Re-visiting the secondary shock

The secondary shock, caused by collapse and re-expansion of an inward-facing shock originating at the edge of the fireball, is a regular feature in blast trials data and the output from numerical models. It has also been postulated that, since the secondary shock travels through the detonation products, it contains important information about the state of the explosive gasses shortly after detonation. Despite this, the secondary shock is often overlooked, and there is, as yet, no established method for predicting its form and magnitude. The current authors recently developed such a method, based on superposition of the UFC positive and negative phase loading from the original charge mass, plus an additional positive phase modelled using a smaller charge at some fraction of the original charge mass (at the same distance). This "dummy charge mass" has been calibrated from 76 experimental tests using 180–350g of PE4, PE8, and PE10 plastic explosives, and is typically around 4–5% of the original charge mass. This paper provides an overview of the method and introduces new data from smaller scale testing (1–3g PE10), as well as additional diagnostics such as high speed video.