Modeling of Ordnance-Induced Pyrotechnic Shock Testing
Design of an ordnance-induced pyrotechnic shock test to meet a specific acceleration based Shock Response Spectrum (SRS) test requirement for a given test article has traditionally been an empirical process. Based on experience, the test engineer will determine a potential test configuration and then, through a trial-and-error process, modify the test parameters and configuration until acceptable SRS levels have been achieved. As a complement to this approach, National Technical Systems (NTS) has developed an explicit finite element based modeling approach to simulate an ordnance-induced pyrotechnic shock test. This tool may be used to assist with test configuration design for particularly challenging test requirements or to streamline the process of arriving at acceptable test levels during the calibration phase of a test program.
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 1000 Hz 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.
Salvaging pyrotechnic data with minor overloads and offsets
The authors are sometimes presented with data with serious flaws, like saturation, over-range, zero shifts, and impulsive noise, including much of the available pyrotechnic data. Obviously, these data should not be used if at all possible. However, they are sometimes forced to use these data as the only data available. A method to salvage these data using wavelets is discussed. The results must be accepted with the understanding that the answers are credible, not necessarily correct. None of the methods will recover information lost due to saturation and over-range with the subsequent nonlinear behavior of the data acquisition system. The results are illustrated using analytical examples and flawed pyrotechnic data.