|dc.description.abstract||Polymer-based particulate composites have a wide range of engineering applications due to their mechanical, chemical, dielectric, and lightweight characteristics as well as simplicity of processing. The mechanical performance of these composites in general, and fracture behavior in particular, are strongly affected by the filler phase characteristics such as filler shape, size, volume fraction and filler-matrix interfacial strength. However, these composites could fail differently in different loading configurations (e.g. symmetric vs. antisymmetric) or loading rates. This dissertation addresses a few of these issues.
In the first part of the dissertation the effect of filler shape (sphere, flake and rod) and filler volume fraction (5%, 10% and 15% Vf) on the dynamic fracture behavior of particulate polymer composites was investigated. The Digital Image Correlation (DIC) technique was utilized to map displacement fields around the crack-tip and extract stress intensity factors. The results showed a pronounced improvement in the crack initiation toughness for rod-shaped fillers, ~145% increase over unfilled epoxy at 15% Vf with flakes and spherical fillers showing ~97% and ~67% improvement, respectively, at the same Vf. For the three different volume fractions – 5%, 10%, and 15% - considered, the rod-shaped fillers produced the highest crack growth resistance followed by flakes and spheres, in that order. In addition, for 10% Vf rod-shaped filler ~113% and ~50% increase in crack initiation toughness for the lowest (dKI/dt =53 x10^3 MPa√m/s) and the highest (dKI/dt =182 x10^3 MPa√m/s) loading rate cases, respectively, were seen compared to that of neat epoxy.
In the second part, mixed-mode dynamic fracture behavior of 10% Vf rod-shaped glass-filled epoxy composites was studied, and compared with the mode-I counterparts. In mode-I, the 10% Vf glass-filled epoxy produced ~128% increase in effective crack initiation fracture toughness compared to the neat counterpart, whereas, in the mixed-mode case, the 10% Vf glass-filled epoxy produced a ~98%-119% increase in the effective crack initiation fracture toughness, relative to the respective neat epoxy cases. Using the crack initiation toughness and mode-mixity measurements, a fracture envelope using Maximum Tensile Stress (MTS) criteria is developed for the 10% Vf composites. A modified fracture envelope equation that fits the measurements better is also demonstrated.
In the third part, the Finite Element Method (FEM) was employed to model the stress-strain behavior of 5% Vf carbon fiber modified epoxy composite under uniaxial tension. The stress-strain response from a Representative Area Element (RAE) of the composite was shown to be in agreement with the experimentally measured stress-strain response for 5% Vf carbon fiber-filled epoxy. Also, the so-called Extended FEM (XFEM) was employed to model crack propagation in this composite and to understand the underlying fracture mechanism by varying the fiber-interface strength. The computations showed that the crack tends to propagate through the fibers when the filler was perfectly bonded to the matrix. However, the crack path circumvented the fiber when it was weakly bonded to the matrix suggesting the potential crack path tortuosity.
In the last part, an ultrafast poptube approach was utilized to grow Carbon Nanotubes (CNTs) on various engineering substrates (hollow glass-microballoons, solid glass spheres, solid glass fibers and carbon fibers) using a microwave. Subsequently, CNT-grown substrates were used as the filler to process a micro-nano hierarchical composite. The hierarchical epoxy compo-site made of CNT-grown microballoons enhanced the critical fracture toughness by ~17% relative to conventional syntactic foam in quasi-static fracture tests. On the other hand, the critical fracture toughness for CNT-grown carbon fiber-filled epoxy decreased by ~15% relative to the traditional counterparts. Fractography revealed that improvements due to CNTs were compromised by the agglomeration of micro-fillers with CNTs grown on them.||en_US