Testing Explosives

English

As new explosive compounds were created, it became evident that there was a need for a way to compare them. Explosives differ in brisance (the ability to shatter something), heave (the amount of material an explosive can move), sensitivity to heat, shock, or friction, and things like melting point, detonation velocity, etc.

Developing standards for these aspects of explosives allowed customers to specify performance benchmarks, and vendors to advertise the advantages of their wares.

Isidor Trauzl, born in Hungary, was an Austrian Army captain, chemist, and explosives expert. In 1870, he established the first Austrian dynamite factory, with Alfred Nobel, and led the company in the years 1882 to 1892.

Trauzl

Trauzl is remembered today for the eponymous Trauzl lead-block test for explosives. It is a way of comparing two explosive mixtures or compounds by measuring how much they can expand a cavity in a lead block.

In the test, a cylindrical block of lead, 20 centimeters high and 20 centimeters in diameter, has a hole drilled in the center of the flat face that is 2.5 centimeters in diameter and 12.5 centimeters deep. Into the hole is placed 10 grams of the test explosive, and a standard number 8 blasting cap. The hole is then filled with fine sand (tamped), and the explosive is set off.

The hole in the lead block expands due to the explosion. Water is poured into the cavity, and then the water is poured into a graduated cylinder to measure the volume. The original volume of the hole is subtracted to get the volume of the expansion. Some part of the expansion is due to the blasting cap, and this is subtracted from the total as well (as determined by setting off the blasting cap with a non-explosive 10 grams of dummy material in a similar block).

Trauzl block

Some part of the explosive energy goes into heating the lead block and the sand. Some part goes into blowing away the sand. Some part escapes as the heat of the gases after they leave the block. So what the test measures is not the total energy of the explosive, but the work it does on the lead block.

Frequently the volumes will be standardized to a particular explosive, such as TNT or Amatol, and the results given in percent of TNT. Both are shown below.

Nitroglycol

610

203.33%

Methyl nitrate

600

200.00%

Blasting gelatin

600

200.00%

Nitroglycerin

530

176.67%

PETN

520

173.33%

RDX

483

161.00%

Nitromethane

458

152.67%

Gelignite

430

143.33%

Ethyl nitrate

422

140.67%

Tetryl

410

136.67%

Nitrocellulose

373

124.33%

ANFO

316

105.33%

Picric acid

315

105.00%

Trinitroaniline

311

103.67%

Trinitrotoluene

300

100.00%

Urea nitrate

272

90.67%

Guanidinium nitrate

240

80.00%

Ammonium nitrate

178

59.33%

 

Other tests followed later. The steel dent test measures the depth of a dent made in a steel plate by 10 grams of explosive placed in a strong steel cylinder sitting on top of the steel plate.

The sand crush test is done by sifting sand through a sieve, and retaining the sand that did not pass through. A small charge of the explosive is then detonated in the sand. Screened once more, this time measuring the weight of the sand that has been crushed enough to fit through the sieve. This is designed to be a measure of brisance. Results are often normalized to a standard explosive, such as TNT.

The ballistic mortar test measures the swing in a pendulum holding a heavy short-nosed mortar. The similar ballistic pendulum test measured the swing of a pendulum when the explosive is detonated next to it. Both tests are prone to experimental variables that are difficult to remove or account for, making comparisons from different laboratories problematic.

In the cylinder test, high-speed photography is used to measure the radial velocity of a metal cylinder filled with the explosive. This measures to some degree the detonation velocity, and the effects of the explosive on the metal case.

The airblast test detonates the explosive in the open, and sensors record the air pressure at different distances from the explosion.

In the drophammer test, 35 milligrams of the explosive is placed on the anvil of the drophammer apparatus. A striker is then placed on the explosive. A 2.5-kilogram weight is then dropped from different heights onto the striker. A microphone records the sound, and detects an explosion by comparing the sound to the sound of the machine when no explosive is in it. The height that will detonate fifty percent of the samples is designated the DH50 value for that explosive. Often the peak height at which no explosions occur is also recorded.

