EFFECT OF LAMINATE ORIENTATION ON THE THERMOMECHANICAL FATIGUE BEHAVIOR OF A TITANIUM MATRIX COMPOSITE

Thermomechanical fatigue (TMF) tests were conducted on the silicon car bide fiber and titanium alloy matrix composite, SCS-6/TIMETAL(R)21S. T hree different laminate orientations were considered: [0]4, [0/90]s, a nd [0/+/-45/90]s. Both in-phase and out-of-phase stress-controlled tes ts were conducted under a temperature cycle of 150-degrees to 650-degr ees-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 (S(max)) by the ultimate tensile strength a t the S(max) temperature of the TMF cycle. For all laminate orientatio ns, the maximum and minimum strain increase during in-phase cycling, w hereas only maximum strain increases during out-of-phase cycling. Dama ge accumulation under in-phase cycling is attributed to a combination of the increasing stress carried by the [0] fibers due to cyclic matri x stress relaxation and the gradual breakage of [0] fibers. The damage during out-of-phase cycling is attributed to matrix cracks which init iate at the surface with the aid of the environment and/or initiate in ternally at the transverse fibers in [0/90]s and [0/+/-45/90]s composi tes. 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 in-phase cycling, but not under out-of-phase cycling. TMF condit ions are shown to be more severe than isothermal fatigue.