Prediction of the Vibration Levels Generated by Pyrotechnic Shocks Using an Approach by Equivalent Mechanical Shock

This paper is concerned with the numerical simulation of mechanical structures subjected to pyroshocks. In practice, the methodology is applied on the pyroshock test facility, which is used by Thales to qualify the electronic equipment intended to be embarked onboard of spatial vehicles. This test facility involves one plate or two plates linked by screw bolts. The tested device is mounted on one side while the explosive charge is applied on the other side. The main issue of this work is to be able to tune, by simulation, the parameters of the facility (number of plates, material of plate, number of bolts, amount of explosive, etc.) so as to get the required level of solicitation during the test. The paper begins by an introduction presenting the state of the art in terms of pyroshock modeling, followed by a description of the shock response spectrum (SRS) commonly used to represent the test specifications of an embarked equipment. It turns out that there is a lack of computational techniques able to predict the dynamic behavior of complex structures subjected to high frequency shock waves such as explosive loads. Three sections are then devoted to the simulation of the pyrotechnic test, which involves on one hand a model of the structure and on the other hand an appropriate representation of the impulsive load. The finite element method (FEM) is used to model the dynamic behavior of the structure. The FEM models of several instances of the facility have been updated and validated up to $1000Hz$ by comparison with the results of experimental modal analyses. For the excitation source, we have considered an approach by equivalent mechanical shock (EMS), which consists in replacing the actual excitation by a localized force applied on the FEM model at the center of the explosive device. The main originality of the approach is to identify the amplitude and duration of the EMS by minimizing the gap between the experimental and numerical results in terms of the SRS related to several points of the facility. The identification has been performed on a simple plate structure for different amounts of explosive. The methodology is then validated in three ways. Firstly, it is shown that there is a good agreement between experimental and numerical SRS for all the points considered to identify the EMS. Secondly, it appears that the energy injected by the EMS is well correlated with the amount of explosive. Lastly, the EMS identified on one structure for a given amount of explosive leads to coherent responses when applied on other structures. A parametric study is finally performed, which shows the influence of the thickness of the plate, the material properties, the localization of the EMS, and the addition of a local mass. The different obtained results show that our pyroshock model allows to efficiently estimate the acceleration levels undergone by the electronic equipment during a pyroshock and, in this way, to predict some eventual electrical failures, such as the chatter of electromagnetic relays.