Abstract
The presence and quality of welds in metallic structures has the ability to influence their likelihood of failure under dynamic loading. This investigation focused on characterising the behaviour of a welded aluminium structure. Samples were taken from the parent metal, heat affected zone (HAZ) and the weld bead and high strain rate characterisation testing was performed to determine the Johnson-Cook (JC) strength and failure model parameters for each material. However, significant scatter was found in the data for the weld bead due to porosity within the samples. Additional tensile tests were performed using a rotating fly wheel machine with four larger samples, which were machined from the welded aluminium structure and contained HAZs on either side of the weld bead, located in the centre of the specimen. Three of the four samples had the weld bead ground flush to the level of the base plate. Digital image correlation was used to determine the surface strain within each region of the sample and identified significant strain localisation at the interface between the weld metal and the HAZ, as well as within the weld bead. Comparisons between the ground welded specimens and those with the weld reinforcement showed a different failure mode between the two specimens. For the ground specimens, the strain localisation in the weld bead initiated failure prior to the strain localisation occurring at the interface between the weld bead and HAZ. Sectioning of the welds indicated that the strain localisation in the weld bead may have been caused by significant levels of porosity within the weld bead. Preliminary numerical simulations of the ground specimens indicated that the force-time history could be well captured. However, as the strain localisation due to porosity is not captured using a JC model, in addition to the scatter in the characterisation data for the weld bead, failure was not accurately predicted numerically.
Highlights
Aluminium alloys may be used in military platforms due to the relative cost of the material and the advantages aluminium can offer in reducing the overall mass of the platform
Failure always occurred within the weld metal, whereas failure was found to occur at the interface between the weld metal and the heat affected zone (HAZ) in the sample with weld reinforcement
The results indicate that there is minimal strain rate sensitivity until beyond a strain rate of 200 s-1
Summary
Aluminium alloys may be used in military platforms due to the relative cost of the material and the advantages aluminium can offer in reducing the overall mass of the platform. The inhomogeneity in the material properties typically results in strain localisation and failure within the welded joint This effect was noted by Wadley et al [4], who conducted material characterisation and blast testing on sandwich panels made using extruded Aluminium (Al) 6061-T6, where under-matched friction stir welds were used to join the Al 6061-T6. Whilst the strength of the weld metal and HAZ was higher than the parent plate, they noted a significant reduction in the failure strain for the weld metal and HAZ When assessing their samples to impact loading, they assessed the strain field using digital image correlation (DIC) and clearly observed strain localisation within the welded region that led to failure of the specimen. These constitutive models are implemented in numerical simulations of the tensile tests on the welded joints to assess the ability of this approach to capture the performance of the welded aluminium structure
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