517.3-1 INTRODUCTION
This Annex provides additional guidelines for pyroshock time history assessment including validation, i.e., to detect any measurement system anomalies that would invalidate the measurement. For massive field shock measurement programs where time and budget constraints do not allow validation of individual pyroshocks, at least one pyroshock time history from the near-field, mid-field, and far-field must be individually validated, and careful examination of the time history for each subsequent shock from the measurement channel be examined for gross anomalies. Consistency relative to the test specification for processed information is acceptable as long as any inconsistency is investigated under pyroshock time history validation. The best indicator of pyroshock accelerometer data quality is the integral or velocity time history. As the examples below show, many anomalies in the accelerometer data cannot be detected from the acceleration plot or the shock response spectrum (SRS), especially if the SRS is only plotted down to 100 Hz.
The sources of pyroshock data contamination have been known for some time (more than 20 years): electromagnetic noise (or other noise sources), digital aliasing, and offsets in the data. Electromagnetic noise is always a potential problem with pyroshock testing, especially when explosives are detonated. The high frequency electromagnetic pulse can be eliminated in some cases, but in many cases, the electromagnetic pulse creates an additional environment that can cause invalid data contaminated by the inadequate response to the pulse by the signal conditioner and/or data acquisition system (DAS). The cause of digital data aliasing is, but not limited to, inadequate analog filtering prior to digitization and inadequate bandwidth of the DAS. Offsets in the acceleration data are generally caused by accelerometer malfunction and, in some cases, DAS problems such as inadequate slew rate capability as shown below.
1.1 Purpose
Pyroshock tests involving pyrotechnic (explosive- or propellant-activated) devices are performed to:
- a. 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.
- b. 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.
1.2 Application
1.2.1 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 fuzes (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 at a duration from 50 microseconds to not more than 20 milliseconds. Acceleration response amplitudes to pyroshock may range from 300 g’s to 200,000 g’s. 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.” 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.
MIL-STD-810H METHOD 517.3
1.2.2 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 use. 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).
1.2.3 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:
- a. Near-the-source stress waves in the structure caused by high material strain rates (nonlinear material region) propagate into the near-field and beyond.
- b. High frequency (100 Hz to 1,000,000 Hz) and very broadband frequency input.
- c. High acceleration (300 g's to 200,000 g's) but low structural velocity and displacement response.
- d. Short-time duration (< 20 msec).
- e. High residual structure acceleration response (after the event).
- f. 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).
- g. Highly localized point source input or line source input.
- h. 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.
- i. Response time histories that are random in nature, providing little repeatability and substantial dependency on the materiel configuration details.
- j. Response at points on the structure that are greatly affected by structural discontinuities
- k. Materiel and structural response that may be accompanied by substantial heat and electromagnetic emission (from ionization of gases during explosion).
1.2.4 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.
- a. 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 10,000g's. A pyrotechnically excited simulation technique is usually appropriate, although in some cases a mechanically excited simulation technique may be used.
- b. 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 10,000g's. A mechanically excited simulation technique other than shaker shock is usually required.
- c. 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 1,000g’s. 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.
1.3 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. This Method does not apply to the following:
- a. 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.
- b. The effects experienced by fuze systems that are sensitive to shock from pyrotechnic devices. Shock tests for safety and operation of fuzes and fuze components may be performed in accordance with MIL-STD-331 (paragraph 6.1, reference c).
- c. 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 that testing at either the high or low operational temperature may enhance the pyroshock environment.
- d. Manned space vehicle testing (see paragraph 6.1, reference a).
- e. Secondary effects such as induced blast, EMI, and thermal effects.
- f. Effects of hostile weapon penetration or detonation. (Refer to Method 522.2, Ballistic Shock.)