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Crack opening loads were determined in load-shedding fatigue crack growth threshold tests on 2024-T3 aluminum alloy at stress ratios R of −2, −1, 0, 0.33, 0.5, and 0.7. The effects of load-shedding procedure and rate were investigated. Values of threshold ΔK were found to vary significantly with R, whereas values of threshold effective ΔK did not. That is, the variation of threshold ΔK with R was almost completely explained by accounting for the measured variation in crack opening load behavior with R. The load-shedding guidelines of ASTM Test Method for Measurement of Fatigue Crack Growth (E 647) produced a threshold ΔK value for R = 0.7 that was in agreement with the value determined using a procedure that should minimize closure effects (constant-Kmax, increasing-Kmin test). At both R = 0 and R = 0.7, high load-shedding rates produced high values of threshold ΔK caused by large closure effects.
fatigue (materials), crack propagation, stress ratio, crack closure, thresholds, aluminum alloys
Research engineer, NASA-Langley Research Center, Hampton, VA