Thermomechanical fatigue damage/failure mechanisms in SCS-6/Timetal 21S [formula omitted] composite

Abstract

The thermomechanical fatigue (TMF) deformation, damage and life behaviors of SCS-6/Timetal 21S [ 0 90 ] s were investigated under zero-tension conditions. In-phase (IP) and out-of-phase (OP) loadings were investigated with a temperature cycle from 150 to 650°C. An advanced TMF test technique was used to mechanically quantify damage progression. The technique incorporated explicit measurements of the macroscopic (i) isothermal static moduli at the temperature extremes of the TMF cycle, and (ii) coefficient of thermal expansion (CTE) as functions of the TMF cycles. The importance of thermal property degradation and its relevance to accurate post-test data analysis and interpretation is briefly addressed. Extensive fractography and metallography were conducted on specimens from failed and interrupted tests to characterize the extent of damage at the microstructure level. Fatigue-life results indicated trends analogous to those established for similar unidirectional [0] reinforced titanium-matrix composite systems. High-stress IP and mid- to low-stress OP loading conditions were life-limiting in comparison to maximum temperature isothermal conditions. Dominant damage mechanisms changed with cycle type. Damage resulting from IP TMF conditions produced measurable decreases in static moduli but only minimal changes in the CTE. Metallography on interrupted and failed specimens revealed extensive [0] fiber cracking with sparse matrix damage. No surface-initiated matrix cracks were present. Comparable OP TMF conditions initiated environment-enhanced surface cracking and matrix cracking initiated at [90] fiber/matrix (F/M) interfaces. Notable static moduli and CTE degradations were measured. Fractography and metallography revealed that the transverse cracks originating from the surface and [90] F/M interfaces tended to converge and coalesce at the [0] fibers.