Experimental investigation and fatigue life prediction for 7475-T7351 aluminum alloy with and without shot peening-induced residual stresses

The effect of shot peening on small crack growth and on the fatigue life of 7475-T7351 aluminum alloy has been investigated. The experimental results show that cracks initiate at second phase particles in the alloy and that fatigue crack growth rates are greatly reduced after shot peening treatment. Stress intensity factors for small cracks subjected both to external loads and to shot peening-induced residual stresses have been determined using weight function methods. Small crack growth rates and fatigue lives of naturally occurring small cracks in shot peened and unpeened specimens were calculated using small crack theory and a crack closure model. The predicted results agree well with the experimental data. The study demonstrates that fatigue life extension by shot peening can be attributed to beneficial compressive residual stresses in the surface layer, that the principle of superposition is applicable to fracture mechanics, and that the effect of residual stress on crack growth can be quantitatively analyzed using weight function methods. It is shown that the total fatigue lives for materials/structures containing residual stresses can be predicted by the crack closure-based small crack theory, provided that residual stresses are properly taken into account.