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The transverse fatigue characteristics of a unidirectional titanium-based metal matrix composite (MMC) (SCS-6/Ti-15-3) were investigated under an isothermal condition. Fatigue tests were performed using a hybrid strain-controlled loading mode. In this hybrid control mode, the specimen was always in a tension-tension state of stress perpendicular to the fiber direction. This prevented any possible buckling effects. A systematic approach that involved fatigue tests, microscopic evaluation, and micromechanical analysis, was taken to characterize the fatigue response (that is, fatigue life, stress-strain response, and so forth) and identify the damage and deformation mechanisms. The analysis involved a unique method to model the fiber-matrix interfacial damage. It was found that the fatigue response was initially dominated by the amount of fiber-matrix interfacial damage that occurred during the first loading cycle. The subsequent response was dependent on the rate at which this interfacial damage progressed, the development and propagation of matrix cracks, and matrix inelastic deformation (plasticity and creep). The chronology and accumulation of these damage mechanisms were dependent on the applied strain level. Using the combined approach involving experiments, microscopy, and analysis, the correlation among applied strain levels, fatigue life, damage mechanisms, and macroscopic response (stress and stiffness) were established in this paper. In Part II, the details and results of the micromechanical analysis are presented.
Wright Laboratory, Materials Directorate,
Professor and head, Air Force Institute of Technology, OH
Stock #: CTR10461J