Probabilistic Material Degradation under High Temperature, Fatigue, and Creep

Abstract

This paper describes the development of methodology that provides for quantification of uncertainty in the lifetime material strength degradation of structural components of aerospace propulsion systems subjected to a number of diverse random effects. The methodology is embodied in the two computer programs PROMISS and PROMISC. These programs form a material‐resistance model used in an aerospace structural‐reliability program NESSUS. A probabilistic material‐degradation model, in the form of a postulated randomized multifactor interaction equation, is used to quantify lifetime material strength. Each multiplicative term in the model has the property that if the current value of an effect equals the ultimate value, then the lifetime strength will be zero. Also, if the current value of an effect equals the reference value, the term equals one and lifetime strength is not affected by that particular effect. Presently, the model includes three effects that typically reduce lifetime strength: high temperature, mechanical fatigue, and creep. The paper also includes the statistical analysis of experimental data for INCONEL 718 obtained from the open literature. This statistical analysis of data provided regression parameters for use as the empirical material constants of the model, thus calibrating the model specifically for INCONEL 718. Model calibration was carried out for three variables, namely, high temperature, mechanical fatigue, and creep. Finally, using the PROMISS computer program, a sensitivity study was performed with the calibrated random model illustrating the effect of each variable upon random lifetime strength.