Development of a new Modified Small-Scale Gap Test
ORAL
Abstract
Small-scale gap tests are useful when comparing the sensitiveness of compositions where formulation parameters and manufacturing conditions are varied. Such gap tests are very economical in terms of quantity of material, time and effort.
The current UK small-scale gap test uses a cylindrical Tetryl donor pellet (density 1500 kgm-3) 12.7 mm diameter and 12.7 mm long combined with a brass shim attenuator. The shape of the acceptor charge is optional, the cross section being either square (12.7 mm side) or circular (12.7 mm in diameter). The acceptor length is 25.4 mm. A 25 or 25.4 mm cube of mild steel is used as a witness block. It can accommodate any explosive with a critical diameter less than 12.7 mm.
However, there are issues with the current test: Tetryl is no longer available and the test explosive needs a detonation pressure sufficient to deform the witness block.
In order to address these shortfalls, we have developed a new Modified Small-Scale Gap Test (MSSGT) that does not require a metal witness plate for test sentencing, instead additional instrumentation in the form of contact pins follow the shock in the explosive and determine whether it transits to detonation. Using this technique it is also possible to observe the run distance to detonation for a given attenuator thickness. From a calibration of the gap test donor and attenuator system the shock pressure entering the explosive can be obtained. From knowledge of the shock pressure and the run distance a ‘Pop Plot’ can then be obtained for the test explosive.
It has also been shown that it is possible to extract the unreacted Hugoniot of the test explosive by capturing the free surface velocity of a thin brass plate placed on the end of the charge using Photon Doppler Velocimetry (PDV) for a number of given input pressures.
In this paper, we describe two variations of the gap test that differ in the diameter (20 and 25mm) and the donor explosive as well as test results on a HMX / Viton (90/10) formulation.
The current UK small-scale gap test uses a cylindrical Tetryl donor pellet (density 1500 kgm-3) 12.7 mm diameter and 12.7 mm long combined with a brass shim attenuator. The shape of the acceptor charge is optional, the cross section being either square (12.7 mm side) or circular (12.7 mm in diameter). The acceptor length is 25.4 mm. A 25 or 25.4 mm cube of mild steel is used as a witness block. It can accommodate any explosive with a critical diameter less than 12.7 mm.
However, there are issues with the current test: Tetryl is no longer available and the test explosive needs a detonation pressure sufficient to deform the witness block.
In order to address these shortfalls, we have developed a new Modified Small-Scale Gap Test (MSSGT) that does not require a metal witness plate for test sentencing, instead additional instrumentation in the form of contact pins follow the shock in the explosive and determine whether it transits to detonation. Using this technique it is also possible to observe the run distance to detonation for a given attenuator thickness. From a calibration of the gap test donor and attenuator system the shock pressure entering the explosive can be obtained. From knowledge of the shock pressure and the run distance a ‘Pop Plot’ can then be obtained for the test explosive.
It has also been shown that it is possible to extract the unreacted Hugoniot of the test explosive by capturing the free surface velocity of a thin brass plate placed on the end of the charge using Photon Doppler Velocimetry (PDV) for a number of given input pressures.
In this paper, we describe two variations of the gap test that differ in the diameter (20 and 25mm) and the donor explosive as well as test results on a HMX / Viton (90/10) formulation.
–
Presenters
-
Malcolm D Cook
Syanco Ltd
Authors
-
Malcolm D Cook
Syanco Ltd
-
Andrew D Wood
Syanco Ltd
-
Christopher Stennett
Cranfield University