Blast attenuation in Cymat foam core sacrificial claddings

The mechanism of air blast attenuation in a sacrificial cladding comprising a steel cover plate and a foam core is examined applying an analytical and numerical analysis. For the range of the analysed pressure pulses, the maximum magnitudes of the induced compaction velocities were usually smaller than the acoustic wave speed in the foam material and caused a non-uniform foam density distribution along the compacted region. A significant density decay measured from the proximal end of the cladding was observed within the compacted foam zone due to a rapid reduction of the compaction velocity. An analytical model was developed to reveal the characteristic features of the foam compaction under a moderate velocity impact. It was shown that waves of strong discontinuity, such as shock waves resulting from an overtaking of the stress increments cannot be initiated in the foam. It was revealed, however, that the stress wave propagation in the form of an unloading plastic wave has a significant dynamic effect on the foam compaction and energy absorption. It was concluded that a quasi-static approach cannot be used to accurately estimate the absorbed energy, at least for partially compacted foam. The predictions of the proposed analytical model are compared with other published analytical models for a foam compaction under a high and low velocity impact and with the results from the numerical simulations of the foam deformation in a finite thickness sacrificial cladding subjected to a blast loading. The numerical model was verified with blast loading tests of 50 mm thick Cymat foam claddings having a density of 253 kg/m³. The influence of the cladding characteristics on the reflected stress from the stationary end of the cladding was also analysed when using the proposed model for foam compaction.