Back-Face-Signature-Monitored Evaluation of Foam-Sandwich Structures as Shock Mitigating Materials

The purpose of the present work is to understand the effect of positive and negative impedance mismatch on shock amplification/attenuation. For this purpose, emergence of shock from the back face of cellular materials like foams and sandwich composites in contact with higher or lower impedance materials like steel and air, respectively, was studied. Shocks of 10-11 bar incident pressure and 1000-1100 m/s velocity were applied to 0.28 g/cc density foam and foam-GFRP (foam-glass fiber-reinforced plastic) sandwiches. The foam as well as the sandwich deformed plastically under the applied shock. Measurement of stress on the higher impedance steel-end-wall against which the foam/sandwich rested showed shock amplification. To assess the emergence of shock in air posterior to the foam/sandwich, a small axial cavity was created in the end-wall, and pressure changes in the cavity as well as back face signatures in the foam/sandwich overlapping the cavity area were tracked. Back face signature in the form of protrusion was developed in case of bare foam, partially filling the cavity, and rapidly increasing the pressure to the level of the applied shock. However, no back face signatures were developed in the sandwich and no shock formed in the cavity. The present technique shows that shock emergence from screening materials into lower impedance air is mediated through back face signatures. Based on these findings, it can be concluded that shock mitigation can be achieved by creating a negative impedance mismatch in the path of the shock, combined with prevention of back face signature development.