Fatigue cracks grown in compact tension specimens of 2024-T851 aluminum at ΔK = 6.93 MN/m^3/2 (R =0.1) were subjected to single-peak overloads of 17.31 MN/m^3/2. After the overload, the specimens were held at zero load for periods of 3 min, 1 h, and 20 or more hours before recycling at ΔK = 6.93 MN/m^3/2. Increasing the rest time had the effect of slightly reducing the crack retardation as measured on the specimen surface.

The load at which the fatigue crack faces fully separate was measured on the specimen surface by laser interferometric techniques and in the specimen interior by through-transmission ultrasonic methods. Crack opening loads measured on the specimen surface were found to increase with application of an overload and then decrease to the original value when the specimen was allowed to rest at zero load, while opening loads measured through the sample by the ultrasonic method did not vary significantly with peak loads or rest times. Varying specimen thickness between 0.64 and 2.54 cm had little effect on surface measured retardation or opening loads. The relationship between applied load and crack surface displacement as measured by ultrasonics varied significantly with specimen thickness.