MIL-STD-810G – Part 21 (Gunfire Shock) Method 519.6

MIL-STD-810G covers Gunfire Shock in Method 519.6. Method 519.6 is comprised of 17 pages with the following annexes:   A – Guidelines for Procedure I (11 pages) B – Guidelines for Procedure II (21 pages) C – Guidelines for Procedure III (12 pages) D – Sine-on-Random Spectrum Prediction (10 pages) E – Guidelines for Gunfire Shock Test Scaling (7 pages)   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. 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 natural frequencies of the mounted materiel (along with its subharmonics and superharmonics) will magnify the adverse effects on the materiel’s overall integrity.   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).   Some of the effects of Gunfire Shock include:

1. changes in materiel dielectric strength, loss of insulation resistance, variations in magnetic and electrostatic field strength;
2. 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);
3. permanent mechanical deformation of the materiel as a result of overstress of materiel structural and non-structural members;
4. collapse of mechanical elements of the materiel as a result of the ultimate strength of the element being exceeded.
5. accelerated fatiguing of materials (low cycle fatigue);
6. potential piezoelectric activity of materials; and
7. materiel failure as a result of cracks and fracture in crystals, ceramics, epoxies, or glass envelopes.

This is a very complicated Method in that the effects of air pressure pulse is combined with shock through the structure with distance from the gun having a significant impact on which input has more effect. In addition, predicting material response may be impossible.   The Method relies heavily on Method 525 (Time Waveform Replication).   There are three procedures:   Procedure I – 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.   Procedure II – 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).   Procedure III – This procedure is ad hoc, lacking necessary field measured time trace information, and a last resort to providing guidelines for 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.   Gunfire Shock testing is performed on a shake table with the ability to follow test waveform inputs.