Fatigue properties of SiC/Al composites

Silicon carbide monofilaments, produced by chemical vapor deposition, have several different classifications depending on the surface treatment imparted to the fiber at the final stage of production. The newest of these fibers, designated SCS-8, is produced specifically for aluminum matrix composites in an attempt to improve transverse mechanical properties over the other types.
This study focuses on the response of SCS-8/6061 Al alloy composites to cyclic loading. Laminates with fibers oriented in the 0°, 90°, and ±45° direction were used in the study. Particular emphasis was on elastic modulus changes due to fatigue. Modulus changes could be observed in as few as ten cycles, depending upon stress level and laminate orientation.
Unnotched specimens were axially loaded in tension-tension (R = 0.1) mode with a sinusoidal waveform at a frequency of 10 Hz. Periodically, cycling was stopped and the specimen elastic modulus determined. Modulus values at N number of cycles, EN, normalized with respect to virgin modulus, E0, were then plotted versus log number of cycles. Conventional fatigue induced fracture behavior was also obtained and reported in the familiar S-N format.
Fractographic observation indicates several failure modes: fiber splitting, fiber/matrix debonding, matrix shear failure, inter-ply delamination, and fiber fracture. Axial tension-tension high cycle fatigue behavior is controlled by the matrix. Low cycle fatigue and static behavior is characterized by progressive failure of the SiC fiber. Modulus change is the result of crack propagation along the fiber-matrix interface.