Effect of periodic overloads in particle reinforced aluminum alloys: the near threshold behavior

The influence of periodic overloads on the fatigue crack propagation behavior in the near threshold regime of physically (extrinsically) short cracks and long cracks is investigated. The overload ratio and the number of base amplitudes were 1.8 and 1000 cycles, respectively. A 20 vol.% SiC particle reinforced 359 cast aluminum alloy and a 17 vol.% SiC particle reinforced 2124 aluminum alloy are examined. In the Paris regime, the two alloys exhibit a different behavior. In the 2124 reinforced alloy, the overloads induce a reduction of the mean crack propagation rate which is typical for ductile metals. On the other hand, the mean crack propagation rate increases in the 359 reinforced cast alloy which is caused by the generation of micro-cracks during the overload, mainly due to particle fracture. In the near threshold regime, the behavior of both alloys is very similar. The effective threshold is not affected by the overload, i.e. if the base amplitudes are smaller than the effective threshold they do not contribute to crack propagation. The initial crack propagation rate of closure free pre-cracks is not influenced by the interaction of the base amplitudes and the overloads. After a relatively short extension of this extrinsically short crack, the periodic overloads begin to dominate the propagation behavior especially for base amplitudes smaller than the long crack threshold of the stress intensity range.