Mechanical response of polymer matrix composites using indentation stress-strain protocols

Abstract

Polymer Matrix Composites (PMCs) are an important material for many advanced applications due to their potential to combine critical mechanical properties with a high strength-to-weight ratio necessary for advancing automotive and aerospace applications. Such materials have been studied extensively for their bulk properties as well as the properties of their individual components. However, there remains an intermediary lengthscale on the order of the arrangement of the reinforcing phase which has yet to be satisfactorily characterized. Understanding the properties of this intermediary lengthscale is critical to the development of multi-scale models with predictive capabilities. To address this need, the use of instrumented indentation on a lengthscale appropriate to the material system presents itself as a viable solution. Indentation is uniquely suited to such an implementation due to its inherent ability to conform to the lengthscale of testing needed and its high throughput nature. Recent advancement in the data analysis protocols for spherical indentation has allowed for the extraction of stress-strain curves from the load-displacement data resulting from indentation tests in a variety of materials including single and polycrystalline metals, natural materials and polymers. The goal of this thesis is to extend these protocols to address the unique challenges presented by their application to polymer matrix composites. For this purpose, two disparate materials systems have been chosen to develop the protocols. One is a laminate composite system made from carbon fibers embedded in an epoxy matrix. Here the intrinsic variation within a single ply is tested within the laminate composite. The second material system is a nano-composite composed of multi-walled carbon nanotubes (CNTs) in a polypropylene matrix. Here the variability in properties due to the presence of agglomerates of the CNTs will be explored. Application to these two different material systems will demonstrate the applicability of these protocols for determining the properties of composites on the desired scale.