CYCLIC BEHAVIOR OF UNIDIRECTIONAL AND CROSS-PLY TITANIUM MATRIX COMPOSITES

Relatively simple and efficient micromechanical models are used to obt ain the uniaxial response of SCS-6/Timetal 21S with [0](4) and [0/90]( s) laminates when subjected to isothermal and thermomechanical fatigue (TMF) loadings. Features of the modeling that are required to obtain the accurate deformation behavior for this class of materials under th ese loadings are highlighted. To this end, a comparison is made betwee n the concentric cylinder model and the uniaxial stress model for repr esenting the [0] laminate. The axial stresses from the two models are very similar under mechanical loading. The greatest differences appear under thermal loading alone. The differences on the composite respons e between a lime-independent elastic-plastic and a viscoplastic matrix constitutive model are also examined. The latter is based on the Bodn er-Partom unified constitutive model. The [0/90] laminate is treated b y adding a parallel element with smeared [90] ply properties to the [0 ] model and invoking axial strain compatibility as well as stress equi librium. The proposed constitutive law for the [90] ply includes both matrix viscoplasticity and fiber/matrix separation damage and is based on damage mechanics concepts. The effect of cyclic frequency on TMF b ehavior is examined. The in-phase TMF life is shown to be very sensiti ve to frequency due to the relaxation of matrix stress and the attenda nt increase in fiber stress.