To prevent rebound, the weight is sometimes made as a hollow steel container filled with lead shot. The device is recalibrated every 20 tests by testing with PETN and Composition B.

The drophammer test is a test of the sensitivity of the explosive to impact. For primary explosives, too low or too high a value will be reason to disqualify the compound. For secondary explosives, high values mean the compound is safer to use, but may require booster explosives (such as PETN) in the primer cap.

Some typical results are shown below.

PETN

15±3

HMX

31±4

RDX

37±6

Composition B-3

50±4

TNT

61±9

 

 

The BAM Friction Sensitivity Test Machine was originally built by the German Bundesanstalt für Materialprufung, which is how it got the name that sounds so appropriate in English — the BAM test.

The device holds a moveable porcelain plate against a fixed porcelain pin. A sample of explosive is placed on the porcelain plate next to the pin. A lever with weights presses down on the pin, allowing masses from half a kilogram to 36 kilograms to press the pin against the plate. The machine slides the plate and sample under the pin a distance of one centimeter.

BAM test

Successive weights are added until the explosive detonates from the friction. The lightest weight that will cause a detonation is the value used. Thus, lower values mean the explosive is more sensitive to friction. A "Go" reaction is anything that can be heard, seen, or smelled. Ten tests are done, and when one in ten tests results in a reaction, that weight is used as the value for that explosive.

Tests are reported either as 1/10 (meaning one explosion out of ten tests) and a weight, or the value 0/10 and a weight of 36 kilograms, indicating the explosive never exploded at the maximum weight.

Some typical results:

ANFO

0/10 36 kg

Black powder

0/10 36 kg

CL-20

1/10 6.4 kg

Composition B

1/10 4.8 kg

HMX

1/10 11.6 kg

Nitrocellulose

1/10 12.0 kg

RDX

1/10 12.4 kg

PETN

1/10 6.4 kg

TNAZ

1/10 11.6 kg

 

The Gap Test (or Zero Gap) places the sample in a steel tube in front of a steel "witness plate", and sets it off with a Pentolite booster charge of known strength. Changes (damage) to the tube and the witness plate are noted.

There are a number of tests that measure thermal stability. One such test is the Simulated Bulk Auto-ignition Temperature or SBAT test. Samples are put into test tubes with thermocouples to monitor their temperatures. All of the samples are then put into a heated metal bath kept at a constant temperature. The temperatures are recorded as the samples heat up. Active samples heat up more than inactive reference samples.

This tests more than just the auto-ignition time (how long it takes the compound to explode or burn at a particular temperature). By monitoring the temperature over time, changes in phase (solid to liquid, liquid to gas) can be tracked. Endothermic mixtures (which heat more slowly than inactive reference materials) and exothermic mixtures (those that produce heat) can be distinguished.

Similar tests heat the sample in a sealed steel tube at a constant rate (e.g. 15° Celsius per minute) until it ignites. Fragmentation of the steel tube is noted. Tests such as these are called "cook-off" tests. Some example data from such a test:

PBXN-109

5.12 hours

169.8° C

1 fragment

LX-10 sample 1

15.82 hours

205.3° C

9 fragments

LX-10 sample 2

13.23 hours

197.1° C

7 fragments

 

The first LX-10 sample in that test was given 10% air headroom (called ullage). The second sample was the same material, but without the extra space.

During these tests, observers note any discoloration, melting, outgassing or smoking.

Still other tests are conducted with similar apparatus to monitor how a material behaves when in contact with a test material, such as a prospective packaging, or weapon material, or transport vehicle material.

Many of these tests are calibrated using some standard explosive, and then reported as a percent of the standard. For many years, TNT was the standard, but more recently, RDX has been used. This metric is reported as the Figure of Insensitivity. RDX itself has a Figure of Insensitivity of 80 on the TNT scale (where TNT is 100).

To measure detonation velocity, a number of approaches have been used. In one method, a piece of resistance wire is placed inside an aluminum tube set next to the explosive. When a length of explosive placed alongside the tube is detonated (from one end), the resistance decreases as the detonation front crushes the aluminum tube against the resistance wire from one end of the tube to another. An oscilloscope is used to record the resistance change over time.

