Energy-based damage descriptions to assess fatigue life of steel samples undergoing various multiaxial loading spectra

The present study evaluates fatigue damage of four steel alloys, mild steel, SS347, SNCM439, and SNCM630, by means of Socie, Ellyin, and Varvani-Farahani damage models. The Socie model assesses fatigue damage through product of maximum principal strain amplitude and maximum normal stress on the maximum principal strain plane. Damage description by Ellyin was developed as both elastic and plastic strain energies over loading spectrum were integrated. The elastic–plastic response of materials is evaluated through use of Garud’s constitutive plasticity model to achieve components of stress/ strain and corresponding stress–strain hysteresis loops. Based on the Varvani-Farahani model, components of stress and strain calculated from largest Mohr’s circles over peak-valley events are employed to assess fatigue damage. Overall damage was calculated on the basis of energy-based models from counted reversals over entire loading blocks and related to fatigue life. The Socie approach overpredicted lives for steel samples. Predicted life data for mild steel and SS347 samples fell below the midline based on the Ellyin’s model. Both Ellyin and Varvani-Farahani models showed a good agreement of predicted lives for steel samples within factors ±3 as compared with experimental data. The choice of damage assessment was highly related to consistency of damage descriptions to crack formation and early growth, stress/strain components, material properties, and loading spectrum.