Volume 1, Issue 10 (November 2004)
Periodic Overloads in the Near Threshold Regime
The effect of periodic overloads on the crack propagation behavior in the near threshold regime of long cracks and physically (extrinsically) short cracks is investigated. A 20 Vol. % SiC particle reinforced 359 cast aluminum alloy and a 17 Vol. % SiC particle reinforced 2129 aluminum alloy are examined. In the Paris regime the two alloys exhibit a different behavior. In the 2124 reinforced alloy a reduction of the mean crack propagation rate, which is typical for ductile metals, is observed. On the other hand the mean crack propagation rate accelerates in the 359 reinforced cast alloy, which is caused by generation of micro-cracks during the overload.
In the near threshold regime and at smaller stress intensity ranges in the physical short crack regime the behavior of both alloys is very similar. The effective threshold is not affected by the overload, i.e., if the constant amplitudes are smaller than the effective threshold they do not contribute to crack propagation. If both, the constant amplitudes and the overload amplitudes contribute to crack propagation, the build up of crack closure is affected by the load history.