Redistribution of internal stresses in composites: An approach for increasing the matrix cracking stress

A concept is proposed to increase the matrix cracking stress of some brittle-matrix composites by taking advantage of the redistribution of internal stresses that occurs when a composite with phases that have dissimilar creep behavior is subjected to thermomechanical loading. The concept is elaborated through the stress analysis of a model unidirectional composite with constituents that exhibit linear viscoelastic behavior. It is shown that when a composite with a matrix that is less creep resistant than the fibers is subjected to a treatment involving both thermal and mechanical loading (e.g., creep test), stresses can be transferred from the matrix to the fibers, resulting in the stress-relaxation of the matrix. It is also shown that at the end of the treatment the matrix can be subjected to compressive residual stresses by the elastic recovery of the fibers, in a process analogous to pre-stressed concrete. The conditions for the viability of this concept are discussed.