High Strain Rate Elastic and Fracture Characterization of Isotropic and Orthotropic Materials with and without Nano Fillers
Type of DegreeDissertation
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A long-bar apparatus for subjecting relatively small samples to mode-I stress-wave loading has been devised for failure characterization. A methodology based on digital image correlation (DIC) used in conjunction with ultrahigh-speed photography and a long-bar impactor has been developed for determining dynamic crack initiation stress intensity factors (SIFs), as well as SIFs for rapidly growing cracks during high-strain rate events in isotropic and orthotropic materials. By altering the material of the pulse shaper to cushion the impact, a range of strain rates has been attained. Commercial grade acrylic was first used to calibrate the device, and then dynamic fracture characterization was carried out for the first time on acrylic bone cement. Despite several key compositional differences, the two materials performed similarly during quasi-static fracture tests; however, under dynamic loading conditions, bone cement exhibited significantly lower crack initiation SIFs, lower dynamic SIFs, and higher crack tip velocities for three different dynamic loading rates. In the second phase, orthotropic cortical bone fracture was characterized under dynamic impact loading conditions. This is the first reported study of dynamic axial fracture of cortical bone, which is considered a naturally occurring nanomaterial. In order to measure SIFs using in-plane displacements for an orthotropic material, all nine independent elastic constants were determined ultrasonically. The measured critical dynamic SIF values were 2.7 MPa*m^0.5 for a crack growing in the axial direction. In the third phase of this research, synthetic carbon nanotubes (CNTs) were dispersed into epoxy with the goal of improving the quasi-static tensile, quasi-static fracture, and dynamic fracture properties of epoxy. Subsequently, the optimally-dispersed CNT-modified epoxy was to be used as the matrix material for 3-phase CNT/epoxy/carbon fiber composites. Despite using many different dispersion techniques to achieve different levels of dispersion using different types of CNTs, epoxy systems, and curing cycles, no tangible mechanical improvements significant enough to warrant elaborate and high-cost material processing were found. Dynamic mechanical analysis pointed towards the possible influence of CNTs on crosslink density, which can significantly affect the mechanical properties. In the last phase, thick carbon fiber reinforced polymers (CFRPs) were built in order to study quasi-static and dynamic interlaminar fracture using DIC and ultrahigh-speed photography. Thick CFRPs were unidirectional, such that interlaminar and intralaminar fracture behaviors could be compared between specimens with identical geometry and cut from the same CFRP plate. Two additional CFRPs containing CNTs or milled carbon fibers (MCFs) were also fabricated in order to examine the size-effects of fillers on the interlaminar properties of reinforced CFRPs. CNTs had little effect on critical SIFs, but they caused +34% and +16% enhancements in critical energy release rate under quasi-static and dynamic conditions, respectively. These were, however, considered insignificant relative to the experimental scatter in these stiff/brittle materials. In the case of MCFs as fillers in the matrix, a +17% improvement in quasi-static critical SIF but a -23% decline in dynamic critical SIF, respectively, were observed. The corresponding critical energy release rates for CFRPs with MCFs had a +106% improvement under quasi-static loading conditions and a -15% reduction under dynamic loading conditions. The decline in the dynamic case was attributed to MCFs acting as microscopic spacers between laminae, thereby contributing to the resin-rich interlaminar region.