A plastic damage model with mixed isotropic-kinematic hardening for low-cycle fatigue in 7020 aluminum

TL;DR

This work addresses fatigue in the high-strength aluminum EN AW-7020 T6 by integrating a plastic-damage framework that combines $J_2$ plasticity with mixed isotropic-kinematic hardening (Chaboche-type) and a dual-damage description featuring a deviatoric/volumetric split. A gradient-enhanced regularization is employed to overcome ill-posedness from softening, and a nonlinear unilateral activation function governs damage under tension, enabling smooth tension-compression transitions. The model uses two damage indices with separate trilinear growth laws, calibrated against dog-bone strain-controlled fatigue data and applied to CT notch fatigue, achieving strong agreement with experiments and mesh-independent predictions. This approach provides a robust tool for predicting low-cycle fatigue in high-strength aluminum, with potential benefits for design against crack initiation and propagation in aerospace and automotive components, where large plastic deformations and damage localization are critical.

Abstract

The paper at hand presents an in-depth investigation into the fatigue behavior of the high-strength aluminum alloy EN AW-7020 T6 using both experimental and numerical approaches. Two types of specimens are investigated: a dog-bone specimen subjected to cyclic loading in a symmetric strain-controlled regime, and a compact tension specimen subjected to repeated loading and unloading, which leads to damage growth from the notch tip. Experimental data from these tests are used to identify the different phases of fatigue. Subsequently, a plastic-damage model is developed, incorporating J2 plasticity with Chaboche-type mixed isotropic-kinematic hardening. A detailed investigation reveals that the Chaboche model must be blended with a suitable isotropic hardening and combined with a proper damage growth model to accurately describe cyclic fatigue including large plastic strains up to failure. Multiple back-stress components with independent properties are superimposed, and exponential isotropic hardening with saturation effects is introduced to improve alignment with experimental results. For damage, different stress splits are tested, with the deviatoric/volumetric split proving successful in reproducing the desired degradation in peak stress and stiffness. A nonlinear activation function is introduced to ensure smooth transitions between tension and compression. Two damage indices, one for the deviatoric part and one for the volumetric part, are defined, each of which is governed by a distinct trilinear damage growth function. The governing differential equation of the problem is regularized by higher-order gradient terms to address the ill-posedness induced by softening. Finally, the plasticity model is calibrated using finite element simulations of the dog-bone test and subsequently applied to the cyclic loading of the compact tension specimen.

Figures (16)

• Figure 1: Dimensions of the specimens in millimeters: (a) dog-bone specimen, and (b) compact tension (CT) specimen.
• Figure 2: Test setups: (a) dog-bone specimen, and (b) compact tension specimen.
• Figure 3: Experimental data of the strain-controlled fatigue test (dog-bone specimen): (a) stress versus strain, (b) peak stress versus cycle number, (c) unloading slope versus cycle number, (d) initial phase, (e) stable phase, and (f) failure phase.
• Figure 4: Applied force versus COD curve of the compact tension specimen test.
• Figure 5: Yield locus in the three-dimensional Haigh--Westergaard stress space and its $\pi$-plane representation.