Pyroshock | Shock da fuoco

MIL STD 810 G – Test Method 517.1 – Pyroshock

 

SCOPE

 

Purpose
Pyroshock tests involving pyrotechnic (explosive- or propellant-activated) devices are performed to:
  1. provide a degree of confidence that materiel can structurally and functionally withstand the infrequent shock effects caused by the detonation of a pyrotechnic device on a structural configuration to which the materiel is mounted.
  2. experimentally estimate the materiel’s fragility level in relation to pyroshock in order that shock mitigation procedures may be employed to protect the materiel’s structural and functional integrity.
Application
Pyroshock
Pyroshock is often referred to as pyrotechnic shock. For the purpose of this document, initiation of a pyrotechnic device will result in an effect that is referred to as a “pyroshock.” “Pyroshock” refers to the localized intense mechanical transient response of materiel caused by the detonation of a pyrotechnic device on adjacent structures. A number of devices are capable of transmitting such intense transients to a materiel. In general, the sources may be described in terms of their spatial distribution – point sources, line sources and combined point and line sources (paragraph 6.1, reference a). Point sources include explosive bolts, separation nuts, pin pullers and pushers, bolt and cable cutters and pyro-activated operational hardware. Line sources include flexible linear shape charges (FLSC), mild detonating fuses (MDF), and explosive transfer lines. Combined point and line sources include V-band (Marmon) clamps. The loading from the pyrotechnic device may be accompanied by the release of structural strain energy from structure preload or impact among structural elements as a result of the activation of the pyrotechnic device. Use this method to evaluate materiel likely to be exposed to one or more pyroshocks in its lifetime. Pyroshocks are generally within a frequency range between 100 Hz and 1,000,000 Hz, and a time duration from 50microseconds to not more than 20 milliseconds. Acceleration response amplitudes to pyroshock may range from 300 g to 300,000 g. The acceleration response time history to pyroshock will, in general, be very oscillatory and have a substantial rise time, approaching 10 microseconds. In general, pyroshocks generate material stress waves that will excite materiel to respond to very high frequencies with wavelengths on the order of sizes of micro electronic chip configurations. Because of the limited velocity change in the structure brought about by firing of the pyrotechnic device, and the localized nature of the pyrotechnic device, structural resonances of materiel below 500 Hz will normally not be excited and the system will undergo very small displacements with small overall structural/mechanical damage. The pyroshock acceleration environment in the neighborhood of the materiel will usually be highly dependent upon the configuration of the materiel and the intervening structure. The materiel or its parts may be in the near-field, mid-field or far-field of the pyrotechnic device with the pyroshock environment in the near-field being the most severe, and that in the mid-field or far-field less severe. In general, some structure intervenes between the materiel and location of the pyrotechnic device that results in the “mid-field,” and “far-field.” There is now agreement on classifying pyroshock intensity according to the characteristics of “near-field,” “mid-field,” and “far-field.” However, the specific frequencies and acceleration amplitudes may differ in various documents. This document reflects the current consensus for three regions according to simulation techniques as “near-field,” “mid-field,” and “far-field” for which the definitions are provided in paragraph 1.2.4.
Pyroshock – momentum exchange
Pyroshock usually exhibits no momentum exchange between two bodies (a possible exception is the transfer of strain energy from stress wave propagation from a device through structure to the materiel). Pyroshock results in essentially no velocity change in the materiel support structure. Frequencies below 100 Hz are never of concern. The magnitude of a pyroshock response at a given point reasonably far from the pyrotechnic source is, among other things, a function of the size of the pyrotechnic charge. Pyroshock is a result of linear elastic material waves propagating in the support structure to the materiel without plastic deformation of large portions of the structure except at the charge point or line. In general, joints and bolted connections representing structure discontinuities tend to greatly attenuate the pyroshock amplitudes. Pyroshock is “designed” into the materiel by placement of pyroshock devices for specific utility. Because to a great extent the pyroshock environment is clearly defined by the geometrical configuration and the charge or the activating device, pyroshock response of materiel in the field may be moderately predictable and repeatable for materiel (paragraph 6.1, reference a).
Pyroshock – physical phenomenon
