Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Shivakumar, Kunigal

Abstract

Eco-Core was previously developed as a fire resistant core material for composite sandwich structures. It was made using a large volume of fly ash (Cenosphere) and a small volume of high char yield binder by a syntactic process. The cellular structure of the material offers a potential for shock and blast mitigation applications. Based on the present study, it was concluded that the processing of Eco-Core is repeatable and the static properties are reproducible. Eco-Core was subsequently modified by a surface coating method and an Impregnation method to enhance its energy absorption capability. The modified Eco-Core was characterized by static confined compression tests. Surface coating with polyurea resulted in an improvement in compression strength by 12%; compression modulus by 64%; and energy absorption density by 14%. The impregnation of Eco-Core with polyurethane enhanced the compression strength by 138% with a penalty of increased density and decreased modulus. The Split Hopkinson Pressure Bar was not available at NC A & T State University for characterizing materials at high strain rates. A compression SHPB test apparatus with 7075 T6 aluminum pressure bars was successfully designed, fabricated, and developed. The SHPB apparatus was verified for 6061-T651 aluminum and polycarbonate specimens and the results were in good agreement with research literature. The dynamic compressive stress-strain response of Eco-Core was measured over strain rates ranging from 1,000/s to 3,100/s with a split Hopkinson pressure bar apparatus. The SHPB test results showed that Eco-Core is not strain rate sensitive over the range of strain rