Effect on Fatigue Performance of Residual Stress induced via Laser Shock Peening in Mechanically Damaged 2024‐T351 Aluminium Sheet

摘要

During manufacture and maintenance the fuselage skin of aircraft are susceptible todamage in the form of scratches. Normally not considered to be of major concern toaircraft structural integrity some airlines discovered fatigue cracks had initiated at theroot of scratches. Crack propagation was in the through thickness direction and if leftuntreated could cause rapid decompression of the passenger cabin. Standard repairmethodology requires patches be riveted around scratch damage and in extremecases could require entire replacement of affected skin panels.Laser shock peening (LSP) is an emerging surface treatment that has been shown toimprove fatigue performance of safety critical components by inducing a surface layerof compressive residual stress. In this work LSP was applied along the scratch damagein an effort to restore pristine fatigue performance. The aim of the project was tomodel the effect on fatigue crack growth rate of residual stress fields induced via LSPand to validate predictions by comparison to experimental test results.The scratches were recreated under controlled laboratory conditions using a diamondtipped tool. This process allowed creation of reproducible V shaped scribes tocontrolled depth, wall angle and root radius. Scribes of depth 50 and 150 μm withroot radius 5 μm were created in dogbone shaped samples of 2 mm thick 2024‐T351clad aluminium. Samples were tested in fatigue at an R = 0.1 and maximum stress of200 MPa. The scribe damage reduced fatigue life compared to the pristine materialby a factor of 22. Scribed samples were processed using LSP treatment from differentproviders that created known residual stress fields in the material. The fatigue life ofscribed samples after peening varied from a further decrease to a 13 times increasedependent on the residual stress field induced.An elastic‐plastic crack closure based finite element model was created to determinethe effect on stress intensity factor and stress ratio of residual stress. Fatigue livescalculated were within a factor of 2 of experimental lives. It was predicted that crackclosure was present during up to 80% of the applied load cycle due to thecompressive residual stress field. However plasticity induced crack closure actuallyreduced after peening because the compressive residual stress field induced a smallerplastic zone at the crack tip and hence reduced the plastic wake.A residual stress based fatigue life sensitivity study was performed to optimise theprofile of the residual stress field for improved fatigue performance. The requiredprofile was created in test samples using LSP. The fatigue life of peened samplesincreased by a factor of up to 15 however pristine life was not fully recovered. Arestriction imposed by the industrial application was peening applied to one face only. This created an unbalanced stress field that resulted in sample distortion tomaintain equilibrium. The distortion induced out of plane bending stresses duringtesting and caused premature crack initiation on the unpeened face. However usinginterrupted fatigue tests it was found that although crack initiation also occurred atthe root of the scribes the cracks were arrested after 24 μm of propagation. This wasconsistent with the findings of the crack growth prediction model.