Pyroshock is a physical phenomenon characterized by the overall material and mechanical response at a structure point from either (a) an explosive device, or (b) a propellant activated device. Such a device may produce extreme local pressure (with perhaps heat and electromagnetic emission) at a point or along a line. The device provides a near instantaneous generation of local, high-magnitude, nonlinear material strain rates with subsequent transmission of high-magnitude/high frequency material stress waves producing high acceleration/low velocity and short duration response at distances from the point or line source. The characteristics of pyroshock are:
  1. near-the-source stress waves in the structure caused by high material strain rates (nonlinear material region) propagate into the near-field and beyond;
  2. high frequency (100 Hz to 1,000,000 Hz) and very broadband frequency input;
  3. high acceleration (300 g to 300,000 g) but low structural velocity and displacement response;
  4. short-time duration (< 20 msec);
  5. high residual structure acceleration response (after the event);
  6. caused by (1) an explosive device or (2) a propellant activated device (releasing stored strain energy) coupled directly into the structure; (for clarification, a propellant activated device includes items such as a clamp that releases strain energy causing a structure response greater than that obtained from the propellant detonation alone);
  7. highly localized point source input or line source input;
  8. very high structural driving point impedance (P/v, where P is the large detonation force or pressure, and v, the structural velocity, is very small). At the pyrotechnic source, the driving point impedance can be substantially less if the structure material particle velocity is high;
  9. response time histories that are random in nature, providing little repeatability and substantial dependency on the materiel configuration details;
  10. response at points on the structure that are greatly affected by structural discontinuities;
  11. materiel and structural response that may be accompanied by substantial heat and electromagnetic emission (from ionization of gases during explosion).
Classification of pyroshock zones
The nature of the response to pyroshock suggests that the materiel or its components may be classified as being in the near-field, mid-field or far-field of the pyrotechnic device. The terms “near-field,” “mid-field,” and “far-field” relate to the shock intensity at the response point, and such intensity is a function of the distance from the pyrotechnic source and the structural configuration between the source and the response point. The definitions that follow are based on simulation techniques consistent with paragraph 6.1, reference b.
  1. Near-field. In the near-field of the pyrotechnic device, the structure material stress wave propagation effects govern the response. A near-field pyroshock test requires frequency control up to and above 10,000 Hz for amplitudes greater than 10,000gs. A pyrotechnically excited simulation technique is usually appropriate, although in some cases a mechanically excited simulation technique may be used.
  2. Mid-field. In the mid-field of the pyrotechnic device, the pyroshock response is governed by a combination of material stress wave propagation and structural resonance response effects. A mid-field pyroshock test requires frequency control from 3,000 Hz to 10,000 Hz for amplitudes less than 10,000gs. A mechanically excited simulation technique other than shaker shock is usually required.
  3. Far-field. In the far-field of the pyrotechnic device, the pyroshock response is governed by a combination of material stress wave propagation and structural resonance response effects. A Far-field pyroshock test requires frequency control no higher than 3,000 Hz for amplitudes less than 1,000gs. A shaker shock or a mechanically excited simulation technique is appropriate.
Distances from the pyrotechnic device have been avoided in these definitions because specific distances restrict structural dimensions and imply point or line pyrotechnic sources with specific weights and densities. The definitions are based on experimental capabilities, but still should be considered guidelines because all structures with their corresponding pyrotechnic devices are different.
Limitations
Because of the highly specialized nature of pyroshock, apply it only after giving careful consideration to information contained in paragraph 6.1, references a, b, c, and d.
  1. This method does not include the shock effects experienced by materiel as a result of any mechanical shock/transient vibration, shipboard shock, or EMI shock. For these types of shocks, see the appropriate methods in this or other standards.
  2. This method does not include the effects experienced by fuze systems that are sensitive to shock from pyrotechnic devices. Shock tests for safety and operation of fuzes and fuse components may be performed in accordance with MIL-STD-331 (paragraph 6.1, reference c).
  3. This method does not include special provisions for performing pyroshock 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 pyroshock environment.
  4. This method is not intended to be applied to manned space vehicle testing (see paragraph 6.1, reference a).
  5. This method does not address secondary effects such as induced blast, EMI, and thermal effects.
  6. This method does not apply to effects of hostile weapon penetration or detonation. (Refer to Method 522.1, Ballistic Shock.)

