Ballistic Shock | Shock balistico

MIL STD 810 G – Test Method 522.1 – Ballistic Shock

 

SCOPE

 

Purpose
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:
  1. provide 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.
  2. 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.
Application
Ballistic shock definition
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. Paragraph 6.1, reference a, discusses the relationship between various shock environments (ballistic shock, transportation shock, rail impact shock, etc.) for armored combat vehicles. 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. 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. The characteristics of ballistic shock are outlined in the following paragraph.
Ballistic shock – momentum exchange
Ballistic shock usually exhibits momentum exchange between two bodies or between a fluid and a solid. It commonly results in velocity change in the support materiel. Ballistic shock has a portion of its characterization below 100 Hz, and the magnitude of the ballistic shock response at a given point reasonably far from the ballistic shock source is a function of the size of the momentum exchange. Ballistic shock will contain material wave propagation characteristics (perhaps substantially nonlinear) but, in general, the material is deformed and accompanied by structural damping other than damping natural to the material. For ballistic shock, structural connections do not necessarily display great attenuation since low frequency structural response is generally easily transmitted over joints. In processing ballistic shock data, it is important to be able to detect anomalies. With regard to measurement technology, accelerometers, strain gages, and shock sensing gages may be used (see paragraph 6.1, reference a). In laboratory situations, laser velocimeters are useful. Ballistic shock resistance is not, in general, “designed” into the materiel. The occurrence of a ballistic shock and its general nature can only be determined empirically from past experience based on well-defined scenarios. Ballistic shock response of materiel in the field is, in general, very unpredictable and not repeatable among materiel.
Ballistic shock – physical phenomenon
Ballistic shock is a physical phenomenon characterized by the overall material and mechanical response at a structure point from elastic or inelastic impact. Such 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 collision of two elastic bodies or a pressure wave applied over a surface. General characteristics of ballistic shock environments are as follows:
  1. 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;
  2. combined low and high frequency (10 Hz – 1,000,000 Hz) and very broadband frequency input;
  3. high acceleration (300g – 1,000,000g) with comparatively high structural velocity and displacement response;
  4. short-time duration (<180 msec);
  5. high residual structure displacement, velocity, and acceleration response (after the event);
  6. 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;
  7. 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;
  8. measurement response time histories that are very highly random in nature, i.e., little repeatability and very dependent on the configuration details;
  9. shock response at points on the structure is somewhat affected by structural discontinuities;
  10. structural response may be accompanied by heat generated by the inelastic impact or the fluid blast wave;
  11. 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.
Limitations
Because of the highly specialized nature of ballistic shock and the substantial sensitivity of ballistic shock to the configuration, apply it only after giving careful consideration to information contained in paragraph 6.1, references a and b.
  1. This method does not include special provisions for performing ballistic shock tests at high or low temperatures. Perform tests at room ambient temperature unless otherwise specified or if there is reason to believe either operational high temperature or low temperature may enhance the ballistic shock environment.
  2. This method does not address secondary effects such as blast, EMI, and thermal.

 

TEST PROCESS

 

