Experimental evaluation of fatigue crack initiation from corroded hemispherical notches in aerospace structural materials

A test program was developed and executed to evaluate the influence of corroded hemispherical notches on the fatigue crack initiation process in aluminium 7075‐T7351, 4340 steel and D6AC steel. Surface enhancements such as shot peening and laser shock peening were also incorporated as part of the test effort with the intent of assessing any improvements in fatigue performance. The aluminium specimens exhibited a relative insensitivity to the surface enhancements for crack initiation in pits ranging from 0.3 to 2.0 mm, and localized yielding was only a factor for smaller pits operated at an elevated load ratio. Residual stresses created by the surface enhancements as well as localized yielding improved the crack initiation behavior from the base of the pits for both the 4340 and D6AC steel specimens. This behaviour was evident at high and low load ratios. In particular, laser shock peening induced residual stresses produced a significant increase in the crack initiation stress even when localized yielding was not a factor.