| Fracture toughness of quasi-brittle ceramics and cement-based materials can be enhanced considerably by the use of fibers. When the volume fraction of fibers is large, they can prevent microcracks from catastrophically propagating, thus allowing the matrix to carry increasing loads. The role of fibers on the crack propagation can thus be observed through an apparent increase in the strength of the matrix, which can be as much as three times stronger under uniaxial tension. This work investigates the characterization of such responses through various theoretical and experimental procedures.;Tensile, flexural, and instrumented impact test methods were developed in order to study the toughness of glass fiber reinforced concrete. Using the developed test procedures, the possible loss of ductility of composites subjected to elevated humidity and temperature conditions was examined.;The role of fibers in the arrest and stabilization of initial cracks, resistance to crack propagation, and distribution of cracking throughout the entire loading history was studied. Acoustic emission activity of polypropylene fiber reinforced concrete specimens was measured over strain ranges of up to 1% strain. An optical fluorescence microscopy technique was developed to examine microcracking in specimens loaded upto and then mechanically frozen at specified strain magnitudes. A reflection LASER holography technique was developed to study the formation of cracks, their propagation, and distributed microcracking. A digital image analysis system was developed to study both holograms and optical micrographs. Automatic, computerized routines were developed in quantitative measurement of internal damage parameters such as crack density, lengths, spacing, and crack-opening profiles. The results are presented using stereological formulations, and measures of internal damage are proposed.;Theoretical formulation was based on the R-curve approach which models the stable crack growth and toughening of the quasi-brittle materials. An approximate closed form solution to the R-curve was developed. Fracture response of cement paste, mortar, rock, and concrete under various geometries, and sizes, as reported by various researchers were satisfactorily predicted using the proposed R-curve formulation. Using the experimentally obtained pullout-slip of fibers, a closing pressure term was introduced during the stable crack growth regime. This enhanced R-curve was shown to predict the increase in matrix response quite favorably. The model was compared to other theoretical predictions, and with the experimental results of various fiber reinforced composites. |
