Thermomechanical Fatigue Behavior of a Quasi-lsotropic SCS-6/Ti-15-3 Metal Matrix Composite

As the speed of new aerospace vehicles pushes the supersonic and hypersonic envelopes, aerodynamic heating and structural strength and weight are becoming even greater design factors. Here, the response of a quasi-isotropic laminate of metal matrix composite, SCS-6/Ti-15-3 in a thermomechanical fatigue (TMF) environment was investigated. To achieve this, three sets of fatigue tests were conducted: (1) in-phase TMF (IP-TMF), (2) out-of-phase TMF (OP-TMF), and (3) isothermal fatigue (IF). The fatigue response was dependent on the test condition and the maximum stress level during cycling. The IF, IP-TMF, and OP-TMF conditions yielded shortest fatigue life at higher, intermediate and lower stress levels, respectively. Examination of the failure mode through the variation of strain or modulus during cycling, and post-mortem microscopic evaluation revealed that it was dependent on the fatigue condition and applied stress level. Higher stresses, mostly with IP-TMF and IF conditions, produced a primarily fiber dominated failure. Lower stresses, mostly with the OP-TMF condition, produced a matrix dominated failure. Also, an empirical model based on the observed damage mechanisms was developed to represent the fatigue lives for the three conditions examined here.