Probabilistic Reliability Analysis of Carbon/Carbon Composite Nozzle Cones with Uncertain Parameters

We describe in this paper a methodology to perform the probabilistic and reliability based design of a novel carbon/carbon rocket nozzle subjected to operational thermal and mechanical loads. In this methodology the nozzle is represented by a multiphysics finite element model capable of predicting the temperature and stress fields of the exit cone. The analysis shows that the most likely failure modes of the exit cone are related to compressive loading along the axial and hoop directions, and interlaminar shear. The probabilistic models used in this methodology account for the uncertainty of the material properties by using uniform and normal distributions and different variances. The reliability analysis is performed by using surface response methods. A global sensitivity analysis is also carried out using polynomial expansion chaos surface response models. A particular novelty of analysis is the use of Sobol indices to rank the importance of the single uncertain parameters in the models. The methodology provides a high level of confidence and robustness in determining that the axial thermal conductivity of the carbon/carbon material is the most critical material property to affect the three main failure modes, while the coefficient of the thermal expansion and the heat capacity play a very marginal role.