 

TEST PROCESS

 

Procedure I – Near-field with actual configuration
  • Step 1. Following the guidance of paragraph 6.1, reference b, select test conditions and mount the test item (in general there will be no calibration when actual hardware is used in this procedure). Select accelerometers and analysis techniques that meet the criteria outlined in paragraph 6.1, reference d.
  • Step 2. Conduct an operational check on the test item. If the test item operates satisfactorily, proceed to Step 3. If not, resolve the problems and repeat this Step.
  • Step 3. Subject the test item (in its operational mode) to the test transient by way of the pyrotechnic test device.
  • Step 4. Record necessary data that show the shock transients when processed with the SRS algorithm are within specified tolerances. This includes test setup photos, test logs, and plots of actual shock transients. For shock-isolated assemblies within the test item, make measurements and/or inspections to ensure these assemblies did attenuate the pyroshock.
  • Step 5. Perform an operational check on the test item. Record performance data. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
  • Step 6. If the integrity of the test configuration can be preserved during test, repeat Steps 2, 3, 4, and 5 a minimum of three times for statistical confidence.
  • Step 7. Document the test series, and see paragraph 5 for analysis of results.
Procedure II – Near-field with simulated configuration
  • Step 1. Following the guidance of paragraph 6.1, reference d, select test conditions and calibrate the shock apparatus as follows:
  1. Select accelerometers and analysis techniques that meet the criteria outlined in paragraph 6.1, reference d.
  2. Mount the calibration load (an actual test item, a rejected item, or a rigid dummy mass) to the test apparatus in a manner similar to that of the actual materiel service mount. If the materiel is normally mounted on shock isolators to attenuate the pyroshock, ensure the isolators are functional during the test.
  3. Perform calibration shocks until two consecutive shock applications to the calibration load produce shock transients that, when processed with the SRS algorithm, are within specified tolerances for at least one direction of one axis.
  4. Remove the calibrating load and install the actual test item on the shock apparatus paying close attention to mounting details.
  • Step 2. Conduct an operational check on the test item. Record performance data.
  • Step 3. Subject the test item (in its operational mode) to the test pyroshock.
  • Step 4. Record necessary data that show the shock transients when processed with the SRS algorithm are within specified tolerances. If requirements are given in terms of more than one axis, examine responses in the other axes to ensure the test specification has been met. This includes test setup photos, test logs, and photos of actual shock transients. For shock isolated assemblies within the test item, make measurements and/or inspections to assure the isolators attenuated the pyroshock.
  • Step 5. Conduct an operational check on the test item. Record performance 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 Steps 1, 2, 3, 4, and 5 three times for each orthogonal axis that is to be tested if the test shock did not meet the test specification in the other axes (see paragraph 2.3.3.3 of this method for guidance).
  • Step 7. Document the test series, and see paragraph 5 for analysis of results.
Procedure III -Mid-field using mechanical test device
  • Step 1. Following the guidance of paragraph 6.1, reference d, select test conditions and calibrate the shock apparatus as follows:
  1. Select accelerometers and analysis techniques that meet the criteria outlined in paragraph 6.1, reference d.
  2. Mount the calibration load (an actual test item, a rejected item, or a rigid dummy mass) to the test apparatus in a manner similar to that of the actual materiel service mount. If the materiel is normally mounted on shock isolators to attenuate the pyroshock, ensure the isolators are functional during the test.
  3. Perform calibration shocks until two consecutive shock applications to the calibration load produce waveforms that, when processed with the SRS algorithm, are within specified tolerances for at least one direction of one axis.
  4. Remove the calibrating load and install the actual test item on the shock apparatus paying close attention to mounting details.
  • Step 2. Conduct an operational check of the test item. If the test item operates satisfactorily, proceed to the first test. If not, resolve the problem and restart at Step 1.
  • Step 3. Subject the test item (in its operational mode) to the test pyroshock.
  • Step 4. Record necessary data that show the shock transients when processed with the SRS algorithm are within specified tolerances. If requirements are given in terms of more than one axis, examine responses in the other axes to ensure the test specification has been met. This includes test setup photos, test logs, and photos of actual shock transients. For shock isolated assemblies within the test item, make measurements and/or inspections to assure the isolators attenuated the pyroshock.
