12.11 Pyrotechnic Shock Simulation

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Many works have been published on the characterization, measurement, and simulation of shocks of

pyrotechnic origin, generated by bolt cutters, explosive valves, separation nuts, and so on (Zimmerman,

1993). The test facilities suggested are many, ranging from traditional machines to very exotic means.

The tendency today is to consider that the best simulation of shocks measured in near-field can be

obtained only by subjecting the material to the shock produced by the real device.

For shocks in the far-field, simulation can be carried out either using the real pyrotechnic source

and a particular mechanical assembly, using specific equipment using explosives, or by impacting

metal to metal if the structural response is more important. When the real shock is practically made up

only of the response of the structures, a simulation on a shaker is possible (when performances by this

means are allowed).

12.11.1 Simulation Using Pyrotechnic Facilities

The simplest solution consists of making functional, real pyrotechnic devices on real structures.

The simulation is perfect but (Conway et al., 1976; Luhrs, 1976):

* It can be expensive and destructive.

* One cannot apply an uncertainty factor (Lalanne, 2002d) to cover the variability of this

shock without being likely to create unrealistic local damage (a larger load, which requires

an often expensive modification of the devices and can be much more destructive).

To avoid this problem, an expensive solution consists of carrying out several tests in a

statistical matter.

There is an infinity of acceleration – time signals with a given spectrum. Several elementary

waveforms can be used to build a signal of acceleration having a given SRS. They give similar

results.

Without particular precaution, the signals thus obtained generally have one duration much

larger and an amplitude much smaller than the shocks which were used to calculate the reference

SRS. A complementary parameter (shock duration, velocity change, etc.) is often specified with the

SRS to limit this effect.

12-58 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

One often prefers to carry out a simulation on a

reusable assembly, the excitation still being pyrotechnic

in nature. Several devices have

been designed, some examples of which are as

follows:

* A test facility made up of a cylindrical

structure (Ikola, 1964), which comprises a

“consumable” sleeve cut out for the test by

an explosive cord. Preliminary tests are

carried out to calibrate the facility while

acting on the linear charge of the explosive

cord and/or the distance between the

equipment to be tested (fixed on the

structure as in the real case if possible)

and the explosive cord.

* A greater number of small explosive charges near the equipment to be tested on the structure in

“flowers pots.” The number of pots to be used on each axis depends on the amplitude of the

shock, the size of the equipment, and the local geometry of the structure. The shape of the shock

can be modified within certain limits by use of damping devices, placing the pot closer to or

further from the equipment, or by putting suitable padding in the pot (Aerospace Systems

pyrotechnic shock data, 1970).

* A test facility made up of a basic rectangular steel plate (Figure 12.68) suspended horizontally.

This plate receives on its lower part, directly or by the intermediary of an “expendable” item, an

explosive load (chalk line, explosive in plate or bread).

A second plate supporting the test item rests on the base plate via four elastic supports (Thomas,

1973). The reproducibility of the shocks is better if the explosive charge is not in direct contact with the

base plate.

12.11.2 Simulation Using Metal-to-Metal Impact

The shock obtained by a metal-to-metal impact has similar characteristics to those of a pyrotechnical

shock in an intermediate field: great amplitude; short duration; high frequency content; SRS comparable

with a low frequency slope of 12 dB per octave, etc. The simulation is in general satisfactory up to

approximately 10 kHz.

The shock can be created by the impact of a hammer on the structure itself, a Hopkinson bar or a

resonant plate (Bai and Thatcher, 1979; Luhrs, 1981; Davie, 1985; Dokainish and Subbaraj, 1989; Davie

and Batemen, 1992).

With all these devices, the amplitude of the shock is controlled while acting on the velocity of impact.

The frequency components are adjusted by modifying the resonant geometry of system (changing the

length of the bar between two points of fixing, adding or removing runners, etc.) or by the addition of a

deformable material between the hammer and the anvil.

