Design is seldom that simple. Usually（通常）, on the basis of（根据）experience, the designer selects a trial（试算） member and then does an analysis to see if that member meets the specified requirements. Frequently（常常）, it does not and then a new trial member is selected and the analysis repeated. This design cycle（设计周期） continues until a satisfactory solution is obtained. The number of cycles（循环次数） required to find an acceptable design diminishes as the designer gains experience.
            正如你已经从提到的各种各样的材料、力和形状所看到的，工程材料力学对所有的工程领域都有价值。工程师利用材料力学的原理来确定是否该材料的性能和构件尺寸足以保证它能安全地承受荷载且没有过多的变形。通常，我们关心的是构件能承受的安全荷载及其相应的变形。如果设计者能通过考虑荷载和材料的力学性能，并利用公式得到合适的构件尺寸，那么工程设计将是一个简单的过程。然而设计很少那么简单。通常，根据经验，设计者选择一个试算构件，然后进行分析，看它是否满足规定的要求。它常常不会满足要求，则再选择一个新的试算构件，再进行分析。这样的设计不断重复，直至得到一个满意的结果。当设计师拥有一定的经验后，为得到一个可接受的设计所需要的循环次数会减少。
      Design of Axially Loaded Members 轴向力构件的设计
           To give you some insight into （使..有一些了解）the design cycle, an extremely simple member will be dealt with first. That member is a prismatic bar with a force, P, acting along its longitudinal axis in the direction（纵轴向）such that it tends to elongate the bar. Such a force is referred to as（称为）an axial tensile load（轴向拉力）, and we can readily imagine it trying to（努力..）pull the fibers apart and to cause failure on a transverse plane（横向平面）. It is safe to assume that all fibers of the bar, in regions remote from（远离）the point of application of the load, are being pulled apart with the same load intensity（荷载强度）. With this assumption, the load intensity or stress is uniform on a transverse plane and is given by  
            为了使你对设计周期有一些了解，首先研究一个非常简单的构件。构件是个棱形的杆件，其上沿着它的纵轴向作用一个力P，这样往往使杆件在该方向上伸长。这样的力称为轴向拉力，我们能容易地想象它在努力地将纤维拉开，导致横向平面的破坏。安全地假定杆件的所有纤维在远离荷载施加点的区域以相同的荷载强度被拉开。在此假定下，荷载强度或应力在横向平面上是均匀的，为 

when P is in（以..为单位）Newtons and A is in square metres, stress,s ,is in Newtons per square metre (N/m2), which is by definition（根据定义）Pascals (Pa).
当P的单位为牛顿、A的单位为平方米时，应力σ的单位为牛顿每平方米（N/m2），根据定义为帕斯卡（Pa）。
            For a given axial load and given dimensions, the stress can be calculated from (4-1) and compared with（与..相比）the stress that can be safely carried by the material. The safe stress, known as（称为）the design stress or allowable stress（许用应力）, is determined by tests performed on material made to（按照） the same specifications as the part being considered. A safety factor（安全系数）, frequently imposed by a legally established code（法规）, is applied to the strength, as determined by tests, to give the allowable stress. The allowable stress, sa , is given by
            对已知的轴向力和（构件）尺寸，可根据公式（4-1）计算出应力，并与材料能安全承受的应力作比较。安全应力，称为设计应力或许用应力，它是通过对材料的试验来确定的，该（试验）材料按照与所考虑（验算）的杆件相同的规范制作。根据法规规定，通常对试验所确定的强度考虑安全系数后得到许用应力。许用应力 sa 为
where sf is the stress at which the material fails (failure to be defined later) and n is the safety factor.
这里，sf  为材料失效（失效在下文有定义）时的应力，而n为安全系数。
            It might at first（起先）seem that the designer would always dimension（选定..的尺寸）the cross section（横截面） so that the stress would exactly equal the allowable stress. However, it may be very costly to produce parts that have nonstandard sizes, so it is usually more economical to waste some material by selecting the next（接近的）larger standard size above that required by the allowable stress. Departure from（背离）standard sizes is justified（合理的） in cases where the penalty（不利后果）for excess weight is very severe, as in aircraft（航天器）or space-ship（宇宙飞船）design.
        起先似乎设计者总是在选定横截面的尺寸，以使应力恰好等于许用应力。但是，生产非标准尺寸部件的成本可能很高，因此，通常人们会选择比按许用应力要求的尺寸大一些的标准尺寸部件，这样尽管浪费了一些材料，但总体上更经济。但不选择标准尺寸的做法在诸如航天器和宇宙飞船的设计中证明是合理的，因为多余重量产生的不利后果是很严重的。