- 2022-06-28 09:38:51
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.
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 the 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.
Exposure to a gunfire shock environment has the potential for producing adverse effects on the structural and functional integrity of all materiel including in-service operational capability. The probability of adverse effects increases with the blast energy of the gun, proximity of the materiel to the gun, and the duration of the gunfire shock environment. The gunfire firing rate and the duration of gunfire shock environment exposure that correspond with the natural frequencies of the mounted materiel (along with its subharmonics and superharmonics) will magnify the adverse effects on the materiel’s overall integrity.
Procedures Of This Method Are As Follows:
Procedure I Direct Reproduction of Measured Materiel Input/Response Time Trace Information under Guidelines Provided in Method 525 for Time Waveform Replication (TWR).
The measured in-service gunfire shock environment for materiel is replicated under laboratory exciter waveform control (Method 525 TWR) to achieve a near-exact reproduction of the measured in-service gunfire shock environment. Test philosophy includes selection of the time trace or traces to be replicated according to the scope of the test. Use the guidelines provided in Annex A and Method 525.
Procedure II Stochastically Generated Materiel Input/Response Based Upon Measured Time Trace Information:
This procedure is based upon either (1) direct stochastic generation of time traces appropriate for Method 525 that are “equivalent” in severity to in-service measured time trace information, or (2) a procedure that may be justified for properly distributing uncertainty, and for conservative testing (but in accordance with the principles of random process theory). It is possible, in the latter case, that measured time trace information is available for a configurationally-similar gun/materiel configuration, and that this can be used with appropriate rationale in the form of predicted time trace information. In general, this procedure requires the use of simulation techniques that preserve the elements of random process theory and allows scaling of time trace information only in accordance with guidelines provided in Method 525 (and summarized in Annex E of this method). Essential information for this procedure, including a detailed discussion of time trace scaling, is provided in Annex B and Annex E.
Procedure III. Stochastically Predicted Materiel Input for Preliminary Design Based Upon Predicted Sine-on-Random Spectrum.
This procedure is ad hoc, lacking necessary field measured time trace information, and a last resort to providing guidelines for the design of materiel to resist gunfire shock environment. Only time trace forms for design are given, and it is not suggested that testing be performed to these forms for materiel qualification purposes. The shortcomings of previous MIL-STD-810 versions and use of prediction methods are outlined in paragraph 2.2.1. The inability to develop a database useful for prediction is unfortunate, and the reluctance to discard what little prediction information that is available has resulted in this procedure. The idea behind this procedure is that the true nature of either air-borne or structure-borne gunfire shock is impulsive in nature at the gunfire rate. Any initial design of materiel must be based on a repetitive shock pulse as opposed to stationary random vibration with added sine components. Annex C with Annex D provides a limited procedure that stochastically generates pulse time traces for preliminary design when no measured gunfire shock information is available.
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