A Shakedown‐Constrained Critical‐Plane Model for Multiaxial Fatigue‐Life Prediction Under Tension–Torsion Loading With Independent Walker Factors

ABSTRACT Accurate fatigue‐life prediction under multiaxial tension–torsion loading remains challenging when mean normal stress, mean shear stress, and nonproportional loading act simultaneously. This study proposes a shakedown‐constrained critical‐plane model in which the admissible stress‐based regime is defined by an elastic shakedown boundary, whereas the fatigue damage parameter is formulated on the maximum‐shear critical plane. Independent Walker factors are introduced for the normal and shear components to describe their direction‐specific mean‐stress sensitivities. The model is calibrated using baseline loading paths and validated against multiaxial fatigue‐limit and finite‐life datasets covering 20 metallic material conditions under proportional and nonproportional loading, with bootstrap‐based sensitivity analysis characterizing parameter identifiability. Compared with established criteria, the proposed model reduces prediction scatter for the evaluated datasets, particularly for loading paths involving complex mean–stress interactions, whereas its validity is explicitly restricted to smooth cylindrical specimens under tension–torsion loading within the elastic shakedown regime.