Date of Award

2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Arun Shukla

Abstract

The use of composites has attracted attention in underwater marine applications due to the array of advantages offered by these materials. Composite materials offer alternatives with reduced weight, improved corrosion resistance, and for submerged structures, greater potential operating depths. In addition, these materials provide improved stealth qualities by having very low thermal, acoustic, and magnetic signatures, increasing their appeal for military applications. For these reasons, the presence of composite materials in marine industries is increasing, and are currently used in several naval applications, such as sonar domes, masts, and hull sheathings. One of the biggest obstacles to widespread adaptation of composite materials is a lack of complete understanding and simple design rules for these materials, especially under extreme loading conditions. For this reason, the present work looks to expand the current knowledge of composite behavior by examining the problem of implosion.

A comprehensive study on the hydrostatic implosion of carbon fiber reinforced epoxy composite tubes is conducted experimentally to examine the failure and damage mechanisms of collapse. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound, braided, and roll-wrapped carbon-fiber/epoxy tubes are studied to explore the effect of geometry and reinforcement architecture on the modes of failure. 3-D Digital Image Correlation technique, which is first calibrated for the underwater environment, is used to capture the full-field deformation and velocities during the implosion event. Dynamic pressure transducers are employed to measure the pressure pulses generated by the event and evaluate its damage potential. The results show that composites with braided fabric reinforcements are found to have more damage potential to adjacent structures than those containing unidirectional reinforcements, as they release pressure waves with significantly greater impulse.

The mechanisms and pressure fields associated with the hydrostatic implosion of glass-fiber reinforced polymer (GFRP) tubes with varying reinforcement are investigated using high-speed photography. Experiments are conducted in a large pressure vessel, designed to provide constant hydrostatic pressure throughout collapse. 3-D Digital Image Correlation (DIC) is used to capture full-field displacements, and dynamic pressure transducers measure the pressure pulse generated by the collapse. Results show that braided GFRP tubes release pressure waves with significantly greater impulse upon collapse as compared to filament-wound tubes, increasing their damage potential.

An experimental study on the underwater buckling of composite and metallic tubes is conducted to evaluate and compare their collapse mechanics. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound carbon-fiber/epoxy, glass/polyester (PE) tubes, and aluminum tubes are studied to explore the effect of material type on the structural failure. 3-D Digital Image Correlation technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulse. The results show that glass/PE tubes release the weakest pressure pulse, and carbon/epoxy tubes release the strongest upon collapse. In each case, the dominating mechanisms of failure control the amount of flow energy released.

An experimental study on the hydrostatic implosion of carbon-fiber reinforced epoxy composite tubes is conducted to explore unique failure and damage mechanisms of collapse. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound carbon-fiber/epoxy tubes are studied using high-speed photography to explore the effect of complex damage on the modes of failure. 3-D Digital Image Correlation technique, which is first calibrated for the underwater environment, is used to capture the full-field deformation and velocities during the implosion event. Fourier Series deformation models are used to extract buckling modes from displacement data. The results reveal that the presence of damage in the structure can cause the mode shape to change as the structure deforms.

An experimental study on the underwater collapse of composite tubes with polymeric coatings is conducted in an attempt to mitigate the implosion pressure pulse released. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure during collapse. Filament-wound carbon-fiber/epoxy tubes are studied with polyurea coatings of different thicknesses on the interior and exterior of the tube to explore the effects of these configurations on implosion pulse mitigation. 3-D Digital Image Correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulses. Local pressure data and DIC results are used to obtain a measure of normalized energy released during implosion. Results show that thick interior coatings significantly reduce the energy released in the pressure pulse by slowing the collapse and softening the initial wall-to-wall contact. In contrast, thick exterior coatings increase this energy by suppressing damage, thereby reducing the energy absorption capacity of the structure.

A comprehensive investigation on the implosion of composite cylinders subjected to a nearby explosion is performed. Experiments are conducted in a large pressure vessel, designed to provide constant hydrostatic pressure throughout the event. Carbon fiber/epoxy filament-wound tubes are studied with constant hydrostatic pressure and varying charge standoff distances to determine the effect of the explosive loading on the mechanisms of collapse. 3-D Digital Image Correlation (DIC) is used to capture the full-field displacements and velocities during the implosion event, and to characterize the initial dynamic response of the tube. Dynamic pressure transducers measure the shock waves generated by the explosive and also the pressure pulse generated by the collapse. Results show that different magnitudes of explosive loading produce drastic differences in the way implosions are initiated, and in the extent of damage to the structure. Experiments with strong explosive loading show immediate collapse of the tube upon the arrival of shock wave. Relatively smaller explosive loading result in collapses due to the additional bubble pulse loading, or after accumulating damage for extended periods of time.

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