Cyclic stress-strain and fatigue behaviour of particulate-reinforced Al-matrix composites

The low-cycle fatigue behaviour of the Al-composite AA6061 reinforced with Al2O3-particles of different volume fractions f(p) (15 and 20vol.%) has been investigated. Cyclic deformation tests were carried out on material in T6-condition at temperatures between T = -30 degrees C and T = 150 degrees C in total-strain control mode. Both cyclic hardening and cyclic softening were observed, dependent on the strain amplitude and test temperature, respectively Cyclic hardening was obtained for all total-strain ranges Delta epsilon(t) (Delta epsilon(t) = 0.004 up to 0.02) at temperatures below and at room temperature. At higher temperatures, cyclic hardening was only observed for Delta epsilon(t) greater than or equal to 0.014. The composite with the higher volume fraction of reinforcement phase exhibited higher cyclic strengths but lower fatigue lives. The number of cycles to failure, N-f, could be related to the applied strain amplitude by the laws of Manson-Coffin and Basquin. In the evaluation of the plastic-strain hysteresis loops, the observed non-linear elastic behaviour was taken into account by extension of the linear Hooke's law by a second-order term. The variation of non-linearity during fatigue was found to be a sensitive indicator of damage.