Quantitative analysis and optimization of residual stress fields near fastener holes under combined factors

Residual stress surrounding the expanded hole, generated by cold expansion process, plays a pivotal role in enhancing fatigue life but is susceptible to multiple factors. Accurate comprehension of the residual stress field is the essential prerequisite for addressing the fatigue failure problem of fastener holes. In this study, 2D-Digital Image Correlation (DIC) and X-ray Diffraction (XRD) were employed to measure the residual strain and stress of hole plate subjected to split sleeve cold expansion (SSCE) under varying conditions. A 3D finite element (FE) model for tapered mandrel expansion was developed and validated against experimental results. A novel index, termed the effective residual stress, was proposed to quantitatively evaluate the residual stress field. Furthermore, the distribution characteristics of residual stress field were predicted and optimized using machine learning (ML) algorithm, considering the combined effect of the degree of cold expansion (DE), thickness of the plate, and edge distance ratio (EDR). Results indicate a nonlinear relationship between the effective residual stress and both thickness and EDR, and the influence of EDR is greater than thickness. For plates with varying thickness and EDR, the optimal DE varies from 3.5 % to 5.0 %. Specifically, when the EDR is below 3.5 and the thickness is below 10 mm, the optimal DE falls within the range of 3.5 % to 4 %.