1、Chapter 6 Mechanical properties,Why study mechanical properties?,As a material engineers, it is incumbent (obligatory有义务的) to understand how the various mechanical properties are measured and what these properties represent. In case that you are called upon( ordered; required) to design structures o
2、r components using predetermined materials such that unacceptable levels of deformation or failure will not occur, the adequate knowledge on mechanical properties of the material is a matter of life.,What should you be able to do after studying this chapter?,know what a tensile-testing apparatus is
3、used for. Know how stress-strain curve is obtained. Name what are the main mechanical properties。,Important terms and concepts:,Anelasticity n.滞弹性 stress n. 应力 ductility n. 延展性，塑性 elastic adj. 弹性的 true stress 真应力 engineering stress 工程应力,Elasticity n.弹性 strain n. 应变 brittlement n. 脆性 plastic adj. 塑性的
4、 true strain 真应变 engineering strain 工程应变,Stress-strain curve 应力应变曲线,elastic recovery 弹性回复 stiffness n. 刚度 modulus of elasticity 弹性模量 deform v. distort/bend/twist： To alter the shape of by pressure or stress.使变形：通过压力或拉力改变的形状 Deformation n.变形,yielding adj. Inclined to give way to pressure, or influenc
5、e. 屈服的。 yield strength 屈服强度 tensile strength 拉伸强度 elongation to fracture 延伸率 Poissons ratio 泊松比 proportional limit 比例极限resilience :The property of a material that enables it to resume its original shape or position after being bent, stretched, or compressed; elasticity.弹性：使某物质能在弯曲、伸展或收缩后恢复原先的形状或位置的物
6、体的性能；收缩性.,safe stress 安全应力 shear 剪切 toughness 韧性 fracture toughness 断裂韧度 impact toughness 冲击韧性 hardness n. 硬度 Rochwell hardness 洛氏硬度 Brinell hardness 布氏硬度 Knoop hardness 努氏硬度 Vichers hardness 维氏硬度 Indenter 压头 Hookes law 虎克定律,ascertained: adj. determined replicate vt. duplicate, copy, reproduce, or r
7、epeat.复制：复印、复写、复制或重复 fluctuate vt. vary irregularly , swing 波动,变动 imperative- urgent Professional society 专业协会 ASTM -American Society for Testing and Materials Confine vt. restrict/limit,Four stress-strain test types:Tension, compression, torsion, shear At the onset-A beginning; a start:开始： the onse
8、t of a cold.感冒初起 Offset-The start or initial stage; the outset.开始，开端,Introduction,Many materials or components, e.g. road wheels of a car, when in service , are subjected to forces or loads. In such situations it is necessary to know the characteristics of the material and to design the member from
9、which it is made such that any resulting deformation will not be excessive and fracture will not occur .The mechanical behavior of a material reflects the relationship between its response or deformation to an applied load or force. Important mechanical properties are strength, hardness, ductility,
10、and stiffness.,Materials and metallurgical engineers are concerned with producing and fabricating （制造）materials to meet service requirements as predicted by these stress analyses. This necessarily involves an understanding of the relationships between the microstructure (i.e., internal features) of
11、materials and their mechanical properties.,Tension test（拉伸试验）,When construct a component or something the like, all these properties of materials should be taken into consideration in the whole process of material selection and structure design.,Stress- strain curve of a low carbon steel specimen,Up
12、per yielding point,Low yielding point,Definition of concepts,Stress （应力）is a measure of an applied mechanical load or force, normalized (归一化)to take into account cross-sectional area. Two different stress parameters were defined-engineering stress and true stress.,缩截面,标距长度,标准圆形截面拉伸试样,(Engineering) s
13、train（工程应变）,Strain represents the amount of deformation induced by a stress; both engineering and true strains are used.in which l0 is the original length（原始长度） before any load is applied, and li is the instantaneous length（瞬时长度）. Sometimes the quantity lil0 is denoted as l, and is the deformation e
14、longation or change in length at some instant, as referenced to the original length.Engineering strain (subsequently called just strain) is unitless（无量纲）, but meters per meter or inches per inch are often used.,Modulus of elasticity（弹性模量）, or Youngs modulus(杨氏模量),The degree to which a structure defo
15、rms or strains depends on the magnitude of an imposed stress. For most metals that are stressed in tension and at relatively low levels, stress and strain are proportional to each other through the relationship =E This is known as Hookes law（虎克定律）, and the constant of proportionality E (GPa) is call
