Observations of fatigue crack growth rate and closure behavior in 7475-T731 aluminum under variable amplitude loading

Simultaneous measures of fatigue crack propagation rate and crack closure were taken from 7475-T731 aluminum alloy center cracked tensile specimens under variable amplitude loading conditions. Results indicate the presence of load interaction in a variety of repeated variable amplitude loading blocks. Load interaction results in retardation of the propagation rate for most loading blocks examined in the study. Closure did not explain observed delay behavior associated with high stress ratio and single peak overloads. Primary influences on the crack opening stress intensity level include the maximum stress intensity of the repeated loading block, the minimum stress intensity of the block, and the number of fatigue cycles between overloads of the block.