Dylan Agius

Aluminum Alloy 7075 Ratcheting and Plastic Shakedown Evaluation with the Multiplicative Armstrong–Frederick Model

This work investigates experimentally and computationally the uniaxial ratcheting strain and plastic shakedown of aluminum alloy 7075-T6. The experimental results illustrate the existence of both p...

Optimising the multiplicative AF model parameters for AA7075 cyclic plasticity and fatigue simulation

Purpose This study presents the improvements of the multicomponent Armstrong–Frederick model with multiplier (MAFM) performance through a numerical optimisation methodology available in a commercial software. Moreover, this study explores the application of a multiobjective optimisation technique for the determination of the parameters of the constitutive models using uniaxial experimental data gathered from aluminium alloy 7075-T6 specimens. This approach aims to improve the overall accuracy of stress–strain response, for not only symmetric strain-controlled loading but also asymmetrically strain- and stress-controlled loading. Design/methodology/approach Experimental data from stress- and strain-controlled symmetric and asymmetric cyclic loadings have been used for this purpose. The analysis of the influence of the parameters on simulation accuracy has led to an adjustment scheme that can be used for focused optimisation of the MAFM model performance. The method was successfully used to provide a better understanding of the influence of each model parameter on the overall simulation accuracy. Findings The optimisation identified an important issue associated with competing ratcheting and mean stress relaxation objectives, highlighting the issues with arriving at a parameter set that can simulate ratcheting and mean stress relaxation for load cases not reaching at complete relaxation. Practical implications The study uses a strain-life fatigue application to demonstrate the importance of incorporating a technique such as the presented multiobjective optimisation method to arrive at robust parameters capable of accurately simulating a variety of transient cyclic phenomena. Originality/value The proposed methodology improves the accuracy of cyclic plasticity phenomena and strain-life fatigue simulations for engineering applications. This study is considered a valuable contribution for the engineering community, as it can act as starting point for further exploration of the benefits that can be obtained through material parameter optimisation methodologies for models of the MAFM class.

Enhanced non-linear material modelling for analysis and qualification of rollover protective structures

Finite element simulations of a rollover protective structure are an important aspect in its design, as it provides a means of structural integrity qualification prior to the required destructive testing. A good understanding of the rollover protective structure behaviour under simulated loading offers engineering practitioners the opportunity to optimize the design. The testing conditions, which are outlined in the applicable standards, result in plastic deformation of the rollover protective structure, associated with material hardening of various areas of the structure. An accurate description of the material behaviour is important for finite element simulations of the structural response. This research examines some of the hardening models commonly used in simulations of rollover protective structures, which are available in most finite element commercial software, including linear and multi-linear isotropic and kinematic hardening models and non-linear kinematic hardening models. The numerical performance of the plasticity models in representing the material behaviour was compared with the experimental data for commonly used rollover protective structure material. Analysis revealed the potential benefits and drawbacks of the various models. Moreover, a damage-induced softening model was implemented at the structure joints in conjunction with the non-linear hardening models. Enhanced computational results were obtained through this modelling variation, highlighting the importance of material modelling at the primary structure and the joints of a rollover protective structure.