stress and strain 应力与应变
In any engineering structure工程结构 the individual components独立构件 or members will (be subjected to)受到,被附加 external forces外力 arising from由。。引起 the service conditions工作环境 or environment in which the component works.
在任何的工程结构中,每个零件,构件将会受到来自工作环境的外力的作用。
If the component or member is in equilibrium平衡, the resultant合力 of the external forces外力 will be zero but, nevertheless不然的话, they together place a load on the member which tends to deform使变形 that member and which must be reacted作用 by internal forces内力 set up within the material由材料提供.
如果这构件是处于平衡状态的话,外力的合力是为零。但是他们必须由材料提供的内力来平衡。++
There are a number of different ways in which load负载 can be applied作用to a member. Loads may be classified分类 with respect to time:
负载作用于构件有许多的形式。相对于时间可以将负载分类为: (a) A static静态 load负载 is a gradually逐渐 applied作
用 load for which equilibrium is reached in a relatively相对 short time.
静态负载是逐渐作用的可以在相对较短的时间内达到平衡的负载。
(b)A sustained load持续载荷 is a load that is constant维持不变 over a long period of很 time, such as the weight of a structure构件重力. This type of load is treated处理,对待 in the same manner as a static load静态载荷; However for some materials对一些材料, and conditions of temperature and stress, the resistance抵抗力 to failure may be different under short time loading and under long time loading.
持续载荷是可以维持很长时间不变的载荷。比如构件的重力。这种类型的载荷和静态载荷是同样的处理方法。但是对于某种材料,在某些特定的温度和外部压力的作用下,构件抵抗失效的能力是不同的。
(c) An impact冲击 load is a rapidly很快的迅速的 applied load (an energy load). Vibration震动 normally results from an impact load, and equilibrium平衡 is not established确定 until the vibration振动 is eliminated消除, usually by natural damping forces阻尼力.
冲击载荷是一种快速作用的载荷。振动常常是有冲击载荷引起。在振动被消除之前,平衡是很难稳定的。而消除振动的一半都是靠阻力。
(d) A repeated load重复载荷 is a load that is applied作用
and removed移动 many thousands of times.
持续作用很多次,移动很多次的载荷称为持续载荷。
(e) A fatigue or alternating load交变载荷 is a load whose magnitude and sign are changed with time.
狡辩载荷是大小和符号都在不断变化的载荷。
It has been noted above that external force applied to a body in equilibrium is reacted by internal forces set up within the material.
上述的外力作用于物体时将会被材料提供的内力抵消而达到平衡。
If, therefore因此, a bar棒子,零件 is subjected to作用 a uniform统一的,制服 tension拉伸 or compression压缩, i.e. 也就是说a force, which is uniformly applied across the cross-section截面, then the internal forces set up are also distributed分布 uniformly and the bar is said to be subjected to a uniform normal stress正应力, the stress being defined定义 as:
因此,如果零件受到统一的拉伸或者压缩,也就是说,力均匀的分布在截面上,此时内力也同样是均匀分布,此时我们就说哦这个构件受到了正应力的作用。这种应力定义为:
stress=press/area=P/A
Stress б may thus be compressive or tensile depending on
the nature of the load and will be measured衡量 in units单位 of newtons牛顿 per square meter每平方米 (N/m2)or multiples倍数 of this.
这个力是压力还是拉力由构件所受的载荷确定。同时,此力可以用牛顿每平方米或者它的倍数来衡量。
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 δL, the strain应变 produced产生的 is defined as follows:
如果构件受到轴向的载荷,此时构件的长度会发生变化。如果构件的本身长度是L,改变了&L则,产生的应变定义为:
strain=chang in lengyh/original length=&l/l
Strain is thus a measure度量 of the deformation变形 of the material and is non-dimensional无量纲的, i.e. it has no unit没有单位; it is simply a ratio比例,比率,比值 of two quantities数量,数值 with the same unit.
因此,应变是材料形变的度量,而且它是无量纲的。也就是说它没有单位。它只是两个有相同的单位的数值之间的比值。
Tensile stress张应力 and strain拉应力 are considered被认为是 positive正的 in sense在某种程度. Compressive压应力 stress and
strain are considered negative负的 in sense. Thus a negative strain produces a decrease in length.
张应力和拉应力在被定义为正的,而压应力被认为是负的。因此负的压应力造成长度的减少。
A material is said to be elastic有弹性的 if it returns to its original, unloaded dimensions大小 when load is removed.
如果材料在外力卸去的时候长度回复到了受力前的正常情况,我们就说这种材料有弹性。
A particular特别的 form形式 of elasticity弹性 which applies to a large range of engineering material工程材料, at least over part of their load range负载范围, produces deformations变形 which are proportional to成比例的 the loads producing them.
很多材料都有这种特性,至少在一定范围内,变形与负荷是成正比的。
Since loads are proportional to成比例 the stress they produce and deformations are proportional to the strains, this also implies意味着 that, whilst material are elastic, stress is proportional to strain. Hooke’s law胡克定律 therefore states that
负载可以产生成比例的应力,而应力又可以产生成比例的形变。也就意味着材料有弹性。用胡克定律表示
This law is obeyed服从 within certain limits by most ferrous alloys铁基合金 and it can even be assumed理论上 to apply to