On the response of ballistic soap to one-dimensional shock loading

The effects of projectile penetration into soft tissue are often studied using the tissue simulant ballistic soap. Consequently, a full understanding of the high strain-rate response of ballistic soap is desirable. Using the plate-impact technique, key shock parameters have been measured for impacts in the range 81-968 m/s, allowing derivation of the Hugoniot equation-of-state for soap in the US-uP and P-uP planes. A linear Hugoniot relationship was found in the US-uP plane, with the general form US = 2.05 + 1.75uP (?0 = 1.107 g/cc). In P-uP space an apparent strengthening phenomenon was observed, with the Hugoniot trending above the hydrodynamic response at impact stresses greater than 1.2 GPa. Embedded lateral manganin stress gauges were employed to interrogate the evolution of lateral stress within the soap. A gradient in lateral stress, whose magnitude increased incrementally with impact stress, was apparent behind the shock for ?x > 1 GPa. This appeared to suggest a continuation of the previously highlighted strengthening mechanism. It is proposed that at higher values of ?x, increased compression of hydrocarbon chains acts to increase the material's resistance to shear, a phenomenon consistent with steric hindrance.