Ductile fracture of aluminium alloys in the low to moderate stress triaxiality range
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This thesis investigates the effect of constituent particles on ductile fracture in 6000-series aluminium alloys in the low to moderate stress triaxiality range. In Part 1, tension tests on smooth and notched specimens were conducted for four 6000-series aluminium alloys to assess the effects of constituent particles on ductile fracture at different stress states. Two of the alloys were engineering materials, while the remaining two were tailor-made to be similar to the engineering materials but with more than three times the amount of constituent particles. The microstructure of both isotropic (cast and homogenised) and anisotropic (extruded) materials was characterised using scanning electron microscopy (SEM), optical microscopy and X-ray diffraction. Axisymmetric smooth and notched specimens were machined from cast and homogenised billets and extruded profiles. All specimens were tested in the artificially aged T6 temper. The specimens were strained in tension until fracture, and the fracture surface of the specimens was later investigated based on SEM images. The experiments show that a high stress triaxiality and a high volume fraction of large-sized constituent particles have a detrimental effect on the ductility. However, the negative effect of the increased particle volume fraction on ductility was markedly reduced after extrusion. The experiments also indicated that a larger amount of constituent particles may improve the ductility under certain conditions. In Part 2, ductile fracture of two high-strength aluminium alloys in temper T6 was investigated using heuristic extensions of the Gurson model with non-quadratic yield criteria. Simulations of tensile tests on smooth and notched specimens made for each alloy in either the cast and homogenised condition with random texture or in the extruded condition with strong deformation texture were performed and compared with experiments from previous studies. The plastic anisotropy and its influence on the predicted fracture behaviour was investigated for the extruded alloys through simulations in several tensile directions. In the heuristic extensions of the Gurson model, the Hershey-Hosford yield function was used for the materials with random texture and the anisotropic Yld2004-18p yield criterion was used for the materials with strong deformation texture. The two alloys had different fractions of constituent particles but were otherwise similar, and as a conservative assumption, the initial porosity was taken equal to the particle fraction in the simulations. For each combination of alloy and processing condition (in total four combinations), the parameters governing the flow stress of the matrix material and the critical porosity at failure were determined using a single test, while the remaining tests were used for validation. Fracture was simulated by element erosion at the critical porosity. The results showed that the heuristic extensions of the Gurson model gave predictions of the tensile ductility with a satisfactory accuracy for the two alloys in both processing conditions. In Part 3, tension-torsion tests were conducted on two under-aged 6000-series aluminium alloys with different area fraction of constituent particles. The two alloys, denoted alloy A and B, had previously been characterized in Part 1 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 notched 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. In Part 4, in-plane shear tests were conducted on two extruded 6000-series aluminium alloys to investigate the influence of constituent particles on ductile fracture. The two alloys have previously been characterized and found to have different area fractions of constituent particles while having similar grain structure, crystallographic texture, strength, and work hardening. Specimens of both alloys were machined from flat profiles with their gauge section oriented along either the extrusion direction or the transverse direction. The specimens were loaded in shear until fracture and an experimental-numerical approach was employed to analyse the stress and strain fields, which involves digital image correlation and crystal plasticity finite element analysis. The fracture mode was determined by examining digital images captured during the experiment, while the fracture surface of representative specimens was examined in the scanning electron microscope. The shear strain across the gauge section at failure was substantially lower for the alloy with higher fraction of constituent particles, and the negative effect of the particle content on the ductility was found to be more pronounced for the tests in the transverse direction. The crystal plasticity finite element analyses showed an extensive texture development within the gauge region and that the local shear strain at failure was somewhat different in the two loading directions.