Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

Abstract

Marine structures are routinely coated with elastomers to enhance their survivability and reliability. These elastomeric coatings are corrosion and abrasion resistant, and substantially mitigate the damage caused by shock, blasts, and impact loading on the structures. These coated structures are exposed to aggressive marine environments (saline water and UV radiation) during service, which can deteriorate and alter the material properties of the elastomer and the strength of the interface over time. The constitutive behavior of the elastomer after exposure to saline water and UV radiation will be studied. In addition, the effects of saline water and UV radiation exposure on the structure/coating interface and the blast/ballistic mitigation capability of the coating will also be investigated and compared to the baseline performance of the coating.

The constitutive behavior of polyurea after prolonged exposure to aggressive marine environments, including saline water, UV radiation, and combinations of both, was investigated in this study. A diffusion study was performed at several temperatures to determine the effect of temperature on saline water ingression into the polyurea using Crank’s method. This diffusion data coupled with Arrhenius’ methodology allowed for the calculation of an acceleration factor relating laboratory exposure time at elevated temperature to real-life service time. Cast cylindrical specimens were exposed to UV radiation, saline water, and various combinations of UV radiation and saline water. These combinations were: a) exposure to saline water followed by UV radiation, b) UV radiation followed by saline water, and c) saline water and then UV radiation followed by saline water. Uniaxial compressive experiments were conducted on both the virgin and exposed specimens at strain rates of 1.7×10-3s-1 2.6×10-3s-1. Under quasi static loading conditions, the elastic modulus of the polyurea dropped by 73% after 84 days of exposure to saline water at 85°C. Specimens exposed to UV radiation showed a maximum increase in the elastic modulus of 64% after 20 days of exposure. When tested under dynamic loading conditions, specimens exposed to saline water for 84 days showed a 48% decrease in strain energy while those exposed to UV radiation showed a 45% increase.

The changes in the peel strength of polyurea/monel 400 interfaces after prolonged exposure to saline water are investigated. The monel family of alloys is extensively used on marine structures due to its resistance to corrosion, and polyurea has shown considerable promise as a coating material for marine structures to mitigate the damage from blast loading and provide corrosion resistance. In the present work, polyurea was cast on sandblasted and chemically treated Monel 400 bars, which were subsequently immersed in saline water at 70°C for 2, 5 and 8 weeks respectively, to accelerate the aging process. This high temperature exposure corresponded to 19, 48 and 76 weeks in the real-life service environment. The peel strengths of the interfaces were measured using the fixed arm peel test at two angles: 90° and 180°. The adhesive energy was computed from the experimentally determined peel strength using models developed by Kendall and Rivlin. The surfaces exposed by peeling were analyzed with the use of an optical microscope to determine the different failure modes at the interfaces. Finally, employing degradation kinetics, a degradation model was established and used to calculate long term adhesive energy and the degradation rate of the interface. Results showed that prolonged exposure to saline water leads to a drop in the peel strength. For 180° peel test, an 83% decrease in the peel strength of the polyurea/monel 400 interface was observed after 8 weeks of exposure at 70°C. Both peel angles gave similar adhesive energies for the same exposure durations indicating the independence of the adhesive energy on the testing angle. The elastic term contribution to the adhesive energy for all experiments was less than 5.5%. Lastly, the exposure time dependence on the adhesive energy followed first order degradation kinetics.

Cables play a crucial role in naval structures serving as a central nervous system. These cables transport energy, transfer control data and serve for radiocommunication. These cables and their connectors are generally molded, encapsulated or potted with polyurethanes to ensure that they are insulated and waterproof. While most of the cables transporting energy and transferring control data are located inside the structures, cables used for radio communication are located outside to avoid any interference. These cables and their connectors are exposed to aggressive marine environment and are largely affected by their weights. To ensure that they are insulated and waterproof, these cables and their connectors are usually molded, encapsulated, or potted with polyurethanes. The premature failure of the polyurethane/metal interface of connectors in the presence of seawater is one key problem faced by the marine industry. The study aims to investigate the coupling effects of seawater and cable weight on the adhesive energy of polyurethane/Monel 400 interface using dead weight peel test under seawater. The results showed that increasing the hanging weights increased the peel velocity, which was linear at lower velocities and nonlinear as the velocities increased. Furthermore, more cohesive failure was observed at the end of the peel process, indicating that peel velocity increased with longer exposure to saline water.

Marine structures operate in environments where there is an inherent risk of the structures being exposed to severe loadings such as shock, explosion, and large hydrostatic pressures, which can cause the structures to implode. Upon their collapse, these structures release damaging pressure pulses into the surrounding fluid. Elastomeric coatings have been seen to mitigate the energy released by such implosion events. However, long-term exposure to aggressive marine conditions, such as salt water and UV radiation, can significantly affect the blast mitigation potential of these coatings. In this work, effects of long-term salt water or UV radiation exposure on the mitigation capabilities of polyurea when applied to Al 6061 cylindrical tubes is investigated. Externally coated tubes were exposed to UV radiation and salt water at 70°C for two and four weeks and then imploded under hydrostatic conditions. Using pressure data from sensors located around the imploding tube and high-speed images taken during the event, the influence of coating ageing and degradation on the transient collapse, damage characteristics, and emitted pressure fields during the implosion of these tubes was examined and compared to a baseline. Results showed that coating the tubes reduced the minimum and peak pressures, as well as the under and over-pressure impulses. In addition, exposure of the coated cylinders to saline water resulted to an increase in the peak pressure and impulse demonstrating that water ingress degrades the coating. No significant changes in the behavior of the coated tubes were observed after exposure to UV radiation.

Available for download on Saturday, April 22, 2023

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