Polar damage sum concept for constant amplitude proportional and nonproportional multiaxial fatigue analysis

•A new multiaxial fatigue damage concept is proposed.•Fatigue damage is defined as the area enclosed by the polar curve of damage parameter.•Fatemi-Socie and Smith-Watson-Topper models are examined.•Results from the polar damage sum concept are better than that of critical plane. This paper proposes a new concept for the evaluation of fatigue damage parameters. It is suggested that evaluation of fatigue damage may not necessarily be confined to a single plane, i.e., a critical plane. Rather, it is suggested here that the culmination of fatigue crack on a particular plane(s) is due to the contributions of damage on all planes. Mathematically, the polar damage sum concept is calculated as the area enclosed by the polar curve of the damage parameter. The proposed concept is examined using multiaxial experimental data for six different alloys. A total of 241 samples are included in this study. Using the proposed concept, fatigue life estimations are improved compared to those obtained from the original critical plane based on Fatemi-Socie and Smith-Watson-Topper models.