英文原文：
Mechanical properties of materials
2007年春节 The material properties can be classified into three major headings: (1) physical， （2） chemical, （3) mechanical
谐音古诗Physical properties
Density or specific gravity， moisture content， etc。, can be classified under this category.
Chemical properties
Many chemical properties come under this category。 The include acidity or alkalinity, react6ivity and corrosion。 The most important of the is corrosion which can be explained in layman’s terms as the resistance of the material to decay while in continuous u in a particular atmosphere。
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Mechanical properties
Mechanical properties include in the strength properties like tensile， compression, shear， torsion， impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load， which the specimen bears by the area of cross-ction of the specimen。
This is a curve plotted between the stress along the This is a curve plotted between the stress along the Y-axis(ordinate) and the strain along the X—axis (abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be en that the deformation disappears. For many materials this occurs op to a certain value of the stress called the elastic limit Ap. This is depicted by the straight line relationship and a small deviation thereafter， in the stress-strain curve （fig。3.1）
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绍兴旅游景点攻略. Within the elastic range， the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality Ap。 In this region， the metal
obeys hookes’s law, which states that the stress is proportional to strain in the elastic range of loading， （the material 关于春节的简介completely regains its original dimensions after the load is removed）。 In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the
elastic limit。 This may be attributed to the time-lagin the regaining of the original dimensions of the material. This effect is very frequently noticed in some non—ferrous metals.
Which iron and nickel exhibit clear ranges of elasticity， copper， zinc， tin, are found to be imperfectly elastic even at relatively low values low values of stress。 Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the nsitivity of the measuring instrument.
When the load is incread beyond the elastic limit, plastic deformation starts。 Simultaneously the specimen gets work—hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called they
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yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i。 e。, yield。 The yield stress is called yield limit Ay。
小学生种绿豆的方法 The elongation of the specimen continues from Q to S and then to T。 The stress—strain relation in this plastic flow period is indicated by the portion QRST of the curve。 At the specimen breaks, and this load is called the breaking load. The value of the maximum load S divided by the original cross-ctional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength Au。
Logically speaking, once the elastic limit is exceeded, the metal should start to yield, and finally break, without any increa in the value of stress。 But the curve records an incread stress even after the elastic limit is exceeded. Two reasons can be given for this behavior:
①The strain hardening of the material；
②The diminishing cross—ctional area of the specimen, suffered on account of the plastic deformation。
The more plastic deformation the metal undergoes， the harder it becomes, due to work—hardening. The more the metal gets elongated the more its diameter (and hence, cross-ctional area） is decread。 This continues until the point S is reached。
After S， the rate at which the reduction in area takes place， exceeds the rate at which the stress increas. Strain becomes so high that the reduction in area begins to produce a localized effect at some point。 This is called necking.
Reduction in cross—ctional area takes place very rapidly； so rapidly that the load value actually drops。 This is indicated by ST。 failure occurs at this point T.
Then percentage elongation A and reduction in reduction in area W indicate the ductility or plasticity of the material:
A=（L—L0）/L0*100％
W=（A0—A）/A0＊100%
