This method includes a set of ballistic shock tests generally involving momentum exchange between two or more bodies, or momentum exchange between a liquid or gas and a solid performed to:

 

a. It provides a degree of confidence that materiel can structurally and functionally withstand the infrequent shock effects caused by high levels of momentum exchange on a structural configuration to which the materiel is mounted.

b. experimentally estimate the materiel's fragility level relative to ballistic shock in order that shock mitigation procedures may be employed to protect the materiel’s structural and functional integrity.

 

 

Photo: Turkish FNSS Kaplan Medium Tank deployed by Indonesian Army Firing Its Main Turret

 

Ballistic shock is a high-level shock that generally results from the impact of projectiles or ordnance on armored combat vehicles. Armored combat vehicles must survive the shocks resulting from large caliber non-perforating projectile impacts, mine blasts, and overhead artillery attacks, while still retaining their combat mission capabilities. Actual shock levels vary with the type of vehicle, the specific munition used, the impact location or proximity, and where on the vehicle the shock is measured. There is no intent here to define the actual shock environment for specific vehicles. Furthermore, it should be noted that the ballistic shock technology is still rather limited in its ability to define and quantify the actual shock phenomenon.

 

Even though considerable progress has been made in the development of measurement techniques, currently used instrumentation (especially the shock sensing gages) is still bulky and cumbersome to use. The development of analytical (computational) methods to determine shock levels, shock propagation, and mitigation is lagging behind the measurement technology. The analytical methods under development and in use to date have not evolved to the level where their results can be relied upon to the degree that the need for testing is eliminated. That is, the prediction of response to ballistic shock is, in general, not possible except in the simplest configurations.

 

 

Photo: Turkish OTOKAR Altay Main Battle Tank (MBT) Firing its Turret with Visible Projectile

 

When an armored vehicle is subjected to a non-perforating large-caliber munition impact or blast, the structure locally experiences a force loading of very high intensity and of relatively short duration. Though the force loading is localized, the entire vehicle is subjected to stress waves traveling over the surface and through the structure. In certain cases, pyrotechnic shocks have been used in ballistic shock simulations. There are several caveats in such testing.

 

Ballistic shock is a physical phenomenon characterized by the overall material and mechanical response at a structure point from an elastic or inelastic impact. Such an impact may produce a very high rate of momentum exchange at a point, over a small finite area or over a large area. The high rate of momentum exchange may be caused by the collision of two elastic bodies or a pressure wave applied over a surface. General characteristics of ballistic shock environments are as follows:

 

 

• near-the-source stress waves in the structure caused by high material strain rates (nonlinear material region) that propagate into the near field and beyond;

• combined low and high frequency (10 Hz – 1,000,000 Hz) and very broadband frequency input;

• high acceleration (300g – 1,000,000g) with comparatively high structural velocity and displacement response;

• short-time duration (<180 msec);

• high residual structure displacement, velocity, and acceleration response (after the event);

• caused by (1) an inelastic collision of two elastic bodies, or (2) an extremely high fluid pressure applied for a short period of time to an elastic body surface coupled directly into the structure, and with point source input, i.e., input is either highly localized as in the case of collision, or area source input, i.e., widely dispersed as in the case of a pressure wave;

• comparatively high structural driving point impedance (P/v, where P is the collision force or pressure, and v the structural velocity). At the source, the impedance could be substantially less if the material particle velocity is high;

• measurement response time histories that are very highly random in nature, i.e., little repeatability and very dependent on the configuration details;
• Shock response at points on the structure is somewhat affected by structural discontinuities;
• The structural response may be accompanied by heat generated by the inelastic impact or the fluid blast wave;
• The nature of the structural response to ballistic shock does not suggest that the materiel or its components may be easily classified as being in the "near field" or "far field" of the ballistic shock device. In general, materiel close to the source experiences high accelerations at high frequencies, whereas materiel far from the source will, in general, experience high acceleration at low frequencies as a result of the filtering of the intervening structural configuration.

 

Procedures Of This Method Are As Follows:

 

 

 

Procedure I – Ballistic Hull and Turret (BH&T), Full Spectrum, Ballistic Shock Qualification. Replication of the shock associated with ballistic impacts on armored vehicles can be accomplished by firing projectiles at a "Ballistic Hull and Turret" (BH&T) with the materiel mounted inside. This procedure is very expensive and requires that an actual vehicle or prototype to be available, as well as appropriate threat munitions. Because of these limitations, a variety of other approaches are often pursued.

