Influence of Reinforcement Geometry on the Very High-Cycle Fatigue Behavior of Aluminium-Matrix-Composites

Abstract

During technical operation, high performance materials are partially exposed to high frequency cyclic loading conditions. Furthermore, the small strains in the very high cycle fatigue (VHCF)-regime lead to accumulative damage which causes crack initiation related to an appropriate local deformation leading to final fatal fracture. At the same time, quite high requirements with regard to high number of cycles without any damage are demanded for many applications. Fields of application of these light-weight, but expensive materials, are e.g. in the automobile industry (e.g. engine blocks, cylinder heads, brakes).The fatigue behavior of Al-matrix composites (Al-MMCs) reinforced by alumina particles (15 vol.% Al2O3) or short fibers (20 vol.% Saffil), respectively, was already intensively studied in the LCF and HCF range. The present study is focusing on investigations in the very high cycle fatigue regime at stress amplitudes up to 140 MPa to reach fatigue life of about 1010 cycles. All experiments were carried out using an ultrasonic fatigue testing device under symmetric loading conditions (R=-1). Fatigue tests were accompanied by in situ thermography measurements to record the temperature of the whole specimen and to find “hot spots” indicating changes in microstructure and therefore the initiation or growth of cracks. Moreover, the resonant frequency as well as the damage parameter were evaluated to determine the beginning of damage. For a better understanding of the damage mechanism (matrix decohesion, matrix failure or failure of reinforcement) all fractured surfaces were investigated by scanning electron microscopy. The combination of these methods contributes to a better understanding of the underlying mechanism of damage in aluminum-matrix-composites.