John C. Thesken

Durability of Polymer Composites in Power and Propulsion Applications

Advancements in polymer matrix composites have made them attractive to developers of power and propulsion equipment for spaceflight and aeronautic applications. However, many of these applications have very unique operational environments that are not easily found in the available design databases. Rapid insertion of these materials through prototype development and concept demonstration programs are hampered by the absence of relevant design data. In such cases, development programs conducted by the NASA Glenn Research Center have found it beneficial to employ pathfinder experimental methods designed to focus on the specific application and operational environment at hand. This chapter describes specialized experimental investigations of composite durability for applications that include flywheel energy storage, combustion chamber, and fan case structures. The experiments were designed to investigate complex thermomechanical and hygrothermal environments posed by these technologies. Beyond cycles-to-failure and residual strength, dimensional stability and stiffness degradation are key, if not primary, concerns in preserving functional capability for the cited applications.

Stress Rupture Life Reliability Measures for Composite Overwrapped Pressure Vessels

Abstract Composite Overwrapped Pressure Vessels (COPVs) are often used for storing pressurant gases onboard spacecraft. Kevlar (DuPont), glass, carbon and other more recent fibers have all been used as overwraps. Due to the fact that overwraps are subjected to sustained loads for an extended period during a mission, stress rupture failure is a major concern. It is therefore important to ascertain the reliability of these vessels by analysis, since the testing of each flight design cannot be completed on a practical time scale. The present paper examines specifically a Weibull statistics based stress rupture model and considers the various uncertainties associated with the model parameters. The paper also examines several reliability estimate measures that would be of use for the purpose of recertification and for qualifying flight worthiness of these vessels. Specifically, deterministic values for a point estimate, mean estimate and 90/95 percent confidence estimates of the reliability are all examined for a typical flight quality vessel under constant stress. The mean and the 90/95 percent confidence estimates are computed using Monte-Carlo simulation techniques by assuming distribution statistics of model parameters based also on simulation and on the available data, especially the sample sizes represented in the data. The data for the stress rupture model are obtained from the Lawrence Livermore National Laboratories (LLNL) stress rupture testing program, carried out for the past 35 years. Deterministic as well as probabilistic sensitivities are examined.