Influence of particle and short-fibre reinforcement on the very high cycle fatigue behaviour of aluminium matrix composites

The fatigue behaviour of aluminium matrix composites (Al-MMCs) reinforced by alumina particles (15 vol% Al2O3) or short-fibres (20 vol% Saffil), respectively, was investigated in the very high cycle fatigue (VHCF) regime at stress amplitudes ≤140 MPa to reach fatigue lives of up to 1010 cycles using an ultrasonic fatigue testing device. Fatigue tests were complemented by in situ thermographic 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. Results obtained in the VHCF-region fit well with results obtained in LCF/HCF-region. A significant influence of test frequency was not found. Surprisingly, the reinforced Al-MMCs are capable to bear up to 109 cycles, although damage starts early during cyclic loading. In addition, although in the LCF/HCF regime the fatigue life and fatigue strength of the short-fibre reinforced Al-MMCs are significantly smaller compared to particle reinforced and unreinforced material, no significant differences to the particle-reinforced and unreinforced Al-MMC were observed at stresses below 140 MPa. Moreover, at stresses below 80 MPa not fatigue failure occurred at all up to 1010 cycles.