An enhanced nonlinear fatigue cumulative damage model based on toughness exhaustion and strength degradation

• Proposed a fatigue model based on material toughness and residual strength as dual state parameters. • Developed a logarithmic equation for residual strength degradation. • Designed a load ratio index for multi-stage load interaction effects. • Damage evolution modeled via state parameters linked to intrinsic material properties. Accurate prediction of fatigue life under variable amplitude (VA) loading remains fundamentally challenged by load-sequence-dependent damage accumulation in metallic structures. This study establishes a nonlinear cumulative damage model integrating toughness exhaustion and strength degradation mechanisms through damage equivalence principles. Multi-level VA experiments on carbon steel, alloy steel, and aerospace aluminum alloys demonstrate that the proposed model reduces life prediction errors to 6.53% (5-level) and 9.43% (8-level), achieving 93.94% and 94.79% accuracy improvement versus the Palmgren-Miner method. Statistical evaluations reveal dominant control of two key factors: material ultimate tensile strength (UTS) and stress amplitude differential between successive cycles. These advancements provide validated engineering tools for durability assessment of aero-engine components and military vehicle systems experiencing complex mission profiles.