A multiscale finite element approach to analyse the effect of shot peening-induced surface roughness on fretting fatigue crack initiation

• A multiscale FE framework is developed to assess the effect of roughness on SP-treated Al 7075-T651. • SP-induced residual stress is introduced as a baseline to isolate the sole effect of surface roughness. • Experimental validation confirms that roughness significantly improves the predicted fretting fatigue lifetime while enhancing prediction accuracy. Shot peening (SP) is a widely used surface treatment technique that enhances the mechanical performance of materials, notably improving fatigue resistance by inducing compressive residual stresses (RS) and modifying surface characteristics. While the overall benefits of SP on fatigue life are well-documented, the specific role of SP-induced surface roughness in fretting fatigue behaviour remains unclear. This study develops a multiscale framework based on finite element method (FEM) to isolate and quantify the influence of surface roughness resulting from SP on fretting fatigue. At the microscale, detailed FEM simulations are conducted to assess how surface topography affects local contact stresses, thereby extracting critical features brought by roughness. These features are then integrated into a macroscale model to construct a SP-treated model that accounts for the effects of surface roughness. Numerical results reveal that while surface roughness has a minimal effect on crack initiation location and orientation, it significantly affects crack initiation lifetime by redistributing contact stresses more favourably. Furthermore, incorporating experimentally measured roughness profiles into the numerical models enhances the accuracy of fatigue life predictions. These findings underscore the importance of considering surface roughness in numerical simulations to achieve reliable predictions of fretting fatigue behaviour in SP-treated materials.