Fatigue data obtained from standard specimens are helpfulto the designers as a guide. Fatigue data are dependent uponenvironmental conditions. Thus, in design calculations,proper consideration must be given to these conditions andalso to the effect of stress concentrations. Realistic actualor simulated end-use testing of a part in service is the bestmethod of evaluating material performance for a specificapplication.Fatigue data for plastics can be obtained by using a Sonntag-Universal machine at constant stress levels. In these tests,a stress is applied repeatedly at 1800 cycles per minuteto a test specimen until failure occurs. Specimens may bestressed in tension only, compression only or in both tensionand compression, which is generally considered the mostsevere situation. In addition, fixtures can be used with thismachine for producing flexure stresses. The picture inFigure 3.51 illustrates how the test specimen is placedin the apparatus for fatigue stressing.Fatigue endurance relates to the useful life expected for amaterial subjected to repeated loading. It is generallysensitivity ratherthan ability to withstand impact.