Micromechanical tailoring of PVA-ECC for structural applications.

Abstract

This dissertation details the development of a low cost polyvinyl alcohol (PVA) fiber reinforced composite to display enhanced strain capacity employing micromechanics-based tailoring of the microstructure as prescribed by the Performance Driven Design Approach (PDDA). Central to the research is the use of the micromechanical model to guide the selection of the microstructure since it can associate the combined confluence of fiber, matrix and interfacial parameters to composite deformation. With the assistance of constraints as defined by the manufacturing of the fiber, economy and composite workability, the final selection of the microstructural parameters is completed. The micromechanics-based design approach attempts to transform the PVA fiber composite possessing quasi-brittle tensile behavior to that of a strain-hardening material. The research can be divided into tasks conducted at the microstructural and composite levels. At the microstructural level, investigation into bond properties for the aligned PVA fiber is conducted. The influence of fiber surface treatment on the fiber/matrix interface properties is quantified through single fiber pullout tests. A result of a systematic study of increasing oiling agent content is the selection of the optimal content that satisfies the targeted bond properties as designated by the micromechanical model. To facilitate accurate modeling of realistic composite behavior with random fiber dispersion, a study of the pullout behavior of inclined PVA fibers is also conducted, utilizing a single fiber pullout model derived specifically for slip-hardening fibers. To ascertain the achievement of the targeted composite performance, experimental studies of the uniaxial tensile behavior of the PVA fiber composites are conducted. Successful translation of the tailored microstructure to increased composite deformation is examined through quantification of the effects of fiber selection, oiling agent content and fine sand content. It will be demonstrated that with adjustment of the oiling agent content to satisfy the designated bond properties, the impact of sufficient fiber/matrix interface on realization of the targeted composite property is significant. Thus, the approach of deliberate selection of the material constituents and the resulting composite tensile performance is justified by the development of another type of Engineered Cementitious Composite (ECC), a high performance PVA-ECC. Theoretical construction of the tensile stress-strain relation is undertaken to provide a constitutive relation necessary for structural element modeling. Fiber/matrix interface characteristics unique to the PVA fiber are incorporated, along with statistical variation of flaw size to predict unsaturated cracking behavior.