Shock initiation of granular explosives pressed to low density

The dependence of the shock initiation of pressed granular explosives on density of the pressing, interstitial gas in the pressing, grain size of the explosive, and characteristics of the initiating shock is discussed. The validity of applying the hydrodynamic equations for a homogeneous material to an inhomogeneous assembly of particles is examined. Various mechanisms for the initiation of granular explosives are compared in terms of current knowledge. Initiation of low density explosives is not satisfactorily explained by uniform shock heating of the pressing nor by shock heating of the interstitial gas, as has long been thought. It appears that initiation is dependent on minute shock interactions and resulting spalling and jetting, with initiation occurring in the fine particles of the spall or when the products of spalling or jetting are stagnated at the surface of an intact grain. Several experiments are suggested with plane wave constant pressure shocks replacing the complicated geometry of the usual gap sensitivity test. Other experiments are suggested for studying small scale details of proposed mechanisms of initiation.