16、ed as modulus of elasticity（弹性模量）.,Elastic deformation（弹性变形）,Deformation in which stress and strain are proportional is called elastic deformation; a plot of stress (ordinate(纵坐标) versus strain (abscissa（横坐标）) results in a linear relationship, as shown in Figure 7.5. The slope of this linear segment
17、 corresponds to the modulus of elasticity E. Stiffness is a materials resistance to elastic deformation. Q：The greater the modulus, the stiffer the material, or the smaller the elastic strain that results from the application of a given stress. (翻译成中文),Plastic deformation（塑性变形）,Yielding and yielding
18、 strength,Yielding phenomenon（屈服现象）: plastic deformation begins, the elasticplastic transition occurs abruptly. Point of yielding （屈服点）may be determined as the initial departure from linearity of the stressstrain curve; this is sometimes called the proportional limit（比例极限）, as indicated by point P i
19、n Figure 7.10a. a convention has been established wherein a straight line is constructed parallel to the elastic portion of the stressstrain curve at some specified strain offset, usually 0.002. The stress corresponding to the intersection of this line and the stressstrain curve as it bends over in
20、the plastic region is defined as the yield strength （屈服强度）y .,Tensile strength,Tensile strength TS (MPa)（拉伸强度） is the stress at the maximum on the engineering stressstrain curve (Figure 7.11). at this maximum stress, a small constriction or neck begins to form at some point, and all subsequent defor
21、mation is confined at this neck, as indicated by the schematic specimen insets in Figure 7.11. This phenomenon is termed necking （颈缩）. Fracture strength （断裂强度）corresponds to the stress at fracture.,Ductility,Ductility （塑性）is another important mechanical property. It is a measure of the degree of pla
22、stic deformation that has been sustained at fracture. Ductility may be expressed quantitatively as either percent elongation （EL延伸率）to failure or percent reduction in area (RA截面收缩率). The percent elongation %EL is the percentage of plastic strain at fracture, or where lf is the fracture length and l0
23、 is the original gauge length as above.,Percent reduction in area (%RA) is defined aswhere A0 is the original cross-sectional area and Af is the cross-sectional area at the point of fracture.A material that experiences very little or no plastic deformation upon fracture is termed brittle （脆性）.,TOUGH
24、NESS,Toughness （韧性）is a mechanical term that is used in several contexts; loosely speaking, it is a measure of the ability of a material to absorb energy up to fracture.Poissons ratio （柏松比）represents the negative ratio of transverse (横向) and longitudinal （纵向）strains.,Question?,What are the two metho
25、ds for expressing ductility ?,Summary,Some of the mechanical characteristics of materials can be ascertained by simple stress-strain tests. There are four test types: tension, compression, torsion, and shear. Tensile are the most common. A material that is stressed first undergoes elastic, or nonper
26、manent deformation, wherein stress and strain are proportional. The constant of proportionality is the modulus of elasticity for tension and compression, and is the shear modulus when the stress is shear. Possions ratio represents the negative ratio of transverse and longitudinal strains.,For metals
27、, the phenomenon of yielding occurs at the onset of plastic or permanent deformation; yield strength is determined by a strain offset （偏移、补偿）method from the stress-strain behavior, which is indicative of the stress at which plastic deformation begins. Tensile strength corresponds to the maximum tens
28、ile stress that may be sustained by a specimen, whereas percents elongation and reduction in area are measures of ductilitythe amount of plastic deformation that has occurred at fracture.,Resilience is the capacity of materials to absorb energy during elastic deformation, modulus of resilience is th
29、e area beneath the engineering stress-strain curve up to the yield point. Also, static toughness represents the energy absorbed during the fracture of a materials, and is taken as the area under the entire engineering stress-strain curve. Ductile materials are normally tougher than brittle ones.,So much for you this time!See you!,
