The relation between grain boundary precipitate formation and adjacent grain orientations in Al-Mg-Si(-Cu) alloys

The occurrence of grain boundary precipitates was investigated with respect to the crystallographic orientation of the adjacent grains in extruded AA6110, AA6063 and AA6061 alloys brought to T6 temper. It was found that the requirement for grain boundary precipitate formation is for the adjacent grains to have Al directions or {100}Al planes parallel to the grain boundary plane. The highest density of grain boundary precipitates was present when this requirement was fulfilled by both adjacent grains.publishedVersio

Unit cell simulations and porous plasticity modelling for recrystallization textures in aluminium alloys

AbstractThe well-known Gurson model has been heuristically extended to incorporate effects of matrix anisotropy on the macroscopic yielding of porous ductile solids. Typical components of recrystallization textures for aluminium alloys were used to calibrate the Barlat Yld2004-18p yield criterion using a full-constraint Taylor homogenization method. The resulting yield surfaces were further employed in unit cell simulations using the finite element method. Unit cell calculations are invoked to investigate the evolution of the approximated micro structure under pre-defined loading conditions and to calibrate the proposed porous plasticity model. Numerical results obtained from the unit cell analyses demonstrate that anisotropic plastic yielding has great impact on the mechanical response of the approximated micro structure. Despite the simplifying assumptions that underlie the proposed constitutive model, it seems to capture the overall macroscopic response of the unit cell. However, to further enhance the numerical predictions, the model should be supplemented with a void evolution expression that accounts for directional dependency, and a void coalescence criterion in order to capture the last stages of deformation

BALLISTIC PENETRATION AND PERFORATION OF LAYERED STEEL PLATES: AN EXPERIMENTAL AND NUMERICAL INVESTIGATION

In the design of protective structures, thin plates of high-strength steel are frequently being used both in civil and military ballistic protection systems. Earlier studies have shown that by changing the target thickness the deformation mode changes accordingly, from thin plate global deformation towards thick plate shear localisation. Thus, the global deformation mode in thin plates may absorb considerable amount of energy, and it can be presumed that layered targets may be a better energy absorber during ballistic perforation than a monolithic target of equal thickness. Some publications in the literature indicate that this is not necessarily true, but the data on impact of layered targets is limited and it is difficult to make comparisons between results. At present the effect of replacing monolithic plates with layered ones is not clear and further work is required. In this study, the response to normal impact of hardened ogival steel projectiles on layered steel plates has been investigated both experimentally and numerically. In the tests, 12 mm thick (monolithic or layered) plates of Weldox 700 E were impacted using a gas-gun at sub-ordnance velocities and the ballistic limit of the different target combinations were obtained. Numerical simulations of the perforation processes were carried out using LS-DYNA. Both qualitatively and quantitatively good agreements were found between experimental and numerical results

Influence of constituent particles on fracture of aluminum alloys under high-triaxiality loading

Single-edge notch bending tests are conducted to study the influence of constituent particles on the fracture resistance of aluminum alloys 6061, 6063, and 6110 under high-constraint loading conditions. The alloys are tested in the as-cast state after homogenization and artificial aging to temper T6. Each alloy type was delivered with two different volume fractions of constituent particles to enable a quantitative assessment of its impact on the toughness of these aluminum alloys. One variant corresponds to the commercial alloy, whereas the other variant is tailor made with an increased amount of constituent particles by adding Fe and Si to the commercial alloy. All alloys exhibit a dendritic structure with particles clustered at grain boundaries and dendrite arm boundaries. The increased content of constituent particles in the tailor-made alloys is shown to be purely detrimental for the toughness and reduces relevant fracture energy parameters by more than 50% in the alloys tested herein. In the plane-strain-dominated regions of the specimens where the stress triaxiality is highest, crack propagation was found to take place on grain boundaries and dendrite arm boundaries due to void nucleation, growth, and coalescence from the constituent particles. Differences in toughness between the alloys are primarily related to variations in the content, size, and spacing of the constituent particles. A comparison between the three different alloy types, i.e. 6061, 6063, and 6110, shows that strength affects the toughness, but it does not follow the commonly reported trade-off between strength and ductility.publishedVersio

A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis

Tensile tests were conducted on dual-phase high-strength steel in a Split-Hopkinson Tension Bar at a strain-rate in the range of 150-600/s and in a servo-hydraulic testing machine at a strain-rate between 10-3 and 100/s. A novel specimen design was utilized for the Hopkinson bar tests of this sheet material. Digital image correlation was used together with high-speed photography to study strain localisation in the tensile specimens at high rates of strain. By using digital image correlation, it is possible to obtain in-plane displacement and strain fields during non-uniform deformation of the gauge section, and accordingly the strains associated with diffuse and localised necking may be determined. The full-field measurements in high strain-rate tests reveal that strain localisation started even before the maximum load was attained in the specimen. An elasto-viscoplastic constitutive model is used to predict the observed stress-strain behaviour and strain localisation for the dual-phase steel. Numerical simulations of dynamic tensile tests were performed using the non-linear explicit FE code LS-DYNA. Simulations were done with shell (plane stress) and brick elements. Good correlation between experiments and numerical predictions was achieved, in terms of engineering stress-strain behaviour, deformed geometry and strain fields. However, mesh density plays a role in the localisation of deformation in numerical simulations, particularly for the shell element analysis

Dynamic response and robustness of tall buildings under blast loading

Recently, extensive research has been focused on the progressive collapse analysis of the multi-storey buildings. However, most of the research is based on the Alternative Path Method (APM) with sudden removal of the columns, ignoring the duration of the blast load working on the structures. In this paper, a 3-D numerical model with the direct simulation of blast load is proposed to study the real behaviour of a 20 storey tall building under the blast loading. A typical package bomb charge of 15 kg was detonated on the 12th floor. The corresponding dynamic response of structure was studied in details. The robustness of the building under blast load was assessed. Comparison between the proposed method and the APM was also made. It is found that, due to the uplift and downw ard pressure working on the slab, the column force under the direct blast simulation method is smaller than that of the alternative path method. The method to enhance the robustness of the buildings is also recommended

The effect of the orientation of cubical projectiles on the ballistic limit and failure mode of AA2024-T351 sheets

This paper presents the results of an investigation of the ballistic limits and failure modes of AA2024-T351 sheets impacted by cubical projectiles. The effect of cube orientation on the ballistic limit and failure modes was considered in detail. Three impact configurations were investigated. Configuration one, two and three considered face, edge or corner impacts correspondingly. The experimental results were complemented with finite element analysis results in order to explain the observations. The lowest ballistic limit (202 m/s) was observed when the cube edge impacted on the target. In the cube face impacts, the ballistic limit was higher (223 m/s), and the highest ballistic limit (254 m/s) was observed for the corner impact. Although the face impact did not have the lowest ballistic limit, this impact configuration resulted in the least amount of projectile energy loss for impacts above the ballistic limit. With the aid of finite element modelling, it was possible to develop a better understanding of the test results and explain that the observed differences in impact response were not just due to a difference in projectile frontal area, but also due to the combination of the localised deformation near the projectile impact point and the resulting global (dishing) deformation