Fatigue behaviour of geometric features subjected to laser shock peening

The current paper reports selected findings from a major collaborative research programme conducted in the United Kingdom into the fundamental understanding of laser shock peening, when applied to key airframe and aero-engine alloys as a means of controlling fatigue performance. It is recognised that relatively deep compressive residual stresses are generated into the surface of a material subjected to LSP which can subsequently resist fatigue crack initiation from that surface. However, it is clear that balancing tensile stress will evolve at some location within the material in order to satisfy equilibrium and under certain circumstances this can lead to sub-surface fatigue initiation. The distribution of the RS field will be highly dependent on LSP process parameters, the structural form and the precise mode and raster pattern of pulse application. Finite element models will be described that predict the RS fields associated with LSP applied to the aerospace grade aluminium alloys Al 2024 and Al 7010 using an eigenstrain approach. Validation of these models is provided by laboratory fatigue experiments on test coupons containing different LSP patch geometries (full width or centre constrained), supplementary stress raising features (drilled holes) and different through thickness dimensions. Particular attention is paid to modelling the application of LSP at edge locations in terms of the angle of incidence and laser pulse sequencing. Interactions between the LSP process and geometric features was found to be key to understanding the location for fatigue crack initiation. Particularly relevant for engineering application, was the fact that not all instances of LSP application provided an improvement in cyclic fatigue life.