(A) study on separation mechanism, pyroshock generation and propagation of pyrotechnic release devices

Abstract

Pyrotechnic release devices such as explosive bolts and separation nuts are prevalent for many applications due to their high reliability, high power-to-weight ratio, and reasonable cost. Explosive bolts are pyrotechnic release devices, which are highly reliable and efficient for built-in release. Although various kinds of explosive bolts have been designed and utilized, most of the design processes rely on experience gleaned from repeated experiments. Based on the theories about shock waves and its interactions, rarefaction waves, spall and detonation of high explosives, the separation mechanism of the ridge-cut explosive bolt, also known as the ridge-cut mechanism, is induced. In order to provide a better understanding of the separation behavior of explosive bolts, a separation behavior analysis environment for ridge-cut explosive bolts is established. Ridge-cut explosive bolts, which are separated via the ridge-cut mechanism (or spall), are analyzed using a commercial hydrocode (ANSYS AUTODYN). Separation behavior analysis of the designed explosive bolts is used to verify the numerical analysis technique and to determine appropriate failure criteria. The numerical analysis of a ridge-cut explosive bolt clearly shows the ridge-cut mechanism including the shock propagation and reflection, and superposition of the release waves. Utilizing the proposed methodology, the separation characteristics of ridge-cut explosive bolts according to confinement conditions and especially the gap distance between the bolt body and the fixture are studied. Degradation in separation reliability due to tight gap distance is observed in separation experiments. This separation phenomenon is specifically clarified by the separation behavior analysis. Based on the numerical study of separation characteristics, some design improvements considering manufacturing tolerance are proposed. At the same time, a separation characteristic study regarding the design parameters of explosive bolts is carried out. Some design parameters are chosen that might affect the separation reliability, and behavior analysis is carried out for several designs. Based on this separation characteristics study, practical design improvements are suggested for reference explosive bolts. The results of this study provide useful information to avoid unnecessary separation experiments related with design parameters. Even though pyrotechnic release devices have been successfully developed, pyroshock which can cause catastrophic failures or malfunctions in electric components is still of concern. Pyroshock, also called pyrotechnic shock, is defined as the response of a structure to high frequency and high magnitude stress waves generated by an explosive event. This study proposes a numerical method for predicting the pyroshock of a ridge-cut explosive bolt using a commercial hydrocode. A numerical model is established by integrating fluid-structure interaction and complex material models for high explosives and metals, including high explosive detonation, shock wave transmission and propagation, stress wave propagation and more. To verify the proposed numerical scheme, pyroshock measurement experiments of the ridge-cut explosive bolts with two types of surrounding structures are performed. A total of five experiments are carried out and the pyroshock are measured at three points for each experiment using laser Doppler vibrometers (LDVs). The predicted pyroshock from the numerical analysis are compared with the measurement results in terms of acceleration and maximax shock response spectra (SRS). The numerical analysis results provide accurate prediction in both the time and frequency domains. In maximax SRS, the peaks due to the vibration modes of the structures are observed in both the experimental and numerical results. The numerical analysis also helps to identify the pyroshock generation source and the propagation routes. Next, this study deals with the pyroshock propagation on the structures. The experimental setup for the pyroshock propagation experiments is developed with the pyroshock excitation using the pyrotechnic initiators. The pyroshock propagation on a simple plate is measured using both LDVs and shock accelerometers. The characteristics of these sensors, including the frequency ranges and the sensor type (contact and non-contact), are studied. The pyroshock propagation analysis environment with a commercial hydrocode is established and verified through comparison with the experiment results. The pyroshock propagates through the thin plates in the form of flexural waves (or the anti-symmetric Lamb waves). By using established numerical and experimental techniques, the effect of pyroshock attenuation by the joints and the washers are studied. Plates connected by the joints with different materials (aluminum alloy, stainless steel and magnesium alloy) and plates connected by the aluminum alloy joints and the washers with different materials and thicknesses are considered. The experimental and numerical results are in agreement: the pyroshock attenuation is highly effective when the joints are made of higher density and stiffness material and when the washers have greater thickness. The major reason for the pyroshock attenuation due to the joints and the washers is flexural wave reflection at the discontinuities caused by acoustic impedance mismatching. This dissertation deals with many engineering challenges for pyrotechnic release devices: lack of a systematic design method, unidentified complex separation phenomena due to the nature of explosive events and the pyroshock issues. In particular, three kinds of numerical techniques using hydrocode were newly established: the separation behavior analysis for the ridge-cut explosive bolts, the pyroshock prediction for the ridge-cut explosive bolts and the pyroshock propagation analysis on the structures. The separation behavior analysis can be used to identify the separation mechanism, evaluate separation reliability and optimize the design of pyrotechnic release devices. The pyroshock prediction analysis can be used to estimate the pyroshock level of pyrotechnic devices early in the design process without experimentation, to identify the pyroshock source and to minimize pyroshock generation. The pyroshock propagation analysis provides a powerful method for the structure design to minimize pyroshock propagation. Without expensive and repetitive experiments, the pyroshock propagation on structures can be studied and the optimal design can be suggested.