On the growth of cracks at the fatigue threshold following compression overloads: Role of load ratio

Studies have been performed on the effect of single compression overload cycles on the role of crack closure during near-threshold fatigue crack growth in ingot metallurgy aluminum alloy 7150-T7, specifically with respect to the role of mean stress or load ratio R (= K min /K max ). It was found that large compression or simple unloading cycles, applied to cracks arrested at the fatigue threshold at R = 0.75, had no detectable effect on crack closure or crack growth behavior. This is in contrast with previous studies at R = 0.10 where arrested threshold cracks subjected to such compression cycles (of magnitude five times the maximum tensile load) were seen to recommence growth immediately, consistent with a measured reduction in closure. Such results are discussed in terms of the major origins of crack closure in this alloy.