重庆大学机械工程专业英语第2章

2 Simple Stress and Strain 应力 和应变 In any engineering structure the individual components or members will be subjected to external forces arising from the service conditions 使用条件 or environment in which the component works If the component or member is in equilibrium 平衡 the resultant 合力 of the external forces will be zero but nevertheless they together place a load 载荷 on the member which tends to 易于 deform that member and which must be reacted by internal forces set up within the material There are a number of different ways in which load can be applied to 应用于 a member Loads may be classified with respect to 关于 time A static load is a gradually applied load 外加载荷 for which equilibrium is reached in a relatively short time A sustained load 持续长期载荷 is a load that is constant 恒定的 over a long period of time such as the weight of a structure This type of load is treated in the same manner 同样地 as a static load however for some materials and conditions of temperature and stress the resistance to failure 抵抗失效的能力 may be different under short time loading and under sustained loading An impact load 冲击载荷 is a rapidly applied load an energy load 能量载荷 Vibration normally results from 由 造成 an impact load and equilibrium is not established until 在 之前 the vibration is eliminated usually by natural damping forces 阻尼力 A repeated load 重复载荷 is a load that is applied and removed 移除 many thousands of times A fatigue or alternating load 疲劳载荷或者交变载荷 is a load whose magnitude 大小 and sign 方向符号 are changed with time It has been noted above 上面已经提到 that external force applied to a body in equilibrium is reacted by internal forces set up within the material If therefore a bar 杆 is subjected to a uniform 均匀的 tension 拉伸 or compression 压缩 i e 也就是 a force which is uniformly applied across the cross section 横截面 then the internal forces set up are also distributed uniformly 分布均匀地 and the bar is said to be subjected to a uniform normal stress 正应力 the stress being defined as A P area load stress Stress may thus be compressive 压缩的 or tensile 拉伸 的 depending on the nature of the load and will be measured in units 单位 of newtons per square meter N m2 or multiples 倍数 of this If a bar is subjected to an axial load 轴向载荷 and hence 因此 a stress the bar will change in length If the bar has an original length L and changes in length by an amount L the strain produced is defined as follows Strain is thus a measure of the deformation of the material and is non dimensional 无量纲的 i e it has no units it is simply a ratio 比值 of two quantities with the same unit Since in practice the extensions 扩张 of materials under load are very small it is often convenient to measure the strains in the form of strain 10 6 i e microstrain 微观应 变 when the symbol 符号 used becomes Tensile stresses 拉应力 and strains are considered positive 正的 in sense 从某种意义上说 Compressive stress 压应力 and strains are considered negative 负的 L L lengthoriginal lengthinchange strain in sense Thus a negative strain produces a decrease in length A material is said to be elastic 有弹性的 if it returns to its original unloaded dimensions 尺寸 when load is removed A particular form of elasticity 弹性 which applies to a large range of engineering materials at least over part of their load range 负载范围 produces deformations which are proportional to 与 成比例 the loads producing them Since loads are proportional to the stresses they produce and deformations are proportional to the strains this also implies 意味着 that whilst 当 的时候 materials are elastic stress in proportional to strain Hooke s law 胡克定律 therefore states that This law is obeyed 服从 whithin certain limits 在特定的 范围 by most ferrous alloys 铁合金铁合金 and it can even be assumed 假定 to apply to other engineering materials such as concrete 混凝土 timber 木材 and non ferrous alloys 非铁合金 with reasonable accuracy 准确性 Whilst a material is elastic the deformation produced by any load will be completely removed when the load is strainstress removed there is no permanent deformation Within the elastic limits of materials i e within the limits in which Hooke s law applies it has been shown that This constant 常数 is given the symbol 符号 E and termed the modulus of elasticity 弹性模量 or Young s modulus 杨氏模量 Thus E is generally assumed to be the same in tension or compression and for most engineering materials has a high numerical value 数值 Typically E 200 109N m2 for steel so it will be observed 注意到 from equation 方程 式 that strains are normally very small In most common engineering appli