Crack closure under high load ratio and test conditions

Fatigue-crack-growth-rate tests on compact specimens were conducted on several materials (2024-T3, 2324-T39, 7050T7451, Inconel-718 and 4340 steel) at load ratios of 0.1 to 0.95, and for several test conditions. Test data were generated from threshold to near fracture using compression pre-cracking constant-amplitude (CPCA) or compression pre-cracking loadreduction (CPLR) test methods in the threshold regime; and constant-amplitude loading at higher rates. Remote back-face strain (BFS) gages were used to monitor crack growth. BFS and local strain gages placed along the crack path were both used to measure crack-opening loads. Previous research had indicated that high load ratio and tests produced crack-closure-free conditions. However, measured crack-closure behavior under high load ratios and test conditions was attributed to residualplastic deformations, crack-surface roughness and/or fretting-debris. From local crack-opening load measurements, the effective stress-intensity-factor range appeared to be uniquely related to crack-growth rate from threshold to higher rate conditions.