A fatigue failure model for life assessment of a structure that incorporates the stress-state dependence and irreversible nature of fatigue damage is presented. In the frame work of cohesive zone model, a stress state dependent traction separation law for plane strain is taken to represent an undamaged ferritic steel. The evolution of damage has two additional fatigue parameters: a stress and a length parameter. Initially a parametric study is done to show that the model is able to reproduce a typical uniaxial fatigue response to stress based cyclic load, that of a stress-life curve and reduction in life due to positive mean stress. The effect of the cohesive fatigue parameters on the characteristics of the stress-life curve is then established. The model is further applied for a range of sinusoidally varying in-phase stress states which are characterised by a fixed bi-axiality ratio. The initiation and growth of damage is shown to be more rapid for higher bi-axiality. Except for stress amplitudes in which the lower bi-axiality case has conditions close to monotonic failure, the effect of bi-axiality is shown to be detrimental to the life expectancy of the material as observed in available experimental literature.

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