RIMI: High Temperature Fatigue of Continuous Fiber Reinforced Silicon Carbide/Silicon Nitride Composites

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Continuous fiber reinforced ceramic composites (CFCC) will be tested under fatigue at room and elevated temperatures to study the variation in fatigue phenomena with temperature. Laminated coupons of SiCf/Si3N4 (silicon Carbide fiber reinforced Silicon Nitride matrix) composites with symmetric layups will be subjected to tensile-tensile fatigue. Prior to fatigue tests, tensile properties will be characterized at various temperatures to determine the strength, and to decide about specific temperatures at which the elevated temperature fatigue tests are to be conducted. Room temperature fatigue tests will be performed at various stress ratios ranging from 0 to 0.5, and which frequencies varying from 1 to 10 Hz, to observe their effects on the fatigue life of the composite. High temperature fatigue tests will be conducted under isothermal conditions to maintain the cyclic loading as the main driving force for the composite failure. NDE and SEM characterization will be performed to evaluate the damage and the failure characteristics of the composite. Parallel to mechanical testing, finite element analysis will be performed to investigate the non-linear response of the laminate under tensile and fatigue loading. Layered shell element developed on the basis of first order shear deformation theory will be used. Both large deformation and plastic strain behavior will be considered to account for the geometric and material non-linearities in the analysis. Cumulative fatigue damage bases on the Miner's rule and the experimental S-N curve, will be simulated in the finite element model to study the laminate response under two and three stress level cyclic loading. Few specimens will be tested under cumulative fatigue to verify the FEM results. The proposed research will be conducted by three faculty associated with the Tuskegee University's Materials Research Laboratory (MRL) which is a multi-disciplinary facility developed for synthesis and characterization of high performance materials including advanced composites. It is proposed to involve six minority graduate students and six minority undergraduate students over a period of three years. The program will also allow the sponsoring department to recruit talented minority faculty and students.