Pyrotechnic shocks caused, for example, by weapons fire, the launch of a missile or the separation of the nose cone of a space launcher are complex waveforms. Their reproducibility in the laboratory requires a great deal of discussion with the client beforehand, in order to prepare the tests in the best possible way, and thus get closer to real-life conditions. The Adetests laboratory, part of the Emitech group, offers to meet the need for pyrotechnic shock simulation using metal-to-metal impact methods on either pendulum-type or guided free-fall shock benches. The levels specified and the mass of the equipment generally dictate which type of rig to use. In addition, questions need to be asked about the influence of zero or non-zero velocity at the moment of impact. For pyrotechnic shock simulation, the time signal alone is not sufficient. In order to compare actual measurements with the tests carried out, a frequency measurement is also used: the Shock Response Spectrum (SRS), a mathematical model originally developed to characterize earthquakes. The SRC is the reference for the MIL-STD810G military standard for defining specifications. The SRC of a metal-to-metal shock is close to that of a pyrotechnic shock.
Two types of impact benches meet every need
In the case of the pendulum-type bench, the EST is fixed to the part receiving the impulse, so its velocity is zero at the moment of impact. In the real world, equipment is most often subjected to a pyrotechnic impact with zero initial velocity, so the pendulum simulation is closer to real-life conditions. Adetests uses two models, one small and one large, to test equipment from 50 g to 50 kg.
With a guided free-fall rig, the equipment under test (UUT) is fixed to the falling platform: the velocity of the UUT is non-zero at the moment of impact, generating the shock wave. A free-fall test rig enables higher shock levels to be achieved than with a pendulum-type rig. Guided free-fall tests should be used with caution for suspended equipment, as the suspensions do not have the same prestressing as in the real case, and shock filtration will therefore not be the same. However, compared with pendulum-type benches, they offer better repeatability and faster set-up times. They can test equipment from 50 g to 10 kg.
The 2 bench types do not achieve the same levels of temporal amplitude and SRC. The pendulum type achieves a maximum of 7,000 g at temporal amplitude and around 1,500 g at 1 kHz on the SRC, while the guided free-fall type achieves a maximum of 30,000 g at temporal amplitude and 5,000 g at 1 kHz.
Knowing the properties of a suspension and the input SRC, the output SRC can be simulated by rigidly attaching the equipment to the shock rig. After receiving the specified SRC, Adetests sends an initial proposal for an SRC covering the specification. The laboratory then carries out fixture tooling and SRC measurements at the EST fixture points, and proposes the results to the customer. Once the proposed SRCs have been validated, the test campaign is launched.
Learn more: www.adetests.com/fr