Evaluation of the Potential of Laser Shock Peening Compared with Cold Expansion for Improving Fatigue Resistance of Riveted Lap Joints of Aerospace Grade 7175 Al Alloy

Abstract

Cold expansion is widely used in repairing and manufacturing rivet lap joints in the aerospace industry. However, it bears inherent disadvantages such as nonsymmetrical residual stress, considerable surface deformation, and tensile zones that may initiate cracks. Limited process flexibility makes it challenging to fully overcome these issues. Given its high penetration depth and flexible parameter adjustments, laser shock peening (LSP) may offer a promising alternative for treating rivet areas. To investigate the effects of LSP parameters on residual stresses, a systematic parameter study is conducted, involving extensive X-ray diffraction residual stress analyses and fatigue testing. Upon determining an optimal parameter matrix, fatigue tests are conducted on laser-peened, cold-expanded, and untreated samples. Subsequently, an energy dispersive synchrotron X-ray investigation is performed in order to understand the effect of LSP on residual stress evolution and the local material behavior in rivet lap joints. For this experiment, laser-peened, cold-expanded, and untreated samples are submitted to fatigue tests at defined cycles. The residual stress distribution around the rivet area is then analyzed for each sample to evaluate residual stress stability and changes post-loading. As a final step, fracture surface analyses are performed via optical and scanning electron microscopy.