机械设计制造及其自动化专业英语翻译第三单元_科技英语翻译机械专业
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Mechanical properties of Materials
材料的机械特性
The materials properties can be claified into three major heading：（i）Physical，（ii）Chemical，（iii）Mechanical。
材料的特性可以分为三个重要的部分：1 是物理特性，2 是化学特性，3 是机械特性。Physical Properties 物理特性
Density or specific gravity, moisture content, etc., can be claified under this category.密度或者比重，水分含量等，都被分为这一类的范畴 Chemical Properties 化学特性
Many chemical properties come under this category.These include acidity or alkalinity，许多化学特性都归入到这个范畴。这些特性包括酸性或碱性 reactivity and corrosion.The most important of these is corrosion which 反应性和腐蚀性。
can be explained in layman’s terms as the resistance of the material to decay while in continuous use in a particular atmosphere 这些中最重要的是耐腐蚀性，它可以被解释为在外行人看来作为材料在特定大气中长期使用时抵抗腐蚀的能力。Mechanical Properties(机械特性)
Mechanical properties include the strength properties like tensile, compreion, shear, torsion, impact, fatigue and creep.机械特性包括拉伸，压缩，剪切，扭转，冲击，疲劳和蠕变等强度特性。
The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of cro-section of the specimen.材料的抗拉强度由试样承载的最大载荷除以试样的横截面积获得。
This is a curve plotted between the stre along the Y-axis(ordinate)and the strain along the X-axis(abscia)in a tensile test.这是一个在拉伸试验中沿着Y轴（纵坐标）的应力和沿着X轴（横坐标）的应变的关系曲线。
A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load.当材料加载时尺寸的变化，取决于载荷大小的变化。
When the load is removed it can be seen that the deformation disappears.当卸载时可以看见变形消失。
For many materials this occurs up to a certain value of the stre called the elastic limitσe.对于许多材料来说这种发生的应力极限值称为弹性极限。This is depicted by the straight line relationship and a small deviation thereafter, in the stre-strain curve（Fig.3.1）.在应力-应变曲线中，直线关系和随后小的偏移描述了这个（加载和卸载的过程）。
Within the elastic range, the limiting value of the stre up to which the stre and strain are proportional, is called the limit of proportionalityσp.在弹性范围内，应力与应变的比例的极限值被称作比例极限。
In this region, the metal obeys Hooke’s law, which states that the stre is proportional to strain in the elastic range of loading(the material completely regains its original dimensions after the load is removed).在这个区域中，金属服从胡克定律，阐述了在加载的弹性范围内应和应变成比例关系（材料卸载后能够恢复到原来的尺寸）。
In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit.在曲线绘制中，载荷的比例极限值比弹性极限值稍低，This may be attributed to the time-lag in the regaining of the original dimensions of the material.这可能由于材料恢复原尺寸时存在时间延迟。这种现象经常出现在有色金属中。
While iron and nickel exhibit clear ranges of elasticity, copper, zinc tin, etc, are found to be imperfectly elastic even at relatively low values of strees.当铁和镍存在明显的弹性范围，铜，锌，锡等，即使在相对低的应力下也表现为不完全弹性。
Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument.实际上弹性极限和比例极限可以区分的更清晰，依靠测试仪器的精确性。
When the load is increased beyond the elastic limit, plastic deformation starts.当在超过了弹性极限载荷时会产生塑性变形。Simultaneously the specimen gets work-hardened.同时试件会发生加工硬化。
A point is reached when the deformation starts to occur more rapidly than the increasing load.到达某点后变形的速度要比载荷增加的速度快一些。This point is called the yield point Q.这一点叫做屈服极限点（Q）。
the metal which was resisting the load till then, starts to deform somewhat rapidly, i.e., yield.一当到达屈服极限后开始一直在抵抗载荷的金属开始迅速地发生形变也就是屈服。
The yield stre is called yield limit.屈服应力叫做屈服极限。
The elongation of the specimen continues from Q to S and then to T.试件的延伸率从Q到S再到T持续地延长。
The stre-strain relation in this plastic flow period is indicated by the portion QRST of the curve.在这个塑性流动期间的应力-应变关系表示为曲线上的QRST段。
At T the specimen breaks, and this load is called the breaking load.在T点试件断裂，此时的载荷称为断裂载荷。
The value of the maximum load S divided by the original cro-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength.最大载荷值S除以试件的横截面积为金属的最大拉伸强度或简单地称为拉伸强度。
Logically speaking , once the elastic limit is exceeded, the metal should start to yield, and finally break, without any increase in the value of stre.逻辑上来说，一旦超过弹性极限，金属应该开始屈服直至最后断裂，在应力值上应该没有增加。
But the curve records an increased stre even after the elastic limit is exceeded.但是实际的曲线却记录了在超过弹性极限后的增加了的应力。
Two reasons can be given for this behavior:
1.the strain hardening of the material;
2.the diminishing cro-sectional area of the specimen, suffered on account of the plastic deformation.这种现象的发生可能有两种原因。1.材料的应变强化。
2.由于塑性变形引起的试件横截面积的缩小。
The more plastic deformation the metal undergoes, the harder it becomes, due to work-hardening.由于加工硬化,金属在发生塑性变形时会变得越来越硬。
The more the metal gets elongated the more its diameter(and hence, cro-sectional area)is decreased.金属拉伸越长它的直径越小。
This continues until the point S is reached.这种现象一直持续到曲线上的S点。
After S, the rate at which the reduction in area takes place, exceeds the rate at which the stre increases.超过S点后,面积减少的速度超过了应力增加的速度。
Strain becomes so high that the reduction in area begins to produce a localized effect at some point.应变变得很大,面积的减少在某些点产生了局部效应。
This is called necking.这种效应称为颈缩。
Reduction in cro-sectional area takes place very rapidly;so rapidly that the load value actully drops.This is indicated by ST.Failure occurs at this point T.横截面积减小的速度非常快以至于实际上载荷降低。
Then percentage elongation and reduction in area indicate the ductility or plasticity of the material: 伸长率和面积减少率表示了材料的延展性: LLδ=100% LAAΨ=100% A0000Where Land L0 are the original and the final length of the specimen;A and A are the original and the final cro-sectional area.0其中，L0和 L为试样的原始和最终的长度；A0 和A为最初的和最后的横截面积。
