•Proposed non-linear fatigue damage accumulation model for fatigue life prediction under multiple load levels.•The isodamage curves converge into one point due to the exhaustion of static toughness.•The proposed model yields better life predictions than the other four models for the five materials. Cumulative fatigue damage is a continuous and irreversible process that refers to the degradation of the component mechanical properties under cyclic loadings. In this work, using the concept of isodamage curves, a new non-linear fatigue damage accumulation model is proposed to account for fatigue damage evolution under multiple load levels. In particular, a computational process is established in which the isodamage curves converge into one point due to the exhaustion of static toughness. Experimental data of five materials are used for model validation and comparison. Results indicate that the proposed model provides better life predictions than the other four models.