Fatigue damage mechanism and life prediction of cold expanded holes

Abstract

Nickel-based superalloy hole structures in aerospace components are prone to premature fatigue failure induced by stress concentration and machining defects, posing significant risks to pressure-bearing critical structures. The cold expansion process (CEP) serves as an effective surface strengthening technique to enhance the fatigue resistance of hole structures. However, the limited understanding of cold expansion strengthening mechanism and the absence of robust fatigue life prediction method constrain the safe operation of the critical components with cold expanded holes. This study presents a multi-stage convex-hull rotary CEP engineered for small hole specimens with diameters varying from 1.0 mm to 3.0 mm. Experiments reveal a threefold improvement in fatigue life when the temperature is below 600 °C and the stress level is less than 800 MPa, providing a strengthening efficacy boundary in terms of the loading conditions for the developed CEP. The microstructure analysis demonstrates that the gradient nanocrystals and compressive residual stress formed on hole root improve fatigue life at lower stress and temperature conditions. Notably, the temperature elevation to 700 °C triggers nanocrystal coarsening and δ-phase stacking fault formation, leading to accelerated fatigue degradation. To predict the fatigue life of expanded hole structures, a data-physics hybrid-driven framework integrating dual-scale modeling with machine learning was developed, achieving both high prediction accuracy and computational efficiency compared with conventional methods. All results were confined within a ±3.0 error band based on the developed method, and the computational speed was improved by approximately four orders of magnitude. This work advances an anti-damage manufacturing technique of hole structures and provides an engineering-oriented life assessment method for surface strengthening components.