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 aluminum 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.
Highlights
Sandwich panel structures constructed from light, stiff, strong face sheets that are widely separated by low density cores are highly efficient at supporting bending loads and widely in weight sensitive applications [1]
At the closest stand-off distance of 15 cm, the sandwich panel fractured and failed catastrophically and the measured deflections consist of a component of deformation accrued while the panel was intact plus that induced as panel failure occurred
The corrugated core sandwich panel tested at the closest stand-off distance had undergone severe bending deformation of both face sheets, exhibited modest buckling of the core webs and had fractured along the central regions of the four clamped sides and at the center in the panel extrusion direction
Summary
Sandwich panel structures constructed from light, stiff, strong face sheets that are widely separated by low density cores are highly efficient at supporting bending loads and widely in weight sensitive applications [1]. Experiments with much higher intensity (more nonlinear) shocks have shown similar deflection improvements when strong honeycomb cores were combined with thick face sheets made from high ductility stainless steels [4] These experiments indicated that both the FSI effect and the panels bending resistance can contribute to reduced panel deflections even in air loading situations. The square panels used in these experiments were fabricated from narrow extruded aluminum alloy sandwich panels that were joined by friction stir welding in the longitudinal (extrusion) direction They were impulsively loaded by an explosively accelerated spherical shell of water saturated synthetic sand (glass microspheres) and their deformation and fracture modes compared to those of a monolithic plate of the same alloy with identical mass per unit area (areal density). This can lead to instabilities and premature failure of test structures at regions of enhanced soil particle reflection
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