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1、崔倩倩PrefaceThe goal of the following pages : To characterize materials in terms of some of the fundamental mechanical properties(基本力学性能） . A qualitative（本质的） distinction between ductile（ 韧性）, brittle（脆性）, and elastic（弹性） materials can be achieved in a relatively simple experiment using the bend test.
2、 A long and comparatively thin piece of the material to be tested is placed near its ends on two supports and loaded at the center. It is intuitively obvious that an elastic material such as wood can be bent to a much higher degree before breakage occurs than can a brittle material such as stone or
3、glass. Moreover, elastic materials return upon elastic deformation（弹性变形） to their original configuration once the stress has been removed. On the other hand, ductile materials undergo a permanent change in shape above a certain threshold load （临界负载）. But even ductile materials eventually break once
4、a large enough force has been applied.Tensile test: To quantitatively evaluate these properties, a more sophisticated device is routinely used by virtually all industrial and scientific labs. In the tensile tester（拉力试 验机）, a rod-shaped or flat piece of the material under investigation is held betwee
5、n a fixed and a movable arm .A force upon the test piece (试样）is exerted by slowly driving the movable cross-head away from the fixed arm. This causes a stress（应力） .On the sample, which is defined to be the force, F, per unit area, A0, that is, Since the cross section (横截面）changes during the tensile
6、test（拉伸试验）, the initial unit area（单位面积 ）A0, is mostly used. If the force is applied parallel to the axis of a rod-shaped material, as in the tensile tester (that is, perpendicular to the faces A0), then is called a tensile stress(拉伸应力）. If the stress is applied parallel to the faces ，it is termed sh
7、ear stress（剪切应力） . Many materials respond to stress by changing their dimensions （纵向尺寸）. Under tensile stress, the rod becomes longer in the direction of the applied force (and eventually narrower perpendicular to that axis). The change in longitudinal dimension in response to stress is called strai
8、n（应变） , that is, where l0 is the initial length of the rod and l is its final length. The absolute value of the ratio between the lateral strain(横向应变) (shrinkage) and the longitudinal strain ( 纵向应变)(elongation) is called the Poisson ratio(泊松比 ). The result of a tensile test is commonly displayed in
9、a stressstrain diagram(应力-应变图). Several important characteristics are immediately evident. During the initial stress period, the elongation of the material responds to in a linear fashion; the rod reverts back to its original length upon relief of the load(卸载). This region is called the elastic rang
10、e. Once the stress exceeds, however, a critical value, called the yield strength(屈服强度), some of the deformation of the material becomes permanent. In other words, the yield point separates the elastic region from the plastic range of materials. This is always important if one wants to know how large
11、 an applied stress needs to be in order for plastic deformation of a workpiece(工件) to occur. On the other hand, the yield strength provides the limit for how much a structural component(结构组件) can be stressed before unwanted permanent deformation takes place. As an example, a screwdriver (螺丝刀)has to
12、have a high yield strength; otherwise, it will deform upon application of a large twisting force(扭曲力). The highest force (or stress) that a material can sustain is called the tensile strength(抗张强度).At this point, a localized decrease in the cross-sectional area(横截面积) starts to occur. The material is
13、 said to undergo necking(缩 颈). Because the cross section is now reduced, a smaller force is needed to continue deformation until eventually the breaking strength. The modulus of elasticity (弹性模量)is a parameter that reveals how “stiff” a material is, that is, it expresses the resistance of a material
14、 to elastic bending or elastic elongation. Stressstrain diagrams vary appreciably for different materials and conditions. As an example, brittle materials, such as glass, stone, or ceramics have no separate yield strength, tensile strength, or breaking strength(断裂强度). In other words, they possess es
15、sentially no plastic (ductile) region and, thus, break already before the yield strength is reached .Brittle materials are said to have a very low fracture toughness(断裂韧度). As a consequence, tools (hammers, screwdrivers, etc.) should not be manufactured from brittle materials because they may break
16、or cause injuries. The tensile test and the resulting stressstrain diagrams have been shown above to provide a comprehensive insight into many of the mechanical properties of materials. For specialized applications, however, a handful of further tests are commonly used. Hardness Test: The hardness test (硬度试验)is nondestructive and fast. A small steel sphere (commonl
