Thermal cycling creep of Ti-6Al-4V/SiC monofilament composites under transverse loading

Laser scanning extensometry has been used to measure creep strain histories of Ti-6Al-4V reinforced with continuous SiC monofilaments. Thermal cycling was carried out under static load on specimens with the fibers oriented either parallel or transverse to the loading direction and also on the unreinforced matrix. It was noted that the unreinforced material did not behave as an isotropic continuum, apparently due to the presence of crystallographic texture. Average strain rates for the composites have been compared with those of specimens held isothermally at the ''diffusional mean'' temperature. The creep rate of the composite under transverse loading is considerably enhanced by thermal cycling. Specimens are also observed to creep in the reverse sense to the applied load during the cooling part of the cycle, indicating that internal stresses from differential thermal contraction can be highly significant compared with those from the external load. Experimental data have been compared with predictions from a model based on a creep rate expression involving the volume-averaged deviatoric stress in the matrix, calculated using the Eshelby method. The influence of stress relaxation phenomena has been simulated by varying the effective stress-free temperature of the composite, Tesf, during the cycle. The main features of the behavior can be explained on this basis, without recourse to modeling in which account is taken of local variations in stress state within the matrix