Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Crack closure was measured in middle crack tension specimens with a crack-tip opening displacement gage located one-half of the specimen thickness behind the crack tip. Five overload ratios from 1.23 to 1.5 were investigated. The stress ratio was 0.7. Closure measurements indicated no closure occurred before or throughout the overload plastic zone following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode case.