A multiaxial, isothermal, continuum damage mechanics model for creep and creep-fatigue interaction of a unidirectional metal-matrix composite (MMC) volume element is presented. The model is phenomenological, stress based, and assumes a single scalar internal damage variable with directional-dependent evolution. The present creep damage model is an extension of the Leckie-Hayhurst creep damage model to unidirectional MMCs and is shown when taken to its isotropic limit, to directly simplify to this previously developed and validated isotropic creep damage model. This extension is accomplished by introducing into the isochronous damage function physically meaningful stress invariants that reflect the local stress and material orientation within a metallic, transversely isotropic material (for example, hexagonally packed unidirectional metal-matrix composite). These invariants are included based on the anticipation that the associated stress may strongly influence void growth at the fiber-matrix interface (as this interface is postulated to play a role, on the mesostructural scale, analogous to that of grain boundaries on the microstructural scale and interfacial degradation); and, consequently may correlate with a creep rupture mechanism based on interfacial degradation through diffusion-related void growth.
Creep-fatigue interaction is accomplished by directly adding together the mechanical effects of creep and fatigue damage. A uniaxial parametric study is performed under pure creep and creep-fatigue conditions, to demonstrate the sensitivity of the various material parameters and the capability of the formulation in attempting to characterize a class of composite materials. Specifically, two interaction regions will be examined, the first being important for low-frequency loading cycles (<5 Hz) and is termed “creep-dominated” interaction; whereas the second is for high-frequency loading cycles with a non-zero mean stress, and is termed “fatigue-dominated” interaction. Results are presented in the form of S-N and damage accumulation curves.