Analysis of crack initiation and propagation in solid rocket motor nozzle throats

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2019
Nigar, Barış
Graphite is a widely used material in high temperature structural applications due to its high melting point and its mechanical strength and integrity at elevated temperatures. However, rocket nozzles made of graphite are subjected to very high temperatures and pressures that cause cracking and eventual failure. This thesis investigates the reasons of failure and explores design alternatives to overcome this problem. The experimental part of the thesis includes compression tests, tensile tests and fracture toughness tests on Mersen-2020 graphite. Compression and tensile tests showed a bimodular response. Fracture toughness tests involved Single Edge Notch Bend (SENB) specimens and demonstrated the brittle nature of graphite. Based on experimentally measured materials behavior of graphite, a two-dimensional finite element model investigated the crack propagation behavior. The analyses were done in three steps. The first step was the computational fluid dynamics analysis of the flow through the nozzle. Using the results of this step, second step predicted the temperature variations within the nozzle. The final and most extensive part of the modeling consisted of Extended Finite Element Method applied on the nozzle geometry experiencing thermal stresses based on the analysis in the second step. The results show that the cracks are mostly mode II cracks induced by the compressive stresses vi next to the flow surface of the nozzle. Within only 5 seconds, these cracks result in graphite pieces breaking off, which adversely affects the flow dynamics of the nozzle. The change in the flow directly impacts the desired thrust force and results in an unreliable performance. As the final part of the analysis, the thesis considered some alternative nozzle designs that utilize a segmented geometry consisting of individual graphite parts. Several geometries were analyzed for their crack propagation behavior. The results show that careful segmentation of the nozzle relieves the stresses in critical regions and delay the failure. The thesis provides insight into the mechanisms of failure in graphite rocket nozzles and presents a design approach for superior and more reliable performance.

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Citation Formats
B. Nigar, “Analysis of crack initiation and propagation in solid rocket motor nozzle throats,” Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Mechanical Engineering., Middle East Technical University, 2019.