Allowable working stress is taken as a proportion of the yield or proof stress; that is, the component is only stressed within its elastic range when in service. For example, consider the screwed fastening shown in Fig. 1.I. When the nut is tightened normally the bolt is stretched slightly and, providing it is stressed within its elastic range, it will behave like a very powerful spring and will pull the joint faces together very firmly. 上投代 Figl.1 Screwed FasteningAllowable working stress is taken as a proportion of the yield or proof stress；that is，the component is only stressed within its elastic range when in service．For example, consider the screwed fastening shown in Fig. 1.l. When the nut is tightened normally the bolt is stretched slightly and，providing it is stressed within its elastic range，it will behave like a very powerful spring and will pull the joint faces together very firmly．