Abstract
This paper presents a method for determining the matrix dominated cyclic fatigue capability of continuous fiber reinforced SCS-6, Ti-15-3 metal matrix composites (MMC). The basis of the fatigue life prediction system relies on the matrix material dominating the fatigue behavior of the MMC. This assumption has been supported by test data and failure mechanisms identified in unidirectional isothermal and out-of-phase thermomechanical fatigue (TMF) specimens which typically show an area of matrix cracking before failure. Since the matrix fatigue capabilities dominate the fatigue behavior of the composite, an MMC thick cylinder model with an anisotropic coefficient of thermal expansion is used to characterize the state of stress in the matrix. The Smith-Watson-Topper fatigue stress damage parameter is applied to the calculated matrix stresses to determine a “corrected” matrix fatigue stress which is isolated from laminate geometry and laminate stress. This method correlates well to room and high temperature SCS-6/Ti-15-3 MMC test data. Since the “corrected” matrix stress fatigue response is independent of fiber volume, stress ratio, and load history, this MMC Life Prediction System can be used to predict the fatigue behavior of any reasonable laminate and loading in which the failure is matrix dominated. With this life system, life analyses can be performed for laminate geometries and loadings at which no test data exists, which will prove valuable when designing engine components where optimization of MMC capabilities are required but testing is limited.
