Biaxial Experiments and Numerical Analysis on Stress-State-Dependent Damage and Failure Behavior of the Anisotropic Aluminum Alloy EN AW-2017A

Abstract

Many experiments indicated the remarkable dependence of the strength and failure behavior of anisotropic ductile metals on the loading direction and on the stress state. These influences have to be taken into account in accurate material models and in the numerical simulation of complex loading processes predicting the safety and lifetime of aerospace structures. Therefore, the present paper discusses the effect of loading direction and stress state on the damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A. Experiments and corresponding numerical analysis with the newly developed, biaxially loaded X0 specimen have been performed and the influence of different load ratios is examined. The formation of strain fields in critical parts of the X0 specimen is monitored by digital image correlation. Different failure modes are visualized by scanning electron microscopy of fracture surfaces. Stress states are predicted by finite element calculations and they are used to explain damage and fracture processes at the micro-level. The experimental–numerical analysis shows that the loading direction and the stress state remarkably affect the evolution of the width and orientation of localized strain fields as well as the formation of damage processes and fracture modes. As a consequence, characterization of anisotropic metals is highly recommended to be based on an enhanced experimental program with biaxial tests including different load ratios and loading directions.

A stress-state-dependent damage criterion for metals with plastic anisotropy

“…Their phenomenological model considers isotropic plastic behavior and is based on experimental and numerical studies on both the micro-and the macro-level examining various mechanisms acting on these scales as well as their interaction. To enhance this continuum model for anisotropic plasticity a series of multiaxial tests has been performed by Bru¨nig et al (2021Bru¨nig et al ( , 2022. These experiments reveal the influence of the loading direction on evolution of inelastic deformations and on damage and failure mechanisms.…”

“…and c denotes the equivalent yield stress of the undamaged material identified by a chosen uniaxial reference test. It should be noted that the Hoffman criterion (2) can be seen as a hydrostaticstress-dependent generalization of the anisotropic Hill condition (Hill, 1948) which has been used by Bru¨nig et al (2021Bru¨nig et al ( , 2022 to analyze the anisotropic plastic behavior of the investigated aluminum alloy without considering the SD effect. For isotropic material behavior stress invariants can be taken into account to characterize the stress state.…”

“…where h is the angle with respect to the rolling direction, see Bru¨nig et al (2021) for further details.…”

“…For studies of stressstate-dependent damage and fracture mechanisms in ductile metals Bru¨nig et al (2015aBru¨nig et al ( , 2015b and Gerke et al (2017) proposed new geometies for biaxially loaded specimens. These new specimens have been extensively used to analyze the influence of non-proportional load paths (Bru¨nig et al, 2019;Gerke et al, 2020) or the effect of loading direction in anisotropic metals (Bru¨nig et al, 2021(Bru¨nig et al, , 2022 on damage and failure behavior.…”

“…In the present paper, results of experiments with the X0-and the H-specimen performed by Bru¨nig et al (2021Bru¨nig et al ( , 2022 are used to develop the anisotropic plastic model and the damage condition which has been applied to the aluminum alloy EN AW-2017A (EN AW-AlCu4MgSi). The experiments clearly showed that the stress state as well as the loading direction influenced the damage mechanisms on the micro-level and, therefore, both dependencies are here taken into account.…”