Procedure I – BH&T
  • Step 1. Select the test conditions and mount the test item in a Ballistic Hull and Turret (BH&T), that may require ‘upweighting’ to achieve the proper dynamic response. (In general, there will be no calibration when actual hardware is used in this procedure). Select measurement techniques that have been validated in ballistic shock environments. See paragraph 6.1, reference g, for examples.
  • Step 2. Perform an operational check on the test item.
  • Step 3. Fire threat munitions at the BH&T and verify that the test item operates as required. Typically, make shock measurements at the mounting location (‘input shock’) and on the test item (‘test item response’).
  • Step 4. Record necessary data for comparison with pretest data.
  • Step 5. Photograph the test item as necessary to document damage.
  • Step 6. Perform an operational check on the test item. Record performance data. See paragraph 5 for analysis of results.
Procedure II – LSBSS
  • Step 1. Mount the test item to the LSBSS using the same mounting hardware as would be used in the actual armored vehicle. Select the orientation of the test item with the intent of producing the largest shock in the ‘worst case’ axis.
NOTE: A ‘dummy’ test item is typically mounted until measurements confirm that the proper explosive ‘recipe’ (i.e., combination of explosive weight, stand-off distance, and hydraulic displacement) has been determined to obtain the shock levels specified in table 522.1-I and on Figure 522.1-1. Then mount an operational test item to the LSBSS.
  • Step 2. Fire the LSBSS and verify the test item is operating as required before, during, and after the shot. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
  • Step 3. Record initial data for comparison with post test data.
  • Step 4. Fire three test shots at the shock level specified in Table 522.1-I.
  • Step 5. Inspect the test item; photograph any noted damage, and record data for comparison with pretest data.
Procedure III – LWSM
  • Step 1. Modify the mounting for the anvil plate to restrict total travel (including dynamic plate deformation) to 15 mm (0.59 inch). Mount the test item to the LWSM using the same mounting hardware as would be used in an actual armored vehicle. Choose the orientation of the test item with the intent of producing the largest shock in the ‘worst case’ axis.
  • Step 2. Perform a pretest checkout and record data for comparison with post test data.
NOTE: Typically, make shock measurements at the ‘input’ location to ensure the low frequency shock levels specified in Table 522.1-I and on Figure 522.1-1 have been attained on the 5-foot drop.
  • Step 3. Perform a 1-foot hammer drop followed by an operational check; record data. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure. Otherwise, proceed to Step 4.
  • Step 4. Perform a 3-foot hammer drop followed by an operational check; record data. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure. Otherwise, go to Step 5.
  • Step 5. Perform a 5-foot hammer drop followed by an operational check; record data. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
  • Step 6. Repeat Step 5 two more times.
  • Step 7. If the worst case axis is unknown (see paragraph 4.1c), repeat Steps 2-6 for each direction of each axis for a total of 18 five-foot hammer drops. See paragraph 5 for analysis of results.
Procedure IV – Mechanical Shock Simulator
  • Step 1. Mount the test item to the Mechanical Shock Simulator using the same mounting hardware as would be used in the actual armored vehicle. Select the orientation of the test item with the intent of producing the largest shock in the ‘worst case’ axis.
  • Step 2. Launch the mechanical shock simulator projectile and verify the test item is functioning as required before, during, and after the shot.
  • Step 3. Record initial data for comparison with post test data.
  • Step 4. Conduct three test shots at the shock level specified in Table 522.1-I
  • Step 5. If the worst case axis is unknown (see paragraph 4.1c), repeat Steps 2-6 for each direction of each axis, for a total of 18 projectile impacts.
  • Step 6. Inspect the test item; photograph any noted damage, and record data for comparison with pretest data. Perform an operational check on the test item. Record performance data. See paragraph 5 for analysis of results.
Procedure V – MWSM
  • Step 1. Modify the supports for the anvil table (by shimming the 4 table lifts) to restrict table total travel (including dynamic plate deformation) to 15 mm (0.59) inch.
  • Step 2. Mount the test item to the MWSM using the same mounting hardware as would be used in an actual combat vehicle. Choose the orientation of the test item with the intent of producing the largest shock in the ‘worst case’ axis (see Step 7 below).
  • Step 3. Perform a pretest checkout and record data for comparison with post test data.
NOTE: Typically, make shock measurements at the ‘input’ location to ensure that the low-frequency shock levels specified in Table 522.1-I and on Figure 522.1-1 have been attained on the ‘Group III’ drop (from MIL-S-901).
  • Step 4. Perform a ‘Group I height’ hammer drop followed by an operational check; record data. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
  • Step 5. Perform a ‘Group III height’ hammer drop followed by an operational check; record data.
  • Step 6. Repeat Step 5 two more times.
  • Step 7. If the worst case axis is unknown (see paragraph 4.1c), repeat Steps 2-6 for each direction of each axis for a total of 18 hammer drops at the Group III height.
Procedure VI – Drop table
  • Step 1. Calculate the expected response of a shock mounted test item (or measurements from field tests may be used) and calculate a shock response spectra (SRS). Choose a half-sine acceleration pulse whose SRS ‘envelopes’ the expected response of the shock mounted item. Note that this approach typically results in an overtest at the lowest frequencies.
  • Step 2. Hard mount the test item to the drop table.
  • Step 3. Conduct an operational check and record data for comparison with post test data. If the test item operates satisfactorily, proceed to Step 4. If not, resolve the problems and repeat this step.
  • Step 4. Test using the appropriate half-sine acceleration pulse three times in each direction of all three axes (18 drops).
  • Step 5. Conduct a performance check and record data for comparison with pretest data. See paragraph 5 for analysis of results.

 

NOTE: Tailoring is essential. Please, ask to your confidence laboratory for further details about tailoring of test methods.

 

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