THERMOMECHANICAL FATIGUE BEHAVIOR OF AN ANGLE-PLY SCS-6 TI-15-3 METAL-MATRIX COMPOSITE

The response of the continuous-fiber-reinforced metal-matrix composite SCS-6/Ti-15-3 with orientation [+/-45](2s) was investigated under fatigue conditions. Three temperature and load profiles were used to characterize the metal-matrix composite, namely: isothermal fatigue (IF) at 427 degrees C, in-phase thermomechanical fatigue (IP-TMF) from 149 degrees C to 427 degrees C, and out-of-phase thermo-mechanical fatigue (OP-TMF) from 149 degrees C to 427 degrees C. At high stress levels (above 400 MPa) the [+/-45](2s) laminate displayed in-phase, out-of-phase, and isothermal fatigue lives which were all within 1000 cycles. At these high stress levels the laminate exhibited the most detrimental fatigue response when subjected to the in-phase TMF profile. The IP-TMF and IF curves displayed equivalent fatigue lives at stress levels below 400 MPa. At these lower stress levels (below 400 MPa) the OP-TMF lives were approximately a factor of two lower than the IP-TMF and IF lives. Mechanical strain measurements, Young's modulus calculations, micromechanical analysis, acetate edge replication, metallography, and fractography were all employed to explain the damage mechanisms and their effects on the fatigue life of the laminate. The initial mode of damage for all specimens tested at all load-temperature profiles was fiber-matrix interfacial failures. Matrix plasticity, matrix cracking, fiber rotation, and fiber failures were evident at all applied stress levels in IP-TMF, OP-TMF, and IF. The degree of each of these mechanisms was dependent on both the applied stress level and the temperature profile.