Processing and Opto-Mechanical Characterization of Transparent Glass-Filled Epoxy Particulate Composites
Type of DegreeMaster's Thesis
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Optically transparent materials have a wide range of applications in areas such as face shields, ground vehicle and aircraft windows, and bullet-resistant enclosures. Glass, the standard material used for applications requiring transparency, is often deficient in providing low weight, high mechanical strength, and toughness. Hence, the development of polymer-based transparent composites has the potential to offer complementary material systems for such applications at a low cost. In this work, a method to produce transparent glass-filled epoxy composites by refractive index matching of an epoxy matrix and rod-shaped E-glass fillers has been developed. These composites were processed at different filler volume fractions (Vf) from 0% to 15%, with transparency maintained at each Vf. The initial investigation included quasi-static three-point bending of edge-cracked specimens to determine the critical stress intensity factor (SIF), KIcr, values. Improvement in KIcr over neat epoxy was seen in each case, with a maximum increase of approximately 30%. Furthermore, this composite was observed to exhibit birefringence, thus making mechanical characterization through photoelastic methods feasible. To determine the mechanical properties, first, the stress optic constant, fσ, had to be evaluated for each sample. This was done using symmetric four-point bending configuration and recording the fringe patterns with a dark-field polariscope and time-lapse photography. Under quasi-static three-point bending, optical fringes in each sample were observed, recorded, and subsequently, stress-intensity factors were evaluated at various load levels and compared to the analytical counterparts. The values determined experimentally matched iii analytically determined ones closely, thus validating the use of photoelastic characterization of this material. Mode-I dynamic fracture tests were also performed at three different loading rates, and the photoelastic stress fringes were recorded using high-speed photography and a laser light illumination. The mode-I stress intensity factor values, KI, were evaluated for each image recorded during the fracture event using crack-tip fringe digitization and least-squares analysis of optical data. For each loading rate, as the volume fraction of the filler material was increased, a monotonic increase in the KIcr values corresponding to crack initiation in this transparent composite was observed. Additionally, a complementary finite element analysis of pre-crack initiation SIF values was performed, showing a good match with the experimental results. Finally, the high-speed photoelastic measurements were extended to mixed-mode (mode-I + mode-II) fracture of these transparent glass-epoxy composites. A simple method of varying the crack length in an eccentrically impacted edge-cracked specimen was adopted to alter the mode-mixity at crack initiation. Isochromatic fringes near the crack tip were recorded throughout the fracture event from which the mode-I and mode-II stress intensity factor values were successfully extracted. The SIF histories for samples of the equivalent filler volume fractions, but different crack length, were compared, and significant variation in the mode-mixity at crack initiation was observed. Additionally, the effect of volume fraction on the mixed-mode fracture behavior was studied, and a monotonic increase in the Keff values at crack initiation with respect to filler volume fraction was observed.