Stress and Plastic Strain Fields during Unconstrained and Constrained Fabrication Cool Down of Fiber-Reinforced IMCs

Abstract

Two mathematical modes are presented for the thermal cool-down phase of composite fabrication and applied to a specific material system (A12O3/Ni3Al). The first is a baseline model of idealized processing conditions. The second extends the process model by including a tooling constraint generally associated with fabrication techniques such as hot isostatic pressing, vacuum hot pressing or die casting. In previous work, little attention has been devoted to the investigation of the constraining effects of the tooling phases, used to fabricate unidirectional metal matrix composites, on the residual stress evolution. Various pressure loading histories can be applied to each model. The stresses and plastic strains that develop during the cool-down are calculated at the micromechanical level by the generalized method of cells, which is embedded into both global models. It is shown that residual stresses, at the end of the process, differ very little due to either the boundary conditions of the models or pressure loading histories. The stresses that evolve during the cool-down, however, are significantly different for the two models. In the unconstrained model the driving mechanism for the developing stresses and plastic strains due to thermal cool-down is the mismatch between the coefficients of thermal expansion of the fiber and matrix materials. In the constrained model this effect becomes secondary to the mismatch between the thermal expansion coefficients of the composite and the constraining can. In the constrained model, in-plane tensile stresses develop which are large enough to produce damage in the form of transverse fiber cracks, matrix cracking or filament breaks within the fiber tow. Since the damage producing stresses predicted by this model can be traced to the difference between the effective coefficient of thermal expansion of the processing can and the composite, steps can be taken to prevent damage by tailoring the properties of the processing can to the composite.