On the utilisation of nonlinear plasticity models in military aircraft fatigue estimation: A preliminary comparison

Strain-life methodologies are commonly employed for fatigue estimation in military aircraft structures. These methodologies rely on models describing the elastoplastic response of the material under cycling. Despite the numerous advanced plasticity models proposed and utilised in various engineering problems over the past decades, the Masing model remains a popular choice in fatigue analysis software, mainly due to its simplicity. However, in the case of military aircraft load spectra including scattered overloads the Masing choice fails to represent adequately transient cyclic phenomena, such as mean stress relaxation and ratcheting. In this study, four well-known constitutive plasticity models have been selected as potential substitutes of the Masing model within a defence organisation in-house developed fatigue analysis software. The models assessed were the well known Multicomponent Armstrong-Frederick Model (MAF) and three of its derivatives: MAF with threshold (MAFT), Ohno-Wang (OW) and MAF with Multiplier (MAFM). The models were calibrated with the use of existing experimental data, obtained from aircraft aluminium alloy tests. Optimisation of the parameters was performed through a genetic algorithm-based commercial software. The models were incorporated in the fatigue analysis software and their performance was evaluated statistically and compared against each other and with the Masing model for a series of different flight load spectra for a military aircraft. In this preliminary investigation, all four models have achieved a drastic improvement in fatigue analysis, with the MAFT model indicating a slightly better performance.