As you can see already from the variety of materials, forces, and shapes mentioned, Mechanics of Engineering Materials is of interest to（对..有价值）all fields of engineering. The engineer uses the principles of Mechanics of Materials to determine if the material properties and the dimensions of a member are adequate to（足以）ensure that it can carry its loads safely and without excessive distortion. In general（通 常）, then, we are interested in both the safe load that a member can carry and the associated （ 相 关 的 ） deformation. Engineering design would be a simple process if the designer could take into consideration（考 虑） the loads and the mechanical properties of the materials, manipulate（利用）an equation, and arrive at（得到）suitable dimensions.
where f is the stress at which the material fails (failure to be defined later) and n is the safety factor. 这里，f 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.
