Investigation of Yield Surfaces Evolution for Polycrystalline Aluminum after Pre-Cyclic Loading by Experiment and...

Current understanding of yield surface evolution in polycrystalline metals has focused mainly on isotropic hardening. Prior research has shown that cyclic loading can alter material behavior, but the role of anisotropic strain hardening remains unclear. While isotropic models are commonly used, they may not fully represent directional deformation effects. This gap motivated the need to integrate anisotropic strain hardening into crystal plasticity frameworks. No prior work had resolved how offset strain definitions influence yield surface evolution. Existing studies lack detailed mesoscopic analysis of plastic deformation heterogeneity. This paper contributes by linking macroscopic observations with microscopic mechanisms. The study introduces a new approach to model directional hardening features.

The primary aim of this work is to incorporate anisotropic strain hardening into crystal plasticity theory. The specific problem addressed is how pre-cyclic loading affects yield surface evolution in polycrystalline aluminum. The motivation stems from the need to better predict material behavior under complex loading histories. The study focuses on mesoscopic-scale yield surface changes after 30 pre-cycles. The researchers sought to validate a model that accounts for directional hardening effects. They aimed to compare experimental and simulation results for yield surface evolution. The study also examines how offset strain definitions influence hardening behavior. The goal is to improve predictive accuracy for material deformation under cyclic loading.

The study used two finite element models at different scales. A global finite element model matched the dimensions of the thin-walled tube specimen used in experiments. A 3D cubic polycrystalline aggregate representative volume element model was also used. Both models operated within a crystal plasticity finite element framework. The researchers applied pre-cyclic loading paths to simulate material behavior. Offset strain definitions were varied to assess their impact on yield surface evolution. Experimental measurements were conducted on the thin-walled tube specimen. Numerical simulations were performed to compare with experimental findings. The models captured plastic deformation heterogeneity across different offset strain conditions.

The subsequent yield surfaces showed significant sensitivity to offset strain and pre-cyclic loading direction. Anisotropic hardening was evident through yield surface translation and distortion. The yield surface exhibited a 'sharp corner' in the pre-deformation direction. A 'flat' region emerged in the reverse direction with small offset strain definitions. With large offset strains, isotropic hardening was observed as von Mises circles distorted into ellipses. Heterogeneous plastic deformation was analyzed across different offset strain conditions. The degree of heterogeneity decreased exponentially with increasing offset strain. Modeling results confirmed the correlation between anisotropic hardening and crystal slip mechanisms.

The study demonstrates that anisotropic strain hardening significantly influences yield surface evolution. The crystal plasticity model accurately captures directional hardening features. The yield surface's shape and size depend strongly on offset strain definitions. Anisotropic hardening is linked to crystal microstructure and slip mechanisms. Isotropic hardening emerges with large offset strain definitions. The model's predictions align with experimental observations of yield surface distortion. Heterogeneous deformation decreases exponentially with increasing offset strain. The findings support the use of crystal plasticity models for predicting directional hardening behavior.