Isothermal fatigue behavior of a titanium matrix composite under a hybrid strain-controlled loading condition

The fatigue response of an eight-ply, unidirectional, titanium-based metal-matrix composite (MMC) (SCS-6/Ti-15-3) was investigated at elevated temperature (427 °C) using a hybrid strain-controlled loading mode. This hybrid control mode did not allow the thin MMC specimen to experience any compressive stress and, thus, prevented buckling. All fatigue testing was conducted at a constant strain rate of 0.2% s−1. Damage mechanisms were systematically identified for the cases when loading was parallel or perpendicular to the fiber direction. When the fibers were parallel to the loading direction, the dominant damage mechanism was either fiber fracture or matrix cracking. Matrix creep occurred at all levels of strain, and matrix plasticity was observed when the strain level was greater than 0.55%. When loading was perpendicular to the fiber direction, the fiber-matrix interfacial damage was the dominant damage mechanism. The severity of this damage varied depending upon the maximum strain level. Matrix cracks also had a critical effect on the fatigue response when the maximum strain level was greater than 0.35%. Plastic deformation in the matrix material occurred for strain levels greater than 0.23%, and matrix creep was a key factor at all strain levels. Fatigue-life diagrams along with dominant deformation and damage mechanisms were established for both cases and are compared with previous studies.