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

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

Local variations in gabion structures

Gabion structures are widely used for force protection as they enable locally available material to be used, reducing logistical expense. The soil fill within these structures provides the blast and ballistic resistance; hence, any localised variation in the contained soil can potentially lead to reductions in protective capability. Specifically, built gabion structures were monitored in internal and external environments to assess the variation of soil moisture content and density over a full year and with changing weather conditions. The gabions were filled with fine sand according to manufacturer’s instructions. Internal and external moisture content readings were recorded at regular intervals, and a continuously monitoring weather station was installed to collect comparative data. LIDAR scanning was used to record the shape and volume of the gabions to estimate variations in the density of the soil fill. The data indicate that moisture content can vary by over 20% between the top and base of the gabion, and by over 5% from face to face and between readings depending on recent weather conditions, while the core of the gabions remains relatively unaffected. This leads to localised variations in density which can impact on both the ballistic performance and blast resistance of the structure

A novel reformulation of the Theory of Critical Distances to design notched metals against dynamic loading

In the present study the linear-elastic Theory of Critical Distances (TCD) is reformulated to make it suitable for predicting the strength of notched metallic materials subjected to dynamic loading. The accuracy and reliability of the proposed reformulation of the TCD was checked against a number of experimental results generated by testing, under different loading/strain rates, notched cylindrical samples of aluminium alloy 6063-T5, titanium alloy Ti–6Al–4V, aluminium alloy AlMg6, and an AlMn alloy. To further validate the proposed design method also different data sets taken from the literature were considered. Such an extensive validation exercise allowed us to prove that the proposed reformulation of the TCD is successful in predicting the dynamic strength of notched metallic materials, this approach proving to be capable of estimates falling within an error interval of ±20%. Such a high level of accuracy is certainly remarkable, especially in light of the fact that it was reached without the need for explicitly modelling the stress vs. strain dynamic behaviour of the investigated ductile metals