Zero Gap Test Description
Test Purpose
The zero-gap test determines whether the material can propagate a detonation from high confinement and defined shock (~200 kbar). If the material can propagate a detonation, it should be classified as an explosive (Class 1.1 if also easily initiated) in a manufacturing operation.
Equivalence
This test is equivalent to UN Test 1 (a) in the UN Manual of Tests and Criteria.
Key Parameters
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Number of trials | Sufficient to ensure a repeatable result | 2 trials | |
| 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 | |
| Confining medium (steel tube) | Provide confinement, increasing the susceptibility of the substance to detonation; evidence of reaction type | UN Gap (new): Cold-drawn, seamless, carbon steel tube 48 ± 2 mm (1.875-in) OD, 4.0 ± 0.1 mm wall (40 ± 2.2 mm (1.5-in) ID), 400 ± 5 mm (16-in) long. UN Gap (legacy): Cold-drawn, seamless, carbon steel tube 47.6mm (1.875-in) OD, 5.6mm wall (36.5mm (1.44-in) ID), 406mm (16-in) long. NOTE: MIL-STD-1751A Method 1041 specifies mild steel (SAE 1015 – 1040); TB 700-2 specifies ASTM 1026 or Drawn Over Mandrel (DOM) |
|
| 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-in square, to reduce the likelihood of shattering the witness plate (an acceptable variation) |
150 ± 10mm (6-in) square and 3.2 ± 0.2mm (1/8-in) thick NOTE: MIL-STD-1751A Method 1041 specifies mild steel (SAE 1015 – 1026); TB 700-2 (2012) specifies ASTM 1020 – 1025 |
| Offset | Offset the witness plate from the ground | 40 x 90 mm (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 | Hole punched through the witness plate: Class 1 |
| Damage to the steel tube | Visual post-test inspection | Complete fragmentation of the tube: Class 1 |
*OR relationship
Example Photo of Witness Plate Not Holed, Pipe Not Completely Fragmented (-)
Example Photo of Holed Witness Plate (+)
