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Dynamic Crushing of Aluminum Foams

Research Objectives

Lightweight cellular materials such as foams exhibit excellent energy absorption characteristics and are widely used in a variety of impact mitigation and blast protection applications. In this study, experimental and analytical efforts are combined to investigate the behavior of an Al-alloy open-cell foam under impact.

Investigators

S. Gaitanaros, A.T. Barnes, S. Kyriakides, K. Ravi-Chandar

Highlights

The experiments involve direct and stationary impact tests on cylindrical foam specimens at impacts speeds in the range of 20-160 m/s using a gas gun. The stress at one end is recorded using a pressure bar while the deformation of the entire foam specimen is monitored with high-speed photography. Specimens impacted at velocities of 60 m/s and above develop nearly planar shocks that propagate at well-defined speeds crushing the specimen (see Movie 1). The shock speed vs. impact velocity Hugoniot, extracted directly from the high-speed images, was found to follow a linear trajectory. Using this relationship, the pressure bar measurements, and the conservation laws across the shock enabled estimation of all problem variables in the range of interest. Thus, for impact velocities above a critical level, the stress behind the shock was found to increase quadratically with impact velocity while ahead of it the stress remains at a constant level that corresponds to the quasi-static compression initiation stress. The strain across the shock increases with impact velocity, asymptotically approaching a strain of about 90%. The compaction energy dissipation across the shock also increases with impact speed reaching levels that are significantly above the quasi-static value.

These dynamic crushing events are simulated numerically using micromechanically accurate foam models (see Movie 2). Skeletal random geometries from soap froth generated with the Surface Evolver are “dressed” with shear-deformable beams of variable cross sections, while the Al-alloy is modeled as a finitely deforming elastic-plastic solid. Utilization of the beam-to-beam contact algorithm within LS-DYNA plays an essential role in the correct simulation of crushing. The models are shown to reproduce essentially all aspects of the dynamic crushing behavior observed in the experiments. It was possible to establish that: (a) for the particular foam of interest, the critical impact speed for shock formation is close to 50 m/s, and (b) for lower velocities crushing occurs at stress and deformation levels that are close to the quasi-static values.

Selected Publications

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