Crack closure under high load-ratio conditions for Inconel-718 near threshold behavior

Fatigue-crack-growth (FCG) rate tests were conducted on compact specimens made of an Inconel-718 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 conditions were suspected to be crack-closure-free and that any differences were attributed to Kmax effects. During a test at a load ratio of 0.7, strain gages were placed near and ahead of the crack tip to measure crack-opening loads from local load-strain records during crack growth. In addition, a back-face strain (BFS) gage was also used to monitor crack lengths and to measure crack-opening loads from remote load-strain records during the same test. The BFS gage indicated that the crack was fully open (no crack closure), but the local load-strain records indicated significant amounts of crack closure. The crack-opening loads were increasing as the crack approached threshold conditions at R = 0.7. Based on these measurements, crack-closure-free FCG data (ΔKeff against rate) were calculated. The ΔKeff-rate data fell at lower ΔK values and higher rates than the constant Kmax test results. In addition, constant R tests at extremely high R (0.9 and 0.95) were also performed and compared with the constant Kmax test results. The constant R test results at 0.95 agreed well with the ΔKeff-rate data, while the R = 0.9 data agreed well with constant Kmax test data in the low-rate regime. These results imply that the R = 0.7 test had a significant amount of crack closure as the threshold was approached, while the R = 0.9 and Kmax test results may have had a small amount of crack closure, and may not be closure free, as originally suspected. Under the high load-ratio conditions (R ⩾ 0.7), it is suspected that the crack surfaces are developing debris-induced crack closure from contacting surfaces, which corresponded to darkening of the fatigue surfaces in the near-threshold regime. Tests at low R also showed darkening of the fatigue surfaces only in the near-threshold regime. These results suggest that the ΔKeff against rate relation may be nearly a unique function over a wide range of R in the threshold regime.