Competition between plasticity-controlled and crack-growth controlled failure in static and cyclic fatigue of thermoplastic polymer systems

Creep rupture (plasticity controlled failure) and slow crack growth are two important failure mechanisms that limit the lifetime of polymer constructions under load. Since both require different approaches to predict lifetime or improve the materials performance, the identification of the active mechanism is essential. Problems arise when the macroscopic failure mode is identical (i.e. brittle) in both cases (e.g. in composite systems). In this study, it is shown that both mechanisms can be distinguished effectively by comparing lifetimes in static and cyclic fatigue. At equal value of the maximum load, plasticity-controlled failure is postponed in cyclic loading, whereas crack propagation is significantly accelerated. The origin of this specific response is discussed, and its generic character demonstrated for a great variety of engineering polymers.