Measurement and Understanding of the Level and Effect of Residual Stresses Induced by the Laser Shock Peening Process

Abstract : This paper describes work undertaken to understand and quantify the benefits of laser shock peening (LSP) in improving the fatigue resistance of components when subject to high, near surface alternating stress fields. The process is known to introduce compressive residual stress fields down to depths of typically 1mm. The geometry of the component being treated is one factor that contributes to the magnitude and distribution of compressive residual stresses. In a simple geometry the measurement of the resulting residual stresses is relatively straightforward. However, practical application involves more complex geometries and the measurement of residual stress levels is more problematic. The first part of this paper describes how finite element modelling can be used to understand residual stress levels in a complex geometry following the application of LSP. In order to maintain equilibrium, sub-surface tensile stresses must accompany the near surface compressive stresses induced by the LSP process. Account must be taken of these tensile stresses in the design process. The second part of this paper describes some work undertaken to predict the effect of the full through-wall residual stress levels on crack initiation lives. A programme of notched 3-point bend specimen testing is reported and an assessment method proposed for predicting fatigue lives. Depending on the applied stress levels crack initiation can occur near-surface or sub-surface. The proposed model can be used to predict both the crack initiation location and the cycles to failure.