  • Step 5. Conduct an operational check of the test item. Record performance 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 Steps 1 through 5 three times for each orthogonal axis that is to be tested if the test shock did not meet the test specification in the other axes (see paragraph 2.3.3.4 of this method for guidance).
  • Step 7. Document the tests, and see paragraph 5 for analysis of results.
Procedure IV – Far-field using mechanical test device
  • Step 1. Following the guidance of paragraph 6.1, reference d, select test conditions and calibrate the shock apparatus as follows:
    1. Select accelerometers and analysis techniques that meet the criteria outlined in paragraph 6.1, reference d.
    2. Mount the calibration load (an actual test item, a rejected item, or a rigid dummy mass) to the test apparatus in a manner similar to that of the actual materiel service mount. If the materiel is normally mounted on shock isolators to attenuate the pyroshock, ensure the isolators are functional during the test.
    3. Perform calibration shocks until two consecutive shock applications to the calibration load produce waveforms that, when processed with the SRS algorithm, are within specified tolerances for at least one direction of one axis.
    4. Remove the calibrating load and install the actual test item on the shock apparatus paying close attention to mounting details.
  • Step 2. Conduct an operational check of the test item. If the test item operates satisfactorily, proceed to the first test. If not, resolve the problem and restart at Step 1.
  • Step 3. Subject the test item (in its operational mode) to the test pyroshock.
  • Step 4. Record necessary data that show the shock transients when processed with the SRS algorithm are within specified tolerances. If requirements are given in terms of more than one axis, examine responses in the other axes to ensure the test specification has been met. This includes test setup photos, test logs, and photos of actual shock transients. For shock isolated assemblies within the test item, make measurements and/or inspections to assure the isolators attenuated the pyroshock.
  • Step 5. Conduct an operational check of the test item. Record performance 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 Steps 1 through 5 three times for each orthogonal axis that is to be tested if the test shock did not meet the test specification in the other axes (see paragraph 2.3.3.5 of this method for guidance).
  • Step 7. Document the tests, and see paragraph 5 for analysis of results.
Procedure V – Far-field using electrodynamic shaker
  • Step 1. Following the guidance of paragraph 6.1, reference d, select test conditions and calibrate the shock apparatus as follows:
  1. Select accelerometers and analysis techniques that meet the criteria outlined in paragraph 6.1, reference d.
  2. Mount the calibration load (an actual test item, a rejected item, or a rigid dummy mass) to the electrodynamic shaker in a manner similar to that of the actual materiel. If the materiel is normally mounted on shock isolators to attenuate the pyroshock, ensure the isolators are functional during the test.
  3. Develop the SRS wavelet or damped sine compensated amplitude time history based on the required test SRS.
  4. Perform calibration shocks until two consecutive shock applications to the calibration load produce shock transients that, when processed with the SRS algorithm, are within specified test tolerances for at least one direction of one axis.
  5. Remove the calibrating load and install the actual test item on the electrodynamic shaker paying close attention to mounting details.
  • Step 2. Conduct an operational check on the test item. If the test item operates satisfactorily, proceed to the first test. If not, resolve the problem and restart at Step 1
  • Step 3. Subject the test item (in its operational mode) to the test electrodynamic pyroshock simulation.
  • Step 4. Record necessary data that show the shock transients, when processed with the SRS algorithm, are within specified tolerances. If requirements are given in terms of more than one axis, examine responses in the other axes to ensure the test specification has been met. This includes test setup photos, test logs, and photos of actual shock transients. For shock isolated assemblies within the test item, make measurements and/or inspections to assure the isolators attenuated the pyroshock.
  • Step 5. Conduct an operational check on the test item. Record performance 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 Steps 2, 3, 4, and 5 three times for each orthogonal axis that is to be tested if the test shock did not meet the test specification in the other axes (see paragraph 2.3.3.6 of this method for guidance).
  • Step 7. Document the tests, and 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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