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

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 Kmax test condition. Previous research had indicated that high R (> 0.7) and constant Kmax test conditions near threshold were suspected to be crack-closure free and that any differences were attributed to Kmax 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, ΔKeff (= U ΔK) against rate, were calculated. These results suggest that the ΔKeff 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 Kmax tests, roughness and fretting debris are suspected to cause crack closure above the minimum load.