12.4 Pyroshocks

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The aerospace industry uses many pyrotechnic

devices such as explosive bolts, squib valves, jet

cord, and pin pushers. During their operation,

these devices generate shocks which are characterized

by very strong acceleration levels at very high

frequencies that can be sometimes dangerous for

the structures, but especially for the electric and

electronic components involved. An example of a

pyroshock is given in Figure 12.21.

Pyroshock intensity is often classified according

to the distance from the point of detonation of

the device. Agreement on classifying

intensity according to this criterion is not

unanimous. Two fields are generally considered

(Table 12.3):

* The near-field, close to the source (material

within about 15 cm of point of detonation

of the device, or about 7.5 cm for less

intense pyrotechnic devices), in which the

effects of the shocks are primarily related to

the propagation of a stress wave in the material.

* The far-field (material beyond about 15 cm for intense pyrotechnic devices, or beyond 7.5 cm for

less intense devices) in which the shock is then propagated whilst attenuating in the structure and

from which the effects of this wave combine with a damped oscillatory response of the structure at

its frequencies of resonance (or the structural response only).

Three fields are sometimes suggested: the near-field, the mid-field (same definition as the far-field

above, between 15 and 60 cm, or 7.5 and 15 cm for the less intense shocks), and the far-field, where only

the structural response effect persists.

An investigation by Moening (1986) showed that the causes of observed failures on the American

launchers between 1960 and 1986 (63 due to pyroshocks) are mainly the difficulty in evaluating these

shocks a priori, especially the lack of consideration of these excitations during design and the absence of

rigorous test specifications.

Such shocks have the following general characteristics.

* The levels of acceleration are very important; the shock amplitude is not simply related to the

quantity of explosive used (Hughes, 1983). The quantity of metal cut by a jet cord is, for example,

a more significant factor than the mass of the explosive.

FIGURE 12.21 Example of a pyroshock. (Source:

Lalanne, Chocs Mecaniques, Hermes Science Publications.

With permission.)

TABLE 12.3 Characteristics of Each Pyroshock Intensity Domain

Field Distance from

the Source (cm)

Intense Pyrotechnic

Devices (cm)

Shock Amplitude

Frequency Content

Near field (stress wave

propagation effect)

, 7.5 , 15 . 5000g, up to 300,000g

above 100,000 Hz

Far-field (stress wave propagation effect

þ structural response effect)

. 7.5 . 15 1000 to 5000g above 10,000 Hz

Source: Lalanne, Chocs Mecaniques, Hermes Science Publications. With permission.

Mechanical Shock 12-17

© 2005 by Taylor & Francis Group, LLC

* The signals assume an oscillatory shape.

* The shocks have very close components according to three axes; their positive and negative

response spectra are curves that are roughly symmetrical with respect to the axis of

the frequencies. They begin at zero frequency with a very small slope at the origin, grow

with the frequency until a maximum located at some kHz, even a few tens of kHz, is

reached, and then tend according to the rule towards the amplitude of the temporal signal.

Due to their contents at high frequencies, such shocks can damage electric or electronic

components.

* The a priori estimate of the shock levels is neither easy nor precise.

These characteristics make pyroshocks difficult to measure, requiring sensors that are able to accept

amplitudes of 100,000g, frequencies being able to exceed 100 kHz, with important transverse

components. They are also difficult to simulate.

The dispersions observed in the response spectra of shocks measured under comparable conditions are

often important, 3 dB with more than 8 dB compared with the average value, according to the authors

(Smith, 1984, 1986). The reasons for this dispersion are in general related to inadequate instrumentation

and the conditions of measurement (Smith, 1986):

* The fixing of the sensors on the structure using insulated studs or wedge which act like mechanical

filters.

* Zero shift, due to the fact that high accelerations make the crystal of the accelerometer work in a

temporarily nonlinear field (this shift can affect the calculation of the SRS).

* Saturation of the amplifiers.

* Resonance of the sensors.

With correct instrumentation, the results of measurements carried out under the same conditions are

actually very close. The spectrum does not vary with the tolerances of manufacture and the assembly

tolerances.

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