The varying densification strain in a multi-layer aluminum corrugate structure: Direct impact testing and layer-wise numerical modelling

Abstract

An aluminum (1050 H14) multi-layer corrugated structure composed of brazed 16 trapezoidal zig-zig fin layers was direct impact tested above the critical velocities for shock formation using a modified Split Hopkinson Pressure Bar. The experimentally measured stress-time histories of the cylindrical test samples in the direct impact tests were verified with the simulations implemented in the explicit finite element code of LSÀDYNA. The quasi-static experimental and simulation deformation of the corrugated samples proceeded with the discrete, non-contiguous bands of crushed fin layers, while the dynamic crushing started from the proximal impact end and proceeded with a sequential and in-planar manner, showing shock type deformation characteristic. The experimental and numerical crushing stresses and the numerically determined densification strains of the fin layers increased with increasing impact velocity above the critical velocities. When the numerically determined densification strain at a specific velocity above the critical velocities was incorporated, the rigid-perfectly-plastic-locking idealized model resulted in peak stresses similar to the experimental and simulation mean crushing stresses. However, the model underestimated the experimental and simulation peak stresses below 200 m s ¡1. It was proposed, while the micro inertial effects were responsible for the increase of the crushing stresses at and below subcritical velocities, the shock deformation became dominant above the critical velocities.

Impact loading and modelling a multilayer aluminium corrugated/fin core: The effect of the insertion of imperfect fin layers

“…It is also noted that the collapse of midsection layers occurs at the later stages of deformation, which leads to the bending of the test sample at increasing strains (Figure a). Although not shown here, the similar bending of the test sample at a larger sample diameter (40 mm) and shorter sample lengths (28.8 and 19.2 mm) was also observed in quasi‐static tests . Layer crushing in the perfect model I starts at the layers near the ends of sample and progresses sequentially to the midsection layers (Figure a).…”

“…Metallic cellular structures deform, after a critical velocity, by forming sequential, planar crush bands starting/propagating at the impact end, known as shock deformation mode. The shock mode of deformation was previously observed in the impact testing wood, urethane foam, aluminium foam, aluminium honeycomb, and multilayer corrugated structures . The shock mode is characterised by an impact‐end stress far greater than the distal end stress, and the difference in‐between increases as the impact velocity increases.…”

“…The numerical stresses on the incident bar were calculated at the same locations with the strain gages in the experimental set‐up. The further details of the quasi‐static compression and impact test models are given elsewhere …”

“…At quasi‐static velocities, crush bands (localised deformation) form randomly, starting most likely at the weakest region of sample and progressing discretely to the uncrushed regions. Because the impact‐end and distal end stresses are nearly the same, it is referred to as “quasi‐static homogenous mode.” The homogenous deformation was previously identified in regular and irregular honeycombs as X‐shaped shear bands, in Voronoi honeycombs and in corrugated layered structure . At intermediate velocities, the deformation is considered as the “transition mode” as the layer crushing is concentrated at the impact‐end.…”

“…The aim of the present study was to investigate the effect of the insertion of imperfect layers on the shock mode of deformation of a multilayer 1050 H14 aluminium corrugated sandwich core. The investigated sandwich core was made of corrugated fin layers and previously shown to exhibit repeatable responses to mechanical forces under compression at low and high velocities . The quasi‐static compression tests were performed at 0.0048 m/s, and the impact tests (Taylor‐like impact) were performed at 135, 150, and 200 m/s.…”