Fatigue-crack propagation characteristics of aluminum alloys in thick sections

Fatigue-crack propagation data have been obtained for a variety of aluminum alloys, tempers, and products in relatively thick sections of interest for large aircraft shapes. For the higher stress-intensity ranges, the alloys rate in about the same order with regard to resistance to fatigue crack propagation as with regard to plane-strain fractare toughness. However, for low stress intensity ranges (i.e. short cracks or low load ranges) the rate of fatigue-crack propagation was lowest for two alloys, 2020-T651 and 2014-T652. which have low plane-strain fracture toughness. The relative order for different specimen orientations was generally consistent with that based upon plane-strain fracture toughness. High humidity results in higher rates of fatigue-crack propagation, with the effect indicated to be largest for those alloys which are susceptible to stress corrosion cracking.