On the effect of grain size on shock sensitivity of heterogeneous high explosives

Analysis of available data on dependence of the critical detonation diameter \(d_{cr}\) of various heterogeneous condensed explosives on mean size of grains and voids demonstrated that in many cases surprising correlations between \(d_{cr}\) and the initial specific surface area of heterogeneous explosives \(A_o\) exist, namely, \(d_{cr}=\alpha _1+\alpha _2/A_o\) or \(1/d_{cr}=\beta _1+\beta _2A_o\). The run distance to detonation in wedge test with sustained strong shock of constant amplitude also linearly correlates with \(1/A_o\), i.e. \(L_{P={\rm Const}}=\gamma _1+\gamma _2/A_o\). At the same time, the shock sensitivity reversal effect is often observed when grain size of HE is reduced. Apart from that Moulard (1989) found that detonation critical diameter of plastic bonded explosive with mono- and bimodal RDX grain size distribution depends nonmonotonously on mean grain size. Complicated dependence of shock sensitivity of heterogeneous explosives on their specific surface area can be explained based on comparison of the critical hot spot size \(a^{*}(P)\) at given characteristic pressure behind shock wave \(P\) with the mean heterogeneity size \(\overline{a}\). At high characteristic pressure (relative to the critical ignition pressure) \(a^{*}\) is small compared with \(\overline{a}\) and all specific surface area of heterogeneous explosive is available for the hot spot growth process in accordance with the grain burn concept. However, when characteristic pressure of shock wave decreases, \(a^{*}(P)\) increases and can become comparable with \(\overline{a}\). In this case only relatively large potential hot spots \((\)with size \(a>a^{*})\) can result in self-supported hot spot growth process and shock sensitivity is controlled by the specific surface area which corresponds to only larger heterogeneities and can be significantly smaller than initial specific surface area.