile strength. The biggest stress before fracture
stress σ-1 Fatigue curve N
Section 2.4 Properties at high temperature
Creep: If a material is undergone a external stress at high temperature,obvious plastic deformation offered by the material can be creased even though the stress is lower than the yield stress.This phenomena is called creep.
Section 2.5 Fracture toughness
Fracture mechanics
is quite different from material mechanics. Materials is considered as inhomogeneous and continuous.
Y----parameter which has relationship with specimen and crack :external stress a: half length of the crack
K1 Y a


K1C Y C a


K1C: material parameter, K1C is called Fracture toughness. Its values can be obtained by testing. Fracture toughness(K1C) only has the relationship with material composition, heat-treatment, hot working process, and so on. It is also the resistance of brittle fracture. K1:mechanical parameter
Impact
test
a.Specimen(10×10×55mm ) b.Impact test machine c.Type of specimen clamping methods d.Absorbed energy
AK=(GH1-GH2 ) ×9.8 (N.m) or aK= GH1 GH 2 (J/mm2 )
Βιβλιοθήκη Baidu
Section2.1 Static properties
拉伸试样
拉伸试验机
Stress-strain figue
(1)definitions
a.Elastic and plastic deformation Elastic deformation:It is that part of the deformed material which when the applied load is removed, would spring back to its normal shape. Plastic deformation: Plastic deformation is permanently set in a material and cannot be regained when the applied load is removed. b.Strain: Strain=change in dimension/original dimension=L1/L0 c.Stress: Stress (σ)=applied load/area of cross section opposing the load σ =P/F(N/mm2 ) d.Elasticity: The ability of recovering its original shape is called elasticity. e. Elastic modulus:E=σ/ε