Initiation of detonation during gap testing of liquids.

Streak-camera observations have shown that wall interactions are frequently responsible for shock initiation of nitromethane in gap-test geometry. Therefore, pressure calibrations of the gap test for liquids made at the attenuator surface along the charge axis should not be used to interpret the test results. Uncritical acceptance of gap-test values for a series of liquids with different properties may lead to erroneous sensitivity ordering. On the other hand, modified gap-test experiments can be used to obtain fundamental initiation data if appropriate instrumentation shows that wall effects are absent. A valid series of reaction time measurements is presented for nitromethane. Experimentally valid initiation data must be interpreted in terms of divergent reactive flow. G AP tests 1 for determining the sensitivities of liquids to shock initiation of detonation have excellent reproducibility, and, moreover, they are simple and economical. The excellence of the reproducibility suggests that it is worthwhile to interpret gap-test data in terms of shock pressure of initiation. Shock pressures, in principle, can be utilized to compute shock temperatures if additional thermodynamic data are available2"5; temperatures, in turn, allow one to discuss chemical reaction rates in initiating shocks. Such information, rapidly available from an experiment as simple as the gap test, would be very useful in studies of the mechanism of shock initiation and ultimately in studying the relation between shock sensitivity and chemical structure. However, there is reason to suspect that a fundamental interpretation of gap-test data can be made only if detailed three-dimensional processes occurring in this somewhat complicated geometry are completely understood. Sensitivity data from gap-test experiments are often reported in terms of the number of cards (usually cellulose acetate sheets 0.010 in. thick) making up the attenuator. The number of cards or the total thickness of plastic is reported as the "50% failure thickness." Thickness is converted to pressure in the attenuator by measuring the shock and free-surface velocities along the axis of the donor-attenuator system, or by measuring only the shock velocity if the Hugoniot of the attenuator material is already known. With some difficulty, attenuator pressure may be measured directly with a gage. 6'7 The entering pressure in the liquid can be calculated from the attenuator pressure by solving the interface equations if the shock velocity in the unreacted liquid is known. The experimental measurements, on which the entering shock pressure depends, are made at the center of the attenuator; thus, it is implicitly assumed that whether the liquid detonates is decided along the charge axis. In this note, experiments are reported which show that conditions on the charge axis do not necessarily control initiation; on the contrary, when the initial shock is too weak to cause detonation directly, initiation occurs at the container walls. The assumption that the axial peak pressure entering the liquid initiates a homogeneous thermal explosion leads to confusing inconsistencies that preclude fundamental interpretation, and in some cases, result in erroneous sensitivity ordering of liquid explosives. The liquid explosive used in these experiments was technical grade nitromethane (NM) manufactured by Commercial