Effect of Laminate Orientation on the Thermomechanical Fatigue Behavior of a Titanium Matrix Composite

Abstract

Thermomechanical fatigue (TMF) tests were conducted on the silicon carbide fiber and titanium alloy matrix composite, SCS-6/TIMETAL®21S. Three different laminate orientations were considered: [0]4, [0/90]s, and [0/±45/90]s. Both in-phase and out-of-phase stress-controlled tests were conducted under a temperature cycle of 150° to 650°C and a stress ratio of 0.1. The fatigue lives for these different orientations can be consolidated within a factor of 3 by normalizing the maximum applied stress (Smax) by the ultimate tensile strength at the Smax temperature of the TMF cycle. For all laminate orientations, the maximum and minimum strain increase during in-phase cycling, whereas only maximum strain increases during out-of-phase cycling. Damage accumulation under inphase cycling is attributed to a combination of the increasing stress carried by the [0] fibers due to cyclic matrix stress relaxation and the gradual breakage of [0] fibers. The damage during out-of-phase cycling is attributed to matrix cracks which initiate at the surface with the aid of the environment and/or initiate internally at the transverse fibers in [0/90]s and [0/±45/90]s composites. A simple model to compute cyclic strain accumulation based on the proposed mechanisms successfully captures the experimentally observed behavior. TMF life is shown to be sensitive to fiber volume fraction under inphase cycling, but not under out-of-phase cycling. TMF conditions are shown to be more severe than isothermal fatigue.