材料科学基础-第5章材料力学性能

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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。
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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True Stresshe 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
（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。
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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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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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.
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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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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
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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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5.1.2 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
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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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Aircraft, such as the one shown here, makes use of aluminum alloys and carbonfiber-reinforced composites.
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The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant.
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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

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.
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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
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”。