A modification of the multicomponent Armstrong–Frederick model with multiplier for the enhanced simulation of aerospace aluminium elastoplasticity

The Multicomponent Armstrong–Frederick (AF) model with Multiplier (MAFM) has demonstrated high simulation accuracy for uniaxial and multiaxial loading conditions for a number of different materials. In this study the MAFM model is modified to improve the phenomenological modelling of aerospace aluminium alloys 7075-T6 and 7050-T7451 under uniaxial constant and variable amplitude loading. In order to recognise the experimentally observed strain amplitude dependency of mean stress relaxation rate, the coefficient of the linear kinematic backstress was modified from a constant to a strain amplitude dependent dynamic term. This modification improved the mean stress relaxation capability of the MAFM model. Additionally, the hysteresis loop evolution has been enhanced via further modification of the MAFM model by improving the monotonic stress-strain evolution of the initial loading branch of cyclic load cases by separating the kinematic backstress coefficients into two parts, the contributions from cyclic and monotonic micro-mechanisms. The monotonic coefficients were allowed to decay with continued cycling, which captured the monotonic to cyclic transition of stress-strain development. Finally, the experimentally observed reversibility of the monotonic stress-strain evolution has been also incorporated successfully through the introduction of a decaying strain range memory parameter, which improved the variable amplitude hysteresis loop evolution. Overall, the modified MAFM model has been successful in improving simulation accuracy of the cyclic elastoplastic response exhibited by both aluminium alloys examined.