Assessment of a Local Fiber Stress Parameter for Thermomechanical Fatigue Life Prediction of Titanium Matrix Composites

摘要

Titanium Matrix Composites (TMC's) are envisioned for use in the next generation of advanced aircraft and their engines. To ensure a smooth transition to industry, life predictio methodologies, which can account for radom variations in mechanical and thermal loads and are base don the physicla dmage processes of the material in question, must be developed. To facilitate the development of such a model, fatigue testing has been conducted at Georgia Tech on (0/+-45/90)_s and (90/+-45/0)_s laminates of SCS-6/Timetal 21S. The tests were done at temperatures of 400, 500 and 650 deg C, with hold times of 1, 10 and 100 seconds superimposed at the maximum stress. The purpose of the tests was to separate the effect of time dependent deformation from the effect of environmental degradation. Using the results of these tests and results generated at NASA-Lewis Research Center (LeRC) and the U.S> Air Force's Wright Laboratories, a model has bene developed which is based on the stress in the load carrying fibers. This stress is modified by an effective stress concentration factor which is due to matrix cracking and a factor which includes the effect of fhole times. This is a single term to matrix cracking and a factor which includes the effect of hold times. This is a single term model that can account for differences in test parameters and would be able to account for any variation in mechanical and thermal load. This model is compared to five methodologies previously developed for life prediction and is shown to have significantly better predictive power while reducing the number of empirical constants and curve fitting parameter necessary to collapse the data.