12.11.3 Simulation Using Electrodynamic Shakers

The limitation relating to the stroke of the electrodynamic shaker is not very constraining for

the pyrotechnical shocks since they are at high frequencies. The possibilities are limited especially

by the acceptable maximum force and then concern the maximum acceleration of the shock

FIGURE 12.68 Plate with resonant system subjected

to detonation. (Source: Lalanne, Chocs Mecaniques,

Hermes Science Publications. With permission.)

Mechanical Shock 12-59

© 2005 by Taylor & Francis Group, LLC

(Conway et al., 1976; Luhrs, 1976; Powers, 1976; Caruso, 1977). If one agrees to cover only part of the

SRS where material has resonant frequencies, then when one makes a possible simulation on the shaker,

which gives a better approach to matching the real spectrum.

Exciters have the advantage of allowing the realization of any signal shape such as shocks of simple

shapes (Dinicola, 1964; Gallagher and Adkins, 1966), but also of reproducing a specified SRS (direct

control from an SRS; see Section 12.10.1).

It is possible, in certain cases, to reproduce the real SRS up to 1000 Hz. If one is sufficiently far away

from the source of the shock, the transient has a lower level of acceleration and the only limitation is the

bandwidth of the shaker, which is about 2000 Hz. Certain facilities of this type were modified to make it

possible to simulate the effects of pyrotechnical shocks up to 4000 Hz. One can thus manage to simulate

shocks whose spectrum can reach 7000g (Moening, 1986).

12.11.4 Simulation Using Conventional Shock Machines

We saw that, generally, the method of development of a specification of a shock consists of replacing

the transient of the real environment, whose shape is in general complex, with a simple shape shock,

such as half-sine, triangle, trapezoid, and so on, starting from the SRS equivalence criterion, with

the application of a given or calculated uncertainty factor (Lalanne, 2002d) to the shock amplitude

(Luhrs, 1976).

With the examination of the shapes of the response spectra of standard simple shocks, it seems that

the signal best adapted is the TPS pulse, whose spectra are also appreciably symmetrical. SRSs of the

pyrotechnical shocks with, in general, averages close to zero have a very weak slope at low

frequencies. The research of the characteristics of such a triangular pulse (amplitude, duration)

having an SRS envelope of that of a pyrotechnical shock led often to a duration of about 1 msec and

to an amplitude being able to reach several tens of thousands of msec22. Except in the case of very

TABLE 12.12 Advantages and Drawbacks of Various Test Facilities for the Pyroshock Simulation

Shock Facility Field Advantages Drawbacks

Real pyrotechnic devices

on real structures

Near Very good simulation Expensive, generally destructive, no

uncertainty factor/test factor

Reusable assembly with

pyrotechnic excitation

Near Good simulation Necessity of preliminary tests,

no uncertainty factor/test factor,

use of explosive (specific conditions

to ensure safety), expensive

Metal to metal impact Near Good simulation,

no explosive charge

Necessity of preliminary tests,

limitations in acceleration and

frequency (approximately 10 kHz)

Electrodynamic shaker Far Easy implementation, control using

any time history signal or SRS,

possibility of using an uncertainty

factor or a test factor

Necessity of one test by axis,

maximum frequency up to

about 1 to 2 kHz

Conventional

shock-test machine

Far Easy implementation, possibility

of using an uncertainty factor

or a test factor

Use of a shock pulse with velocity

change instead of an oscillatory

shock pulse (over test at low

frequency), necessity of one test

by axis, shock duration higher

than 2 msec (0.1 msec using a

specific device for very light

test item), limitation in

amplitude, useable for very

small test items only

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© 2005 by Taylor & Francis Group, LLC

small test items, it is in general not possible to carry out such shocks on the usual drop tables due to

certain limitations:

* Limitation in amplitude (acceptable maximum force on the table).

* Duration limit: the pneumatic shock simulators do not allow it to go below 3 to 4 msec; even with

the lead shock simulators, it is difficult to obtain a duration of less than 2 msec and a larger shock

duration leads to a significant overtest at low frequency.

* The SRSs of the pyrotechnical shocks are much more sensitive to the choice of damping than

simple shocks carried out on shock machines.

A comparison of some pyroshock-test facilities is given in Table 12.12.

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