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MIL STD 810 G – Test Method 519.6 – Gunfire Shock
SCOPE
Purpose
Gunfire shock tests are performed to provide a degree of confidence that materiel can structurally and functionally withstand the relatively infrequent, short duration transient high rate repetitive shock input encountered in operational environments during the firing of guns.
Application
Use this method to evaluate the structural and functional performance of materiel likely to be exposed to a gunfire shock environment in its lifetime. This test method is applicable when materiel is required to demonstrate its adequacy to resist a “gunfire schedule” environment without unacceptable degradation of its structural integrity and functional performance (“gunfire schedule” here refers to the firing rate and the number of rounds fired in a given firing). The gunfire environment may be considered to be a high rate repetitive shock having form of a substantial transient vibration produced by (1) an air-borne gun muzzle blast pressure wave impinging on the materiel at the gun firing rate, (2) a structure-borne repetitive shock transmitted through structure connecting the gun mechanism and the materiel, and/or a combination of (1) and (2). The closer the materiel surface is to direct pressure pulse exposure, the more likely the measured acceleration environment appears as a repetitive shock producing high rise time and rapid decay of materiel response, and the less role the structure-borne repetitive shock contributes to the overall materiel response environment. The farther the materiel surface is from direct pressure pulse exposure, the more the measured acceleration environment appears as a structure-borne high rate repetitive shock (or a substantial transient vibration) with some periodic nature that has been filtered by the structure intervening between the gun mechanism and the materiel. Repetitive shock applied to a complex multi-modal materiel system will cause the materiel to respond (1) at forced frequencies imposed on the materiel from the external excitation environment, and (2) to the materiel’s resonant natural frequencies either during or immediately after application of the external excitation. Such response may cause: materiel failure as a result of increased or decreased friction between parts, or general interference between parts;
changes in materiel dielectric strength, loss of insulation resistance, variations in magnetic and electrostatic field strength;
materiel electronic circuit card malfunction, electronic circuit card damage, and electronic connector failure. (On occasion, circuit card contaminants having the potential to cause short circuits may be dislodged under materiel response to gunfire environment);
permanent mechanical deformation of the materiel as a result of overstress of materiel structural and non-structural members;
collapse of mechanical elements of the materiel as a result of the ultimate strength of the element being exceeded.
accelerated fatiguing of materials (low cycle fatigue);
potential piezoelectric activity of materials; and
materiel failure as a result of cracks and fracture in crystals, ceramics, epoxies, or glass envelopes.
Limitations
This method provides limited information with regard to the prediction of input levels to materiel based only on the gun parameters and the geometrical configuration between the gun and materiel. Procedure III is provided for purposes of preliminary materiel design when no other information is available. The shock form of time trace this is not a recommended practice. It may not be possible to replicate some operational service gunfire materiel response environments because of impedance mismatches. In particular, laboratory fixture limitations or other physical constraints may prevent the satisfactory application of gunfire-induced excitation to a test item in the laboratory. In addition:
This method does not provide guidelines for separating air-borne from structure-borne excitation input to materiel. It is important that a trained structural dynamicist examine the structural configuration and any measured data to determine the transmission path(s) from the gun excitation source to the materiel.
This method does not provide guidance on techniques for isolation of the materiel from the source of excitation.
This method does not provide guidance on materiel design to avoid unacceptable structural or functional materiel degradation during gun firing, e.g., shock isolation.
This method does not include the repetitive shock effects experienced by large extended materiel, e.g., airframe structural systems over which varied parts of the materiel may experience spatially correlated external excitation. For this type of repetitive shock, with degrees of input and response spatial correlation from the external excitation, specialized tests based on experimentally measured data must be employed.
This method does not include provisions for performing gunfire tests at high or low temperatures including the extreme temperature environment directly related to the gunfire pressure wave emission and subsequent materiel absorption of this thermal energy. Perform tests at standard ambient temperature unless otherwise specified. However, thermal energy generated from the gun blast pressure wave may be an important design consideration for materiel close to the gun muzzle.
This method is not intended to simulate blast pressure or acoustic effects on materiel as a result of exposure to gunfire environment. This method assumes materiel acceleration as the measurement variable but does not limit consideration to other materiel input/response variables, e.g., force.
In general this method provides limited guidance on materiel response to gun excitation from simultaneous firing of more than one gun
This method does not address benign gunfire shock environments where materiel input or response may be a form of transient random vibration with peak root-mean-square levels below the levels of materiel qualification to stationary random vibration as determined by the square root of the area under the Autospectral Density Estimate (ASD).
