材料科学基础-第5章材料力学性能

图5－6 穿晶断裂与 沿晶断裂示意图
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5.1.3 材料的断裂及其性能指标
3．剪切断裂与解理断裂 (1)剪切断裂 材料在切应力作用下沿滑移面分离而造成的 断裂称为剪切断裂。又分为滑断（纯剪切断裂）和微孔聚集 型断裂。某些纯金属，尤其是单晶体金属可产生纯剪切断裂 ，其断口呈锋利的楔形。如低碳钢拉伸断口上的剪切唇。大 块单晶体的纯剪切断口上，用肉眼便可观察到很多直线状的 滑移痕迹。微孔聚集型断裂是通过微孔聚合而导致材料分离 。由于实际材料中常同时形成许多微孔，故微孔聚集型断裂 是材料韧性断裂的普通方式。其断口在宏观上常呈现暗灰色 、纤维状，微观特征是断口上分布大量“韧窝”。
shear stress-shear strain curve.
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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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5.1.2 材料的变形及其性能指标
Elastic limit Tensile strength, Necking Hooke’s law Poisson’s ratio Modulus of resilience (Er) Tensile toughness Ductility
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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5.1.1 材料的拉伸曲线
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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
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5.1.2 材料的变形及其性能指标
1. 材料变形的实质
(1)弹性变形的实质
(2)塑性变形的实质 (3)超塑性 2.材料变形的性能指标 比例极限σp 弹性极限（Elastic limit） σe 弹性模量E 屈服极限σs (σ0.2) 抗拉强度（Tensile strength ）σb 断裂强度Sk 延伸率δ 断面收缩率ψ

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
材料科学基础
李谦– 宁向梅主讲
Chapter 5 –材料的力学性能
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Chapter Outline
5.1 材料承受静载荷时的力学性能 5.2 材料承受冲击载荷时的力学性能 5.3 材料的疲劳 5.4 材料的断裂韧性 5.5 材料的磨损性能 5.6 材料的蠕变性能
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Technological Significance
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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.
Leabharlann Baidu
Strain - Elongation change in dimension per unit length.
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Figure Dimples form during ductile fracture. Equiaxed dimples form in the center, where microvoids grow. Elongated dimples, pointing toward the origin of failure, form on the shear lip
Figure The relation between the true stress-true strain diagram and engineering stressengineering strain diagram. The curves are identical to the yield point
Figure
Aircraft, such as the one shown here, makes use of aluminum alloys and carbonfiber-reinforced composites.
Figure
The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant.
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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. Engineering stress - The applied load, or force, divided
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Figure (a) Determining the 0.2% offset yield strength in gray cast ion, and (b) upper and lower yield point behavior in a low-carbon steel
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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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5.1.3 材料的断裂及其性能指标
2．穿晶断裂与沿晶断裂 穿晶断裂（Transgranular）可以是韧性断裂，也可以是 脆性断裂；离子键晶体的断裂往往以穿晶解理为主。 沿晶断裂（Intergranular）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。共价键陶瓷 晶界较弱，断裂方式主要是晶界断裂。
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5.1.3 材料的断裂及其性能指标
(3)准解理断裂 准解理断裂是解理断裂的变种，断口微观形态相似于解理河 流状花样，但准解理裂纹不是严格地沿着一定晶面扩展，其 刻面不是晶体学解理面，不属于真正的解理，故称为准解理 。解理裂纹一般源于晶界，而准解理裂纹则常源于晶内硬质 点，形成从晶内某点发源的放射状河流花样。准解理断裂常 见于淬火、回火处理的钢中。 （4）高分子材料的断裂 高分子材料的断裂也分为脆性断裂和韧性断裂两大类。玻璃 态聚合物在玻璃化温度 以下主要为脆性断裂，聚合物单晶 体可以发生解理断裂，属于脆性断裂。而温度以上的玻璃态 聚合物以及通常使用的半晶态聚合物，断裂时伴随有较大的 塑性变形，属于韧性断裂。
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5.1.3 材料的断裂及其性能指标
(2)解理断裂 在正应力作用下，材料原子间的结合键被破 坏，从而引起沿特定晶面发生的穿晶断裂称为解理断裂。解 理断口由许多大致相当于晶粒大小的解理面集合而成。这种 以晶粒大小为单位的解理面称为解理刻面。解理裂纹往往沿 着一族相互平行，但位于“不同高度”的晶面扩展。不同高 度的解理面之间存在台阶，众多台阶的汇合便形成河流状花 样。解理台阶、河流花样和舌状花样是解理断口的基本微观 特征，
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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 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。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。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 （如淬火钢、灰铸铁、陶瓷、玻璃等脆性材料的断口。 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。
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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.
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Figure When a ductile material is pulled in a tensile test, necking begins and voids form – starting near the center of the bar – by nucleation at grain boundaries or inclusions. As deformation continues a 45° shear lip may form, producing a final cup and cone fracture
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Figure Tensile stress-strain curves for different materials. Note that these are qualitative
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Figure The stress-strain curve for an aluminum alloy
单向静拉伸试验是广泛应用的材料性能检测方法。 负荷-伸长曲线。 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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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).