When do small fatigue cracks propagate and when are they arrested?

Abstract

Abstract The formation of small and long cracks and their propagation or arrest are treated drawing special attention to the (a) impact of environment and (b) several loading parameters (R-ratio, stress/strain-amplitude, constant-amplitude and variable-amplitude loading, superimposed loading, ultrasonic fatigue loading, and frequency effects) for three groups of metallic materials (two high-strength steels, 7075 and 2024 Al alloys, polycrystalline copper). The influence of these parameters on lifetimes and fatigue crack propagation behavior being determined by microstructural features on all levels of magnification (ranging from several millimeters to nanometers) is presented. A review of the state of knowledge according to literature is given in the introduction. The following results were obtained, and models for their interpretation were presented: The development (growth or arrest) of small cracks into a long crack is driven by several competing processes (due to material, way of loading, environment, etc.). The environment plays a predominant role. Especially in the high-cycle and very-high cycle regime, the complexity of interacting processes needs further – mainly experimental – investigations. Some further studies relate to different loading conditions and possibilities of testing considering newly developed material. Experiments in high vacuum have to form the basis for studying environmentally assisted fatigue response.