Use of Prandtl operators in simulating the cyclic softening of Inconel 718 under isothermal low-cycle fatigue loading

In this article, a new approach is proposed for modelling the stress-strain response of the Inconel 718 super alloy under isothermal Low-Cycle Fatigue (LCF) loading. The proposed constitutive model is based on the Prandtl operator approach, in which a set of modifications is introduced in order to simulate strain range dependent cyclic softening. A new simulation capability is introduced by evolving the yield strains of the individual hysteresis operators with an accumulated plastic strain. In addition, the effect of the strain range dependency of cyclic softening is introduced into the proposed constitutive model by coupling its parameters with the concept of the plastic strain memory surface. These introduced modifications preserve the main advantages of the Prandtl operators, such as a small number of model parameters, their fast determination from the cyclic stress-strain curve, and a high computational speed, when used to simulate complex non-linear mechanical behaviour. Finally, the prediction capability of the proposed model is illustrated by various strain controlled tests performed at 500 C, including block spectrum loading and variable strain amplitude loading.