The Modified Gap Test

As acceptors used in underwater tests were 50/50-Pentolite, TNT, 60/40-Cyclotol, PBX 9404-03 and LX-04-1. The results gave thresholds for burning which are comparable in magnitude to Susan and Skid tests, but lower than those obtd by the modified gap test. This showed that burning can be produced at lower shock levels if the pressure were held longer and the wave front made flatter 

In Table 6, reproduced from p 494 of the paper, the pressures, P, of shocks required to produce burning just detectable in underwater test and modified gap test are compared 

Explosive Loading Density g/cc Pb Underwater, kbar Pb Mod Gap, kbar 

Murray J. Plant, A Method for the Study of Solid Explosives and Other Solid (Including Porous) Materials When Subjected to Shock Waves , 4tbONRSympDeton (1965), 355-65. Description is given of a method for determining simultaneously the pressure, density and velocity in shock waves in solid (including porous) materials. The shock was poduced in a cylinder of material using the solid barrier technique which is usually 

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T.P. Liddiard S.J. Jacobs, "initiation of Reaction in Explosives by Shocks , NOLTR 64-53(1965). The "Modified Gap Test , illustrated here in Fig 8, is identical with the test arrangement described in Ref 58, except that two charge set-ups were used in order to conserve light sources. A... 

The following data (Ref 30) show the differences between the pressures required to produce burning or detonation in pressed or cast TNT, using the Modified Gap Test ... 

Note that underwater defloration thresholds are about 1 /3 those in the modified gap tests. Liddiard (Ref 8a) ascribes this to longer duration and more planar shocks in the underwater expts. 

In the past these tests were rather qualitative and one of the chief disadvantages was the lack of knowledge of the pressure transmitted to the acceptor. With the advent of calibration, however, the significance of gap tests was greatly increased. After discussing briefly the work on calibration done by various scientists between the years 1949 1965. Liddiard Price stated that the purpose of their work was to use the improved experimental and data reduction techniques developed in the few years prior to 1965 in order to obtain a calibration with the best data available. The report describes two test assemblies "NOL Standardized Gap Test [Fig 1(A)land "Modified Gap Test [Fig 1(B)]. The "Standardized Test , also known as "LSGT (Large Scale Gap Test), is described in Refs 48 54. For description of "NOL Modified Test , see Refs 59 68 Other modification developed at NOL is described in Ref 52... 

Ref 55b) and for the "Susan Test" (Refs 55a, 61 67). It was of interest to determine if the above threshold can be lowered by applying shocks of longer duration and more nearly plane as used in the NOL modified gap test. In this test the pressure falls rapidly because of the action of strong rarefaction waves and in addition, the shock entering the explosive is quite curved. An experimental arrangement which gives a more... 

Modified Influence (Propagation) Test. Gawthrop (Ref 1) and others used a modified gap test to determine the relative ability of a shielded donor charge to transfer detonation over an air gap to an acceptor charge. Clark (Ref 2) also used this method for the determination of the gap in the transmission of detonation from a small charge (0.5 to 2g) of HE to a cartridge of 40% straight dynamite... 

Modified Gap Test results are given in the following tabulation (which shows that DDNP is an efficient donor) ... 

Comparison of Pressure, P, at Which Burning ic juc+ Detectable in the Underwater and Modified Gap Tests... 

Modified Gap Test for the Determination of the Shock Wave Sensitivity of Igniting and Initiating , Explosivst 20 (7—8), 143—50 (1972)... 

Donna Price T.P. Liddiard, Jr, "The Small Scale Gap Test Calibration and Comparison with the Large Scale Gap Test", NOLTR 66-87(1966), White Oak, Maryland [The SSGT, first described in 1952 by Dim-mock, Jr (Ref 26), has been used in its modified form, described in 1961 by Ayres (Ref... 

A method for interpolation of calculated vapor compositions obtained from U-T-x data is described. Barkers method and the Wilson equation, which requires a fit of raw T-x data, are used. This fit is achieved by dividing the T-x data into three groups by means of the miscibility gap. After the mean of the middle group has been determined, the other two groups are subjected to a modified cubic spline procedure. Input is the estimated errors in temperature and a smoothing parameter. The procedure is tested on two ethanol- and five 1-propanol-water systems saturated with salt and found to be satisfactory for six systems. A comparison of the use of raw and smoothed data revealed no significant difference in calculated vapor composition. 

If the hierarchy of low-temperature/high-frequency (unrelaxed) G values corresponds more or less to the hierarchy of cohesive energy densities, it is completely modified at ambient temperature/low frequency - i.e., in the usual mechanical testing conditions - owing to the importance of the modulus gap at Tp (Fig. 11.5). 

To test these theoretical predictions a number of initiation experiments using both a modified version of the NOL large scale gap test and also the Rotter drop weight impact test have been conducted. The experimental details of these tests have been described in a previous publication (Ref 22). 

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