Deformation and fracture of impulsively loaded sandwich panels

Abstract

Light metal sandwich panel structures with cellular cores have attracted interest for multifunctional applications which exploit their high bend strength and impact energy absorption. This concept has been explored here using a model 6061-T6 aluminium alloy system fabricated by friction stir weld joining extruded sandwich panels with a triangular corrugated core. Micro-hardness and miniature tensile coupon testing revealed that friction stir welding reduced the strength and ductility in the welds and a narrow heat affected zone on either side of the weld by approximately 30%. Square, edge clamped sandwich panels and solid plates of equal mass per unit area were subjected to localized impulsive loading by the impact of explosively accelerated, water saturated, sand shells. The hydrodynamic load and impulse applied by the sand were gradually increased by reducing the stand-off distance between the test charge and panel surfaces. The sandwich panels suffered global bending and stretching, and localized core crushing. As the pressure applied by the sand increased, face sheet fracture by a combination of tensile stretching and shear-off occurred first at the two clamped edges of the panels that were parallel with the corrugation and weld direction. The plane of these fractures always lay within the heat affected zone of the longitudinal welds. For the most intensively loaded panels additional cracks occurred at the other clamped boundaries and in the center of the panel. To investigate the dynamic deformation and fracture processes, a particle-based method has been used to simulate the impulsive loading of the panels. This has been combined with a finite element analysis utilizing a modified Johnson-Cook constitutive relation and a Cockcroft-Latham fracture criterion that accounted for local variation in material properties. The fully coupled simulation approach enabled the relationships between the soil-explosive test charge design, panel geometry, spatially varying material properties and the panel's deformation and dynamic failure responses to be explored. This comprehensive study reveals the existence of a strong instability in the loading that results from changes in sand particle reflection during dynamic evolution of the panel's surface topology. Significant fluid structure interaction effects are also discovered at the sample sides and corners due to changes of the sand reflection angle by the edge clamping system. 2

Effects of heat treatment on the ballistic properties of AA6070 aluminium alloy

“…A modified version of the Johnson-Cook (MJC) constitutive relation was used to model the target materials (see [1][24] [25]). The constitutive behaviour is assumed to be isotropic and modelled with the von Mises yield criterion even though the materials exhibit some anisotropy (but mainly in the strain to failure, see Figure 3).…”

Penetration of granular materials by small-arms bullets

“…3 [25][32] [34]). This method, which is based on a Lagrangian formulation, has several advantages over coupled Lagrangian-Eulerian approaches as both numerical advection errors and severe contact problems are avoided, while at the same time keeping the computational time at a reasonable level.…”

“…It works with discrete, rigid, spherical particles that transfer forces between each other through a penalty-based contact formulation. Modelling the sand as discrete particles allows for a numerically simple and robust treatment of its interaction with structural parts represented by finite elements (see [25] and [34]), and several parametric studies for increased understanding of this particular penetration problem have been carried out.…”

Unsteady penetration of a target by a liquid jet

“…This raises the question of whether fluidstructure interaction (FSI) between the fluid-like projectile and the deformable target might offer another possible explanation for the dwell phenomenon. FSI effects have been studied in the context of sand sprays impacting deformable structures (14,15). Similar to the impacting long rod, the loading by the sand is also primarily inertial in nature.…”