The effect of pitting corrosion on split sleeve cold hole expanded, bare 7075-T651 aluminium alloy

As part of an aircraft fleet fatigue life improvement program, investigation has been carried out into the effect pitting corrosion has on bare 7075-T651 which had undergone split sleeve cold hole expansion. Constant amplitude sinusoidal loading was applied to fatigue test coupons which had pitting corrosion induced upon them by a modified cyclic immersion process using a 3.5% NaCl solution. A pit depth of 39–58 μm was found to significantly reduce the increased fatigue life gains achieved by carrying out cold hole expansion. At 137.9 MPa fatigue life was reduced from achieving run out of ten million cycles to an average 371 × 103 cycles, while at 165.5 MPa average fatigue life was reduced from 810 × 103 to 65 × 103 cycles. The fracture surfaces were analysed under a scanning electron microscope where each displayed an individual crack initiation site located on the material surface within the zone of residual circumferential stress.