Creep deformation of unidirectional metal matrix composites: modeling and experimental observations

Abstract

The creep response of elastic fiber-reinforced metal matrix composites is modeled by establishing governing equations that link the composite's behavior under creep tests to that of the unreinforced creeping matrix in the stress relaxation test. These findings diverge from widely accepted models, which are well-acknowledged on the priori assumption of extending the steady-state creep method from constant stress to decreasing stress conditions. The unreachability of the steady-state creep of the creeping matrix with elastic reinforcements is demonstrated, as continuous stress transfer occurs from the creeping matrix to the elastic fibers. Experimental validation through stress relaxation tests reveals significant discrepancies, including underestimation in the short term and overestimation in the long term, driven by altered stress evolution paths for the creeping matrix. In this work, the relationships of governing equations among stress relaxation tests of unreinforced matrices, creep tests of composites and stress relaxation tests of composites are established, emphasizing the condition of the creeping matrix under decreasing stress. The difference in the effect of broken fibers on the strain increase between localized strain within the stress recovery distance and averaged strain over the extensometer gauge length is discussed. An experimental validation method is proposed and conducted by examining the elastic modulus variation of the composite in the loading and unloading stages during repeated creep tests.