Friction Sensitivity Apparatus

The friction testing of explosives is not as common as impact testing in the U.S. It is difficult to devise a test which delivers a simple frictional stimulus without also impacting the sample and indirectly heating it by contact with other sliding components of the apparatus. Friction tests even then commonly do not produce reactions in any but the most sensitive explosives. Brown [77] found that there were many varieties of friction sensitivity test, but that they could be grouped into three categories  

Those which shear a thin layer of explosive between two rigid plates of steel or other materials of construction some impart linear, or single-pass, motions and some impart rotary and continuous motion  

Those which rub a block of explosive violently on a hard or abrasive surface  

Those which subject a sample to extreme deformation in an impact or extrusion event. 

Only a few tests fit the last two categories the only test that fits the last description is the Susan test [12], in which one pound of explosive in a special Projectile is driven against a hard target at selected velocities. It is really a form of impact test and is used only with secondary explosives. 

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Armour, Carl and Smith, Lloyd A. The Invention of a New Type of Friction Sensitivity Apparatus," ROTR No. 60, 11 June 1965, U.S. Nava Arrmumtion Depot, Crane, IN... 

Kinoshita and Arimura [84] utilized a Yamada-type friction sensitivity apparatus [85] to test combinations of explosives and materials in designing manufacturing equipment. Tests were conducted using friction test pieces of emery, ebonite, bakelite, hardened vinyl, copper, brass, iron, lead, aluminum, wood, polyethylene, and hard rubber. Some of the results are listed in Table XVII. 

The BAM Friction Sensitivity Apparatus is shown in Figure 2.11. The operation mode of the apparatus is as follows. Friction is created electromechanically between the cylindrical porcelain pistil and the plate bearing the sample. The pistil and the plate are of definite roughness. The size and the shape of the plate and the pistil are shown in Figure 2.12. 

Figure 2.11. BAM friction sensitivity apparatus (JULIUS PETERS KG, Berlin, Germany)...

Pendulum, Friction, Apparatus. See under Friction Sensitivity Tests in Vol 6, F204-L to F206-L... 

The chemistry involved in this explosively unstable system is reviewed [1]. The mechanism of the trigger reactions that initiate the exothermic decomposition of chlorate-sulfur mixtures has been studied. Mixtures containing 1-30% of sulfur can decompose well below the m.p. of sulfur, and addition of sulfur dioxide, the suspected chemical trigger, causes immediate onset of the reaction [2], Autoignition of stoichiometric mixtures can be as low as 115°C, with frictional sensitivity at 5N, the lowest load the test apparatus permitted. Both were dependent upon the history of the sulphur used [3],... 

Determination of friction sensitivity is applicable to solids, pastes, and gel-type substances. To determine the friction sensitivity, a thin sample is placed under a load between two roughened surfaces, and the surfaces are then rubbed together in a controlled manner. The load can be varied. Results from this action, such as smoke, cracking, or discoloration, are observed. Examples of apparatus of this type are the BAM friction apparatus, shown in Figure 2.30, the rotary friction test, and the ABL friction test. 

The truly unnerving characteristic of HMTD was its sensitivity to friction. Earlier, the test apparatus used to determine friction sensitivity was briefly described. Usually, some form of pressure needs to be exerted on the wheel in order to create enough frictional force to initiate an explosive. In the case of HMTD, a sample could be placed on the plate and the wheel placed over it with no pressure exerted at all. The minute the plate was slid under the wheel, the HMTD detonated. 

Friction sensitivity >350N for purified FOX-7 (RDX-120N) as recorded on a Julius Peters apparatus. 

Friction sensitivity on Julius Peters apparatus Highly sensitive (exploded at 50 g load) Less sensitive (did not explode even at 200g load)... 

Friction sensitivity by J.P. Apparatus Does not function at 300gm but functions at 350 gm Does not function at 150gm but functions at 200 gm... 

The determination of friction sensitivity can be carried out using different types of apparatuses discussed at length in the literature which are briefly outlined here. 

Friction Sensitivity can be detd by pendulum Friction Apparatus, as briefly described in Vol 1 (Ref 9> PP XIII XIV) and given in detail in BurMines Bull 346 (Ref 5)> pp79-84... 

Most friction tests are those in the first category. The explosive shear between two srrrfaces, which may be smooth, rough, or covered with abrasive. If the two surfaces come in contact before or at the same time as acting on the explosive, they can be heated sufficiently to cause ignition by simple thermal mechanisms. This possibility has led some to question whether the tests correctly characterize friction sensitivity in this view the apparatus are merely crude heat machines. 

Tests were also conducted to determine the effect of temperature on the friction sensitivity of the same materials [16]. None of the primary explosives revealed any marked temperature effect however, the fact that no large temperature effect was observed led Copp et al. to conclude that the action of the apparatus was mechanical rather than thermal. 

Table XVII. Results of Friction Sensitivity Test Utilizing Yamada Friction Apparatus [85]...

As with the impact sensitivity, one of the major difficulties in assessing friction sensitivity lies in the lack of knowledge of the mechanisms and influences to which the explosive is subjected during "friction" tests. This, combined with limited data on the intrinsic properties of the explosive, results in the crudest measures of sensitivity and potential hazards being available. The various friction tests attempt to simulate specific hazards that may be encountered in practice, but the data obtained are pertinent only in comparison with those for other materials using the same apparatus under the same conditions. 

Attempts to correlate the data from various types of apparatus have not been successful, and again a paucity of information on the properties of azides and the action of the stimulus has prevented the development of a universal standard test. To progress from the barely qualitative determinations of friction sensitivity... 

Example of very simple and very inexpensive friction sensitiveness screening apparatus... 

Friction sensitivity (Julius Peters Apparatus) Insensitive upto 8 kg Insensitive upto 36 kg... 

Table 9.1 Impact (drop-hammer machine, 2.5 kg hammer, 50 % probability) and friction sensitivity (BAM apparatus, 1 event in 10 trials) of powdered CP [6]...

There are two versions of the friction test apparatus in operation, a standard size apparatus and a small size version. The small device is particularly designed to test sensitive materials such as primary explosives. It can be operated with different weights on the porcelain bolt holder allowing loads in the range of 0.1 to 10 N. On the standard BAM friction test apparatus higher forces on the pin varying from 5 to 360 N can be applied. 

Two types of results are obtained from mechanical sensitivity tests (1) no reaction, or (2) decomposition with or without an explosion. The magnitude of friction and the impact sensitivity reported is the smallest load at which a positive result has been noted. The objective of mechanical sensitivity testing is to establish whether or not the substance is sensitive under normal handling conditions. However, this objective may not be reached. The test results may not truly reflect process conditions because most testing is carried out at ambient temperature and pressure. Since results are dependent on the type of test apparatus used, the interpretation of the results for use in practical applications requires much experience in this field of testing. 

H. Kast (104-07) Determination of Sensitivity to Friction (Medicion de la sensibili-dad al rozamiento) (107) Determination of Sensitivity to Initiation by Detonation (110-12) Determination of Sensitivity to Initiation by Influence (112-13) Determination of Power of Explosives using Trauzl Test (113-17), Small Lead Block Test (117), Quinan Apparatus (118), Guttmann Apparatus (118-19), Ballistic Pendulum (119-20), Mortar (Mortero probeta) (120-21) Determination of Efficiency of Initiating Devices by Lead Plate Test (121-23), Nail Test (123), Sand Test (124) and Acoustic Tests (124) Determination of Characteristics of Flames Produced on Explosion (125-29)... 

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