On the influence of stress state on ductile fracture of two 6000-series aluminium alloys with different particle content

Tension-torsion tests were conducted on two 6000-series aluminium alloys with different area fraction of con-stituent particles. The two alloys, denoted alloy A and B, have previously been characterized and found to have similar matrix material, albeit the three times higher area fraction of constituent particles in alloy B than in alloy A. Single notch tube specimens of the two alloys were subjected to fifteen proportional load paths by varying the ratio of axial force and twisting moment, probing stress states from torsion to plane-strain tension. The overall failure strain in the notch was estimated analytically based on the experimental data, whereas finite element simulations were used to determine the stress and strain fields within the notch region and to estimate the local failure strain. The experiments showed that the increased particle content led to a reduction in the local failure strain of alloy B compared with alloy A that varied from 16% to 60%, depending on the stress state, with an average reduction of 39%. While the overall trend was an increasing failure strain with decreasing stress triaxiality, significant influence of the Lode parameter was observed, and thus the increase was not monotonic. Applying a porous plasticity model, localization analyses were conducted to examine the underlying mechanisms for the complex variation of the failure strain with stress state.