A finite-element analysis of the inelastic relaxation of thermal residual stress in continuous-fiber-reinforced composites

In an effort to develop a methodology for interpreting in situ high-temperature neutron diffraction measurements of thermal residual stresses in composites, a finite-element model was developed and the implications of various material and experimental parameters on the residual stress evolution were studied. The model composite comprised a continuous Al2O3 fiber, unidirectionally aligned in a NiAl matrix. The effects of temperature-dependent elastic, plastic or creep properties, fiber volume fraction and cooling/heating rates were explored on the relaxation mechanisms of the residual stress during an initial cooling and a subsequent heating. Thermal path-dependency in the stress evolution was investigated by comparing the cooling and re-heat cycles. The result shows that the effect of the time-dependent deformation (creep) becomes more significant as the fiber content increases and the cooling/heating rate decreases. Furthermore, the path-dependency in stress evolution becomes stronger (i.e. considering the actual thermal history in the model becomes more important) as the total inelastic relaxation increases during the cooling and/or the subsequent re-heating cycles due to; (i) the presence of creep in addition to plastic deformation, (ii) increased fiber volume fraction, and (iii) slower cooling/heating rates. It was also demonstrated that using an estimated stress-free temperature in elastic–plastic models can be problematic in predicting the high-temperature behavior due to; (i) the simple assumption of reduced ΔT, and (ii) the thermal path-dependency.