Indentation fatigue behaviour of polycrystalline copper

Conventional indentation experiments have been widely used to extract the mechanical properties of materials from a load-depth curve. However, most of them are focused on static loading conditions. In the present study, the detailed indentation fatigue behaviour of polycrystalline copper under cyclic loading was studied experimentally. The experimental work emphasized indentation depth propagation behaviour such as the influence of overloading and underloading. It was shown that an increase in the maximum load can accelerate indentation depth propagation, while a decrease in the maximum load can retard indentation depth propagation. Further experiments showed that a sudden increase in maximum load after achieving a steady state followed by cycling at normal loading conditions can also delay indentation depth propagation, while a sudden drop in maximum load had a contrary effect. Those experimental phenomena implied that there were some similarities in the behaviour of indentation fatigue depth propagation and conventional fatigue crack propagation. In the following analysis, optical microscopy (OM) and scanning electron microscopy (SEM) were used to investigate the microstructures of the indentation cross-sections. The results revealed that the nucleation and accumulation of cavities to develop cracks was promised to be the main damage mechanism during the indentation fatigue.