材料的力学性能

图5－6 穿晶断裂与沿 晶断裂示意图
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Figure Localized deformation of a ductile material during a tensile test produces a necked region. The micrograph shows necked region in a fractured sample
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Figure Range of elastic moduli for different engineered materials. (Source: Reprinted from Engineering Materials I, Second Edition, M.F. Ashby and D.R.H. Jones, 1996, Fig. 3-5, p. 35, Copyright © 1996 Butterworth-Heinemann.
Figure
The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant.
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Terminology for Mechanical Properties
Stress - Force or load per unit area of cross-section over which the force or load is acting. Strain - Elongation change in dimension per unit length. Young’s modulus - The slope of the linear part of the stress-strain curve in the elastic region, same as modulus of elasticity. Shear modulus (G) - The slope of the linear part of the shear stress-shear strain curve. Viscosity ( ) - Measure of resistance to flow, defined as the ratio of shear stress to shear strain rate (units Poise or Pa-s). Thixotropic behavior - Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time. 5
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Chapter Outline
5.1 5.2 5.3 5.4 5.5 5.6 材料承受静载荷时的力学性能 材料承受冲击载荷时的力学性能 材料的疲劳 材料的断裂韧性 材料的磨损性能 材料的蠕变性能
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Technological Significance
Figure Aircraft, such as the one shown here, makes use of aluminum alloys and carbonfiber-reinforced composites.
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5.1.3 材料的断裂及其性能指标
（一）断裂的类型及断口特征 根据断裂前后材料宏观塑性变形的程度，分为脆性断裂与韧性断裂； 根据晶体材料断裂时裂纹扩展的途径，分为穿晶断裂和沿晶(晶界)断 裂；根据微观断裂机理，分为解理断裂和剪切断裂等。 材料的断裂表面称为断口。用肉眼、放大镜或电子显微镜等手段对材 料断口进行宏观及微观的观察分析，称为断口分析。 1．韧性断裂与脆性断裂 韧性断裂是材料断裂前产生明显塑性变形的断裂过程。韧性断裂的断 口往往呈暗灰色、纤维状。塑性较好的金属材料和高分子材料，室温 下的静拉伸断裂具有典型的韧性断裂特征。 脆性断裂是材料断裂前不产生明显的塑性变形。脆性断裂的断口，一 般与正应力垂直，宏观上比较齐平光亮，常呈放射状或结晶状。淬火 钢、灰铸铁、陶瓷、玻璃等脆性材料的断口常具有上述特征。 实际上，金属的脆性断裂与韧性断裂并无明显的界限，脆性断裂前也 会产生微量塑性变形。因此，规定光滑拉伸试样的断面收缩率小于5％ 为脆性断裂；大于5％为韧性断裂。 20
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Section 5.1 材料承受静载荷时的力学性能 5.1.1 材料的拉伸曲线
Load - The force applied to a material during testing. Strain gage or Extensometer - A device used for measuring change in length and hence strain. Glass temperature (Tg ) - A temperature below which an otherwise ductile material behaves as if it is brittle. Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material. Engineering strain - The amount that a material deforms per unit length in a tensile test.
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Figure Tensile stress-strain curves for different materials. Note that these are qualitative
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The stress-strain curve for an aluminum alloy
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Figure Typical yield strength values for different engineered materials. (Source: Reprinted from Engineering Materials I, Second Edition, M.F. Ashby and D.R.H. Jones, 1996, Fig. 8-12, p. 85. Copyright © Butterworth-Heinemann曲线
单向静拉伸试验是广泛应用的材料性能检测方法。
负荷一伸长曲线-材料的拉伸曲线。 整个拉伸过程中的变形可分为弹性变形、屈服变形、均匀塑性变 形及不均匀塑性变形四个阶段。 应力—应变曲线(工程应力—应变曲线) 真实应力—应变曲线
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Figure A unidirectional force is applied to a specimen in the tensile test by means of the moveable crosshead. The cross-head movement can be performed using screws or a hydraulic mechanism
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Figure (a) Tensile, compressive, shear and bending stresses. (b) Illustration showing how Young’s modulus is defined for elastic material. (c) For nonlinear materials, we use the slope of a tangent as a variable quantity that replaces the Young’s modulus constant
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True Stress and True Strain
True stress The load divided by the actual cross-sectional area of the specimen at that load.
True strain The strain calculated using actual and not original dimensions, given by εt ln(l/l0).
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5.1.3 材料的断裂及其性能指标
2．穿晶断裂与沿晶断裂 根据材料(包括金属、陶瓷及结晶高分子)发生断裂时裂纹扩展的路径 ，分为穿晶断裂和沿晶(晶界)断裂两种。 穿晶断裂可以是韧性断裂，也可以是脆性断裂；而沿晶断裂则多为脆 性断裂，断口呈结晶状；沿晶断裂是晶界结合力较弱的一种表现。例 如共价键陶瓷晶界较弱，断裂方式主要是晶界断裂。离子键晶体的断 裂往往以穿晶解理为主。
Figure The relation between the true stress-true strain diagram and engineering stressengineering strain diagram. The curves are identical to the yield point
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5.1.3 材料的断裂及其性能指标 Microstructural Features of Fracture in Metallic Materials
Transgranular - Meaning across the grains (e.g., a transgranular fracture would be fracture in which cracks would go through the grains). Microvoids - Development of small holes in a material. Intergranular - In between grains or along the grain boundaries. Chevron pattern - A common fracture feature produced by separate crack fronts propagating at different levels in the material.