Micromechanical analysis of multiple fracture and evaluation of debonding behavior for fiber-reinforced composites

Enhanced stress capacity during multiple matrix cracking in unidirectional, continuous fiber-reinforced brittle matrix composites subjected to uniaxial tension has been investigated by using the energy approach of fracture mechanics, in which the bridging stress of the fibers in the matrix crack is determined by the inclusion method. The interactions among the multiple fracture, the interfacial debonding and the frictional sliding are discussed. Theoretical predictions for the stresses at the end point of multiple cracking and the debonding lengths have been derived. To verify the validity of the theoretical model, an experimental study was conducted with cement-based composites made with different volume fractions of steel fibers. The steel fiber reinforced specimens were loaded under uniaxial tension to various pre-determined stress (deformation) magnitudes, and then the deformations in the specimen were "frozen" by gluing rigid steel blocks on the specimen. The technique of optical fluorescence microscopy was used to acquire the extent of debonding length quantitatively from thin sectioned samples obtained by cutting the "frozen" specimen. A "stable growth" of debonding was observed in the study. The theoretical predictions are compared with the experimental results and a reasonable agreement is shown.