A more modern method uses optical fibers placed a known distance apart on a length of explosive. The charge is detonated from one end, and the time between flashes seen by the optical fibers is noted. Older forms used electrical sensors instead of fiber optic cable.

Detonation velocities are often given in millimeters per microsecond, since the equipment measures millimeter lengths and microsecond or nanosecond times, but this gives the same number as kilometers per second (just multiply each by one million).

An old method, the Dautriche Method uses detonation cord placed from one end to the other of a length of the explosive to be measured. The middle of the cord is placed on an aluminum or lead plate. The explosive is detonated from one end, and the detonation cord starts detonating at that end. The detonation wave in the sample under test then reaches the other end, and starts that end of the detonation cord exploding. The place where the two racing detonation waves in the detonation cord meet is recorded by the metal plate. Knowing the speed of detonation of the detonation cord allows the tester to calculate the speed of the wave in the sample under test.

High-speed photography of the explosion can give a direct measurement of the detonation velocity.

Sandia National Laboratories developed a system where a coaxial cable (a wire inside a tube) is connected as a delay element in an electronic oscillator. As the cable was crushed by the detonation shock wave, the frequency of the oscillator increased (because the delay is shorter as the tube is collapsed). This was used to measure the shock wave velocity in nuclear weapons tests.

A similar idea is behind another type of sensor. In a coaxial cable, any pulse reaching the open end of the cable is reflected back. An instrument sends a pulse down the cable and times how long it takes to see the echo. As the cable gets shorter, less time is needed for the echo to return. This does not depend on the cable collapsing and shorting out. Instead, the cable is progressively disintegrated. At the detonation front, the hot plasma conducts electricity, so it looks like a short circuit. But since a short circuit or an open end both reflect the pulse, this method is very reliable.

Both of these methods, like the resistance wire and photographic methods, gives a graph of the detonation velocity over time, instead of a single measurement.

Detonation velocity is affected by the confinement (and thus pressure) of the explosive. Some of the above methods can be used to measure detonation velocities inside boreholes in rock, to get a more accurate measurement of real-world conditions than laboratory testing can give. The detonator is placed at the bottom of the borehole, connected to wires leading up to the surface. Probes (optical fibers, coaxial cables, or electrical sensors) are placed at locations along the borehole, with cables leading to measuring equipment at the surface.

Resistance wire sensors can be as simple as insulated resistance wire twisted together. The detonation wave plasma shorts them out as it travels up the borehole. Equipment at the surface records resistance changes over time. The slope of the recorded line is the detonation velocity.

The detonation velocity is one of the tests that provides a consistent dataset to high resolution. Combined with the density of the explosive (a measurement very easy to make), the so-called CJ pressure (Chapman-Jougnet pressure) of the detonation can be calculated. This value (the CJ pressure) is one of the better metrics to use when comparing explosives, so many lists of explosives give both the density and the VOD (velocity of detonation).

Gap tests are used to tell how easily an explosion in one munition will cause an explosion in a nearby munition. Knowing this information allows supplies to be stacked at a sufficient distance that an accident in one will not cause the rest to be lost. Since space is often at a premium (especially at sea or in the air), knowing how closely munitions can be stored is important. In laying a minefield, if the explosives are too close to one another, all of them might go off, when only one is triggered. This is not considered optimal, unless you are on the other side, engaging in mine clearing operations.

In civilian mining operations, the separation of blast holes from one another must be carefully considered so as to reduce the likelihood of sympathetic detonations. The bores need to be close to one another to properly fracture the rock, but not so close that they set one another off. Each blast should be initiated by a blasting cap and nothing else, or excessive ground shaking and ejected material can result. Relief holes can be bored between explosive-filled holes to hinder the detonation wave propagation through the rock.

In the gap test, an explosive, called the donor, is detonated next to the explosive under test, called the receptor. A gap between the explosives can be air, or some material such as metal or concrete, that is being tested for resistance to sympathetic detonations. The distance between the explosives when 50% of the receptor charges detonate is the figure of merit.