Crack-closure behavior of 2324-T39 aluminum alloy near-threshold conditions for high load ratio and constant Kmax tests

Abstract Fatigue-crack-growth rate tests were conducted on compact specimens made of 2324-T39 aluminum alloy to study the behavior over a wide range in load ratios (0.1 ⩽ R ⩽ 0.95) and a constant K max test condition. Previous research had indicated that high R (> 0.7) and constant K max test conditions near threshold were suspected to be crack-closure free and that any differences were attributed to K max effects. During the tests, strain gages were placed near and ahead of the crack tip to measure crack-opening loads from local strain records on all tests, except R = 0.95. In addition, a back-face strain gage was used to monitor crack lengths and also to measure crack-opening loads from remote strain records. From local gages, significant amounts of crack closure were measured at the high- R conditions and crack-opening loads were increasing as the threshold condition was approached. Crack-closure-free data, Δ K eff (= U Δ K ) against rate, were calculated. These results suggest that the Δ K eff against rate relation may be nearly a unique function over a wide range of R even in the threshold regime, if crack-opening loads were measured from local strain gages and not from remote gages. At low R , all three major shielding mechanisms (plasticity, roughness, and fretting debris) are suspected to cause crack closure. But at high R and K max tests, roughness and fretting debris are suspected to cause crack closure above the minimum load.