The yield stress for a given number of cycles is termed the fatigue strength. The fatigue life of a part is the number of cycles to failure at a given fluctuating load. The most critical choice for tests is between load controlled or displacement controlled cyclic loading. Other test variables are temperature, mean stress, amplitude of fluctuation and frequency. Elevated temperatures hasten failure.
Higher frequencies shorten the test time.
Hence the test frequency should be as high as possible without having significant heating due to viscous dissipation in the polymeric material. Such heating is less of a concern with fiber filled plastics than with neat plastics. The breaking stress is lowered as the number of cycles is increased. This means that semi-crystalline polymers perform better with respect to fatigue.