Test Description
Test Purpose
The DoD has used the NOL card gap test as one of the criteria to distinguish whether an explosive material is a 1.1 or a 1.3. Instead of placing the booster right next to the sample, an attenuator is placed between the shock source and the material such that the sample is subjected to a reduced shock event. The attenuation level is varied to determine the 50% probability between detonation and no detonation. For in-process materials, a substance with explosive characteristics is considered a mass explosive (Division 1.1) unless shown to have a detonation sensitivity value of less than 70 cards.
This test is similar to the UN gap test except that a shorter tube of material is used and a varied attenuator is placed between the test material and the pentolite booster. The attenuators provide a reduced shock source. The criteria for a “+” is that a clean hole is punctured through the witness plate. Typically 12 trials are required to determine the 50% “+/-” level. The test determines if a material will propagate an attenuated detonation.
The shock pressure imparted to the sample using 160 grams of Pentolite (Lot 718) is recorded in MIL-STD-1751A Method 1041: NOL Card Gap Test Calibration Data as shown below in the table where 0.01 inch is equivalent to 1 card.
Key Parameters
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Initiating device | Repeatably initiate the booster | #8 cap or equivalent | |
| Booster | Provide a strong, repeatable, stable shock front to the top of the sample | 160 grams of RDX/wax (95/5) or PETN/TNT (50/50), 50 mm diameter, ~50mm length | |
| Gap | Attenuate the shock pressure | MIL-STD-1751A Method 1041: PMMA was initially chosen as the gap material because
Additional advantage of PMMA, particularly over metal gaps, is that PMMA forms no damaging fragments. The latter situation complicates the estimation of the shock wave transmitted from the gap to the test material which has already been pre-compressed by the elastic wave. The disadvantage of PMMA is its viscoelastic behavior and the resultant uncertainty of its relaxation times. Hence, in the low pressure range there is still some uncertainty about whether a pressure lower than the equilibrium value should be used. |
Standard attenuator sizes are 0.010, 0.050, 0.100, 0.250, 0.500, 0.750, 1.00, and 1.50 inches |
| Confining medium | Provide confinement, increasing the susceptibility of the substance to detonation. | Cold-drawn, seamless, mild steel (SAE 1015 – 1040) tube: 47.6mm (1.875-in) OD, 5.6mm wall (36.5mm (1.44-in) ID), 139.7mm (5.5-in) long. NOTE: Same pipe as UN Gap (legacy) or TB 700-2 (2012) |
|
| Substance density | Determine whether the substance in its in-process form is sensitive to the test stimulus | Substance to be tested as near as possible to the in-process density | |
| Spacer | Separate the steel tube from the witness plate | MIL-STD-1751A Method 1041: The small stand-off (0.063-inch gap) between the acceptor and the witness plate was introduced to prevent the witness plate from shattering and thereby facilitate interpretation of test results. | 1.6 ± 0.2mm (1/16-in) thick |
| Witness plate specifications | Evidence of reaction type | MIL-STD-1751A Method 1041: Some laboratories prefer to use witness plates with larger length and width, e.g., 9″ square, to reduce the likelihood of shattering the witness plate. This is an acceptable variation. | Mild steel (e.g., SAE 1015 – 1026) witness plate 6-inch square by 0.375-inch-thick steel |
| Offset | Offset the witness plate from the ground | 2 x 4 in wood blocks at the edge of the witness plate |
Indicators
| Indicators | Detection Method | Assessment |
|---|---|---|
| Damage to the witness plate | Visual post-test inspection | “Neat” hole punched through the witness plate: [IP Division 1.1 if sensitive to detonation when shock pressure is attenuated by greater than 70 cards (70 kbar)] |
Setup Example
Example Photo of Witness Plate Not Holed
Example Photo of Neatly Holed Witness Plate
