Analysis of Damage and Failure in Anisotropic Ductile Metals Based on Biaxial Experiments with the H-Specimen

Abstract

Background Dependence of strength and failure behavior of anisotropic ductile metals on loading direction and on stress state has been indicated by many experiments. To realistically predict safety and lifetime of structures these effects must be taken into account in material models and numerical analysis. Objective The influence of stress state and loading direction on damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A is investigated. Methods New biaxial experiments and numerical simulations have been performed with the H-specimen under different load ratios. Digital image correlation shows evolution of strain fields and scanning electron microscopy is used to visualize failure modes on fracture surfaces. Corresponding numerical studies predict stress states to explain damage and fracture processes on the micro-scale. Results The stress state, the load ratio and the loading direction with respect to the principal axes of anisotropy affect the width and orientation of localized strain fields and the formation of damage mechanisms and fracture modes at the micro-level. Conclusions The enhanced experimental program with biaxial tests considering different loading directions and load ratios is suggested for characterization of anisotropic metals.

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.…”

“…On the other hand, series of experiments with biaxially loaded specimens have been performed to investigate the stress-state-dependent damage behavior as well as to study the effect of the loading direction on damage and failure mechanisms in anisotropic metals. In particular, the X0-and the H-specimen have been extensively tested by Bru¨nig et al (2021Bru¨nig et al ( , 2022 considering different load ratios leading to a wide range of stress states in notched parts where damage occurred. They also studied the influence of the orientation of the specimens with respect to the principal axes of anisotropy during loading and concluded that in both specimens loading in RD and TD leads to more and larger micro-voids whereas during loading in DD more pronounced micro-shear-cracks are formed.…”

“…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.…”