Modelling of Dynamic Material Behaviour and Fracture of Aluminium Alloys for Structural Applications

Abstract

The present thesis aims to establish a validated thermoelasto-thermoviscoplastic model and a failure criterion for several extruded anisotropic aluminium alloys. The materials investigated are AA6060, AA6082, AA7003 and AA7108, all in temper T6, which are relevant for application in automotive crash components, e.g. in crash boxes and bumper beams. AA6060-T6 has recrystallized grain structure, while the other three alloys have fibrous grain structure. The stress-strain behaviour and plastic anisotropy of the materials at a wide range of strain-rates were obtained through tensile tests. The dynamic fracture behaviour of the investigated alloys was investigated using an instrumented Charpy test machine and V-notch specimens. The numerical study was carried out with the non-linear finite element code LS-DYNA.

Tensile tests at low to medium strain-rates were performed with a standard tensile test machine; while a split-Hopkinson tension bar (SHTB) was used to carry out tests at high rates of strain. Both fibrous and recrystallized extruded aluminium alloys have anisotropic mechanical properties. Tests were therefore done in three directions, i.e. 0°, 45° and 90°, with respect to the extrusion direction in order to characterize the plastic anisotropy of the extruded alloys. Fracture characteristics were obtained from measurements of the original area and fractured area of the gauge section.

Based on the test results, an anisotropic thermoelastic-thermoviscoplastic constitutive relation and a simple fracture criterion were calibrated for the alloys and used in explicit finite element analyses of the SHTB tests for all four alloys. Using a combination of implicit and explicit solvers, the numerical predictions were found to represent the observed behaviour, including fracture, in the experimental tests fairly well.

To investigate dynamic fracture, instrumented Charpy V-notch impact tests were conducted. For each alloy, the influence of orientation of the specimen with respect to the extrusion direction was investigated with the V-notch either parallel or normal to the thickness direction of the profile. Metallurgical studies were also carried out to investigate the physical mechanisms governing the macroscopic material behaviour. The experiments showed that the dissipated energy is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the fibrous alloys the dissipated energy decreases with increasing angle between the longitudinal direction of the specimen and the extrusion direction, i.e. the dissipated energy is lower when the notch is parallel to the fibrous grain structure.

Finally, 3D numerical analyses of the Charpy V-notch tests were carried out on AA6060-T6 and AA7003-T6 using LS-DYNA. The work hardening behaviour in the first part of the test was well predicted in the simulation, while the peak load was underestimated in almost all simulations. The results indicated that the fracture parameter is important for an accurate estimation of the fracture energy.