Date of Award

2022

Degree Type

Thesis

Department

Mechanical Engineering and Applied Mechanics

First Advisor

Arun Shukla

Abstract

Polymeric materials are an important part to marine structures as they provide protection from the harsh underwater environment. While they can be used in many other applications, they can be found as a coating on propellers, boat and ship hulls, and even protective coatings for electronics. Given the service environment of these polymers, it is important to understand how they are affected by their surroundings. Three polymers were studied in different conditions; two polyurethanes and a polyurea. Each polymer underwent different experiments to determine how they would be affected by the environmental factors they would be exposed to.Polyurea is a very durable polymer with excellent mechanical and chemical properties. It is wear and corrosion resistant which makes it an optimal choice for naval applications. The US Navy uses polyurea as a protective coating on marine structures which expose it to aggressive marine environments, i.e. low temperatures, sea water, and possible high strain rate loadings. This work investigates the effects of low temperature coupled with saline water weathering at multiple strain rates. For this study, a range of tests were conducted: low temperature (-2°C) and room temperature, virgin and weathered specimens, low and high strain rates, and a combination of those mentioned. Specimens which were weathered were exposed to saline water for 2 weeks in an accelerated weathering facility. This simulates 4.4 months in real life time at 17°C and 12 months at -2°C. An insulated box with cold flowing air was used to obtain the low temperature. A universal testing machine and a modified Split Hopkinson Pressure Bar were used to obtain strain rates of 10-3 to 103. Results show polyurea's response in highly sensitive to the given parameters, where weathering has the dominant effect.

In the marine industry, polyurethanes are a well-known material for use as they are water and tear resistant. Their properties allow the material to be an excellent choice for encapsulants for cable connectors and circuitry which during its service life may come in contact with sea water. In order to better understand how this material changes over time when exposed to sea water, a polyether-based polyurethane was studied after long term exposure to saline water using accelerated weathering techniques. The specimens were weathered in 3.5 % saline water at 70°C for 10, 48, 85, and 129 days simulating 1 month, 4.5, 8 and 12 months in real life service time, respectively. The quasi-static (2 x 10-3 /s) tensile and compressive behavior and the dynamic (2 x 103 /s) compressive behavior was studied. It was found that the exposure to saline water caused softening in the material which decreased mechanical properties. Under quasi-static conditions the Young's modulus and strain energy decreased by a maximum of 57% in compression and 51% and 50% in tension. The dynamic strain energy also decreased by 83% after 129 weathering days. Cable connectors are an integral part of submarine communication systems. Some of these cables can be found on the exterior of ship hulls exposing them to the aggressive marine environment. The sea water along with of factors, such as the cable weight, may accelerate the failure of these cables. In this study, a polyurethane and primer subject to peel tests using a static load of 2.27 kg (5 lbs) while submerged in saline water and in air at two temperatures of 20 and 55°C. The contact angles of the Monel, primer and polyurethane surface were taken to understand the mode of failure when comparing the underwater to air experiments. Conventional peel tests were conducted to determine the peel strength of the system while in air at room temperature. When submerged under saline water, it was found that the average time to peel significantly increased when they were placed in air at room temperature when compared to the other conditions.

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