This method does not include engineering guidelines related to unplanned test interruption as a result of test equipment or other malfunction. If interruption occurs during a short duration gunfire test, repeat the portion of gunfire test. Care must be taken to ensure stresses induced by an interrupted gunfire test do not invalidate subsequent test results. It is incumbent on all test facilities that, data from test interruptions be recorded and analyzed before continuing the test sequence. In addition, the materiel must be inspected prior to test to ensure pre-gunfire test materiel integrity.
TEST PROCESS
Procedure I – Direct Reproduction of Measured Materiel Input/Response Time Trace Information under Guidelines Provided in Method 525 for Time Waveform Replication (TWR)
Step 1. Precondition in accordance with paragraphs 4.2 and 4.4.1.
Step 2. Choose control strategy and control and monitoring points in accordance with paragraph 2.5.
Step 3. Perform operational checks in accordance with paragraph 4.4.1.
Step 4. Mount the test item on the vibration exciter or use some other means of suspension in accordance with paragraph 4.4.1.
Step 5. Determine the time trace representation of the vibration exciter drive signal required to provide the desired gunfire shock materiel acceleration input/response on the test item. (Refer to Annex A).
Step 6. Apply the drive signal as an input voltage and measure the test item acceleration response at the selected control/monitoring point.
Step 7. Verify that the test item response is within the allowable tolerances specified in paragraph 4.2.1.
Step 8. Apply gunfire shock simulation for on and off periods and total test duration in accordance with the test plan. Perform operational checks in accordance with the test plan. If there is failure in test item operational performance stop the test, assess the failure and decide upon the appropriate course of action to proceed with testing to complete the test plan. Follow the guidance in paragraph 4.3.2.
Step 9. Repeat the previous steps along each of the other specified axes, and record the required information.
Procedure II – Stochastically Generated Materiel Input/Response Based Upon Measured Time Trace Information
Step 1. Generate a stochastic representation of the field measured materiel input/response data. In general, this will involve an off-line procedure designed to generate an ensemble of pulses based on measured data for input to the vibration exciter as a single time trace of concatenated pulses or a single stochastic time trace (refer to Annex B).
Step 2. Precondition in accordance with paragraphs 4.2 and 4.4.1.
Step 3. Choose control strategy and control and monitoring points in accordance with paragraph 2.5.
Step 4. Perform operational checks in accordance with paragraph 4.4.1.
Step 5. Mount the test item on the vibration exciter (or use some other means of suspension) in accordance with paragraph 4.4.1.
Step 6. Determine the time trace representation of the vibration exciter drive signal required to provide the desired gunfire shock materiel acceleration input/response on the test item. (Refer to Annex B).
Step 7. Apply the drive signal as an input voltage and measure the test item acceleration input/response at the selected control/monitoring point.
Step 8. Verify that the test item response is within the allowable tolerances specified in paragraph 4.2.2.
Step 9. Apply gunfire shock simulation for on and off periods, and total test duration in accordance with the test plan. Perform operational checks in accordance with the test plan. If there is failure in test item function performance stop the test, assess the failure and decide upon the appropriate course of action to proceed with testing to complete the test plan. Follow the guidance in paragraph 4.3.2.
Step 10. Repeat the previous steps along each of the other specified axes, and record the required information.
Procedure III – Stochastically Predicted Materiel Input for Preliminary Design Based Upon Predicted Sine-on-Random Spectrum
Step 1. Specify the gun/materiel parameters and generate the predicted Sine-on-Random spectrum (See Annex D.)
Step 2. Generate a Random-Modulated-Pulse time trace with the specified Sine-on-Random spectrum.
Step 3. For materiel design considerations analyze the Random-Modulated-Pulse time trace according to procedures appropriate for a repetitive shock and use this analysis for consideration in preliminary materiel design. Typically
transient vibration root-mean-square peak levels along with a normalized ASD estimate will be used in specifying the acceleration environment for the materiel design, or
SRS estimates will be made on the Random-Modulated-Pulse time trace (either under ensemble representation or as an overall time trace) and be used in specifying a shock environment for materiel design.
Step 4. If testing is required generate the equivalent Random-Modulated-Pulse time trace environment. (refer to Annex C.), and go to Procedure II for testing while recording the required information.
NOTE: Tailoring is essential. Please, ask to your confidence laboratory for further details about tailoring of test methods.