 

Procedure II – Large Scale Ballistic Shock Simulator (LSBSS). Ballistic shock testing of complete components over the entire spectrum (10 Hz to 100 kHz) defined in Table 522.1-I and in Figure 522.1-1 can be accomplished using devices such as the Large Scale Ballistic Shock Simulator (LSBSS). This approach is used for components weighing up to 500 kg (1100 lbs), and is considerably less expensive than the BH&T approach of Procedure I.

 

Procedure III - Limited Spectrum, Light-Weight Shock Machine (LWSM). Components weighing less than 113.6 kg (250 lb) and shock mounted to eliminate sensitivity to frequencies above 3 kHz can be tested over the spectrum from 10 Hz to 3 kHz of Table 522.1-I and Figure 522.1-1 using a MIL-S-901 Light Weight Shock Machine (LWSM) adjusted for 15 mm (0.59 inch) displacement limits. Use of the LWSM is less expensive than full spectrum simulation, and may be appropriate if the specific test item does not respond to high-frequency shock and cannot withstand the excessive low-frequency response of the drop table (Procedure V).

 

Procedure IV - Limited Spectrum, Mechanical Shock Simulator. Mechanical shock simulators have been constructed to test very light-weight components (0.5 to 1.8 kg (1 to 4 lb) for the smallest machine; higher for other contractor machines). These machines produce a shock that lies within the envelope of the default shock response spectrum described in paragraph 2.3.4 up to 10 kHz. Shock content is present above 10 kHz, but it is not well defined. Use of a Mechanical Shock Simulator is less expensive than full spectrum simulation, and may be appropriate for light-weight items that are sensitive to shock up to 10 kHz.

 

Procedure V - Limited Spectrum, Medium Weight Shock Machine (MWSM). Components weighing less than 2273 kg (5000 lb) and not sensitive to frequencies above 1 kHz can be tested over the spectrum from 10 Hz to 1 kHz of Table 522.1-I and Figure 522.1-1 using a MIL-S-901 Medium Weight Shock Machine (MWSM) (paragraph 6.1, reference c) adjusted for 15 mm (0.59 in.) displacement limits. Use of the MWSW may be appropriate for heavy components and subsystems that are shock mounted and/or are not sensitive to high frequencies.

 

Procedure VI - Drop Table. Lightweight components (typically less than 18 kg (40 lbs)) which are shock mounted can often be evaluated for ballistic shock sensitivity at frequencies up to 500 Hz using a drop table. This technique often results in overtest at the low frequencies. The vast majority of components that need shock protection on an armored vehicle can be readily shock mounted. The commonly available drop test machine is the least expensive and most accessible test technique. The shock table produces a half-sine acceleration pulse that differs significantly from ballistic shock. The response of materiel on shock mounts can be enveloped quite well with a half-sine acceleration pulse if an overtest at low frequencies and an undertest at high frequencies is acceptable. Historically, these shortcomings have been acceptable for the majority of ballistic shock qualification testing.

 

 

 

 

Effects of ballistic shock.

 

In general, the ballistic shock has the potential for producing adverse effects on all electronic, mechanical, and electro-mechanical systems. In general, the level of adverse effects increases with the level and duration of the ballistic shock and decreases with the distance from the source (point or points of impact) of the ballistic shock. Durations for ballistic shock that produce material stress waves with wavelengths that correspond with the natural frequency wavelengths of micro-electronic components within various system components will enhance adverse effects. Durations for the ballistic shock that produce structural response movements that correspond with the low-frequency resonances of mechanical and electro-mechanical systems will enhance the adverse effects. Examples of problems associated with ballistic shock include:

 

• system failure as a result of the destruction of the structural integrity of micro-electronic chips including their mounting configuration;

• system component failure as a result of relay chatter;

• system component failure as a result of circuit card malfunction, circuit card damage, and electronic connector failure. On occasion, circuit card contaminants having the potential to cause short circuits may be dislodged under ballistic shock. Circuit card mounts may be subject to damage from substantial velocity changes and large displacements.

• material failure as a result of cracks and fractures in crystals, ceramics, epoxies or glass envelopes.

• system component failure as a result of sudden velocity change of the structural support of the system component, or the internal structural configuration of the mechanical or electro-mechanical system.

 

 

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