Witness plate

In general, detonation test apparatus consists of a steel tube that is filled with the substance under investigation. One end of the tube is provided with a booster charge consisting of an electric detonator covered by detonative material. The other end is either closed or provided with a witness plate. One type of steel tube apparatus is provided with a velocity probe to record the shock wave velocity as shown in Figure 2.28. 

Tests must be performed in a bunker. The booster charge is detonated and, after the event, observations are made of the fragmentation pattern of the tube and/or the condition of the witness plate. 

In the steel tube test, the fragmentation pattern is a measure of whether or not a substance detonated, for example, a substance is capable of detonation if the tube is completely fragmented. The velocity of the reaction front is also a measure of detonation, that is, a detonation has occurred if this velocity is higher than the velocity of sound in the substance. When a witness plate is used, a substance is said to be capable of detonation if either a hole is punched through the plate or the tube is fragmented along its entire length. 

A mild steel "witness plate served as the criterion of "detonation". A test was considered positive if a hole was punched. As a measure of charge sensitivity,served the length of gap at which there is 50% probability according to the above criterion. Results of tests are given in the report... 

MaJfek is similar to the "NOL Gap Test Assembly", previously described in NOL Reports such as ItfavWepsRept 7401 (1961) (Our Ref 48). It is shown here as Fig 4. Tetryl pellets served as a donor, the gap consisted of several cellulose acetate cards 0.25 mm thick, the acceptor was unconfined and the witness plate was of steel. For given donor and gap materials and for fixed geometries of both the donor and the acceptor, the gap thickness which gives the marginal shock strength, called the "gap value"... 

Fio 4 The Naval Ordnance Laboratory gap test for solids. The gap consists of a varying number of cellulose scetate cards, 0.25 mm. thick. The witness plate is steel... 

The bottom of tube was placed on an Al witness plate, 0.5-inch thick, while on the top of pellets was placed a Tetryl booster (0.5-inch diam and 0.5-inch long) connected with a No 8 detonator and a safety fuse. 

Detonation as evidenced by hole punched in witness plate... 

Witness Plates (Plate Denting Tests). See under Plate Denting Tests in Vol 1, XIX XX, under Brisance in Vol 2, B266 to B295 (Table... 

His results are presented in the form of output energy (dent in witness plate) vs input shock (exit pressures in the barrier) curves. His curve for RDX, Fig 4, agrees with the writer s observation that coarse RDX is easier to initiate by shock than fine RDX... 

Probably the most frequently used shock sensitivity test is the so-called gap test, in which the shock generated by a standard donor is attenuated in an inert barrier before it enters the test expl (acceptor). Shock strength is varied by varying the thickness of the inert barrier, and results of the test are gaged by terminal observation of the effects of the acceptor on the witness plate or block on which it is placed... 

Bartels (Ref 39) has described a gap-test which he claims is applicable to measuring the shock sensitivity of expls as well as potentially explodable pyrot compns. This test differs from the NOL SSGT (Fig 4) in that confinement of the acceptor is much reduced and w is used as the inert barrier. Instead of a witness plate Bartels used an A1 witness-rod ... 

Fig. 1.18e Witness plate of SSRT of lead azide (left) and K2DNABT (right).

A modified Small Scale Shock Reactivity Test (SSRT) was performed with K2DNABT as well as with lead azide for comparison. In these tests, 500 mg of each compound was ignited by an igniter and it was found that I<2DNABT showed more indentation than lead azide on the witness plate (Fig 1.18e). 

The PDV diagnostic measures the accelerating witness plate, which serves as the primary measure of explosive performance for DAX. A very detailed analysis of the plate velocity allows accurate estimates of the CJ pressure, product gas energies and JWL EOS s. 

Measurements of airblast overpressure and impulse were made at 12 gage locations along a double blast line (Fig k). The gages were spaced at selected scaled distances ranging from approximately 2-20 ft/lbsl/3. The pressure transducers were installed flushed with the top surface of a concrete slab in mechanically isolated steel plates. The test item was placed on a steel witness plate located on the surface of the slab. Fastax motion pictures were taken of all tests. 

Similar to Df, but for plates or slabs of explosive, there is a failure thickness that can and has been measured. These experiments, to determine failure thickness, are run on tapered explosive wedges initiated at the thicker end. The tests are conducted using a brass witness plate to indicate where failure occurred. 

The witness plate has also been used to determine the effect of varying the density and the quantity of lead azide in typical detonators [33]. Figure 16 illustrates the effect of lead azide quantity on a small detonator of the type shown in Figure la, but of dimensions 0.147 in. (0.373) diam and 0.143 in. (0.363 cm) long. The most significant consequence of reducing the weight... 

In 1917 Wohler and Martin [37] compared the minimum quantities of silver and lead azides required to initiate various secondary explosives of importance at that time, and their tests were conducted in blasting caps using lead witness plates. Abbreviated details of their findings are also given in Table VII. 

After the initiation of the nitrcguanidine prc >ulsion charge, the steel plate is propelled at a velocity of 122 m/s. Depending on the sensitivity of the explosive, the impact of the steel plate can cause its initiation. Whether or not the initiation has occurred m be concluded on the basis of (tent depth or other detectable damage in the steel witness plate. 

The test is conducted in the following manner 200 mg of hexogen are press-loaded at 1115 kp/cm in a polymethylmetacrylate (PMMA) holder. Above it, a primary explosive is filled in the state at which its initiating strength is to be tested freely poured or pressed to a desired density. The initiation of the primary explosive is performed by means of a safety fuse. If the depth of the dent produced after initiation in the steel witness plate of 70-90 Rockwell B hardness is greater than 0.76 mm, the complete detonation of hexogen is... 

The comparative initiating strength of different types of primary explosives may be obtained in a similar way—on the basis of the dent depth produced in the steel witness plate after the detonation of the primary explosive alone. For this, 300 mg of primary explosive alone is press-loaded at 1115 kp/cm into a PMMA sample holder, wliich is then placed on the steel witness plate. The initiation of the primary explosive is performed by means of the safety fuse. The depth of the dent produced in the steel witness plate after the detonation of the primary explosive is connected with its initiating performance and serves for the comparative evaluation of the initiating strength of different types of primary explosives. 

Another cap sensitivity test was developed in Germany (cited in Kohler and Meyer, 1993). According to this test, a high explosive is filled into a 200 mm long cardboard tube having an inside diameter of 80 mm. The determination of completion of the detonation is accomplished on the basis of the clean hole cutting in the 1 mm thick steel witness plate. 

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