Crack-Closure Behavior of 7050 Aluminum Alloy near Threshold Conditions for Wide Range in Load Ratios and Constant K_max Tests

Fatigue-crack-growth rate tests were conducted on compact specimens made of a 7050-T7451 aluminum alloy to study the behavior over a range in load ratios (0.1≤R≤0.9) and constant K_max test conditions. Previous research had suggested that differences in the threshold regime at high load ratios were attributed to K_max effects. But recent measurements of crack-closure behavior under high R and constant K_max test conditions near threshold conditions on a variety of materials have indicated that these tests may not be crack-closure free as suspected. Strain gages were placed near and ahead of the crack tip to measure crack-opening loads from local strain records. In addition, a back-face strain (BFS) gage was used to monitor crack sizes and to measure crack-opening loads from remote strain records during the same tests. The 7050 alloy produced very rough crack-surface profiles. For R=0.1, the BFS and local gages indicated very similar high crack-opening loads. For R≥0.7 and K_max test results in the threshold regime, the BFS gages indicated lower crack-opening loads than the local gages. Based on local measurements, crack-closure-free fatigue-crack-growth data (ΔK_eff against rate) were calculated. These results indicated that the ΔK_eff against rate relation is nearly a unique function over a wide range of R values even in the threshold regime, if crack-opening loads were measured from local strain gages. At low R, all three major shielding mechanisms (plasticity, roughness, and fretting debris) are suspected to cause crack closure. But for high R and K_max tests, roughness and fretting debris are suspected to cause crack closure above the minimum load. A strip-yield model was also used to correlate the data over a wide range in load ratios and rates, but required a very low constraint factor (α=1.3), due to the high crack-opening loads.