Micromechanics Analysis and Life Prediction of Titanium Matrix Composites

The response of a unidirectional SCS-6/Ti-6Al-4V composite is evaluated under a range of isothermal fatigue (IF) and thermomechanical fatigue (TMF) conditions. Both processing and thermomechanical test conditions are simulated using a cylinder in cylinder code, FIDEP, which treats the fiber as thermoelastic while the matrix is characterized using a recent version of the Bodner-Partom constitutive law. Computed stresses and strains in the constituents are obtained after 10 cycles of loading and are used as input into a new and simpler version of a life fraction model. Applied stress at temperature and computed matrix strains are used for the time-dependent and matrix fatigue terms in the model, respectively. Cyclic stress-strain behavior of the matrix material is also computed and used, together with fatigue life data, to identify the governing mechanisms and to explain the observed trends in fatigue life as a function of frequency and applied stress level. Comparisons of fatigue behavior with that of SCS-6/Timetal 21S illustrate differences when tested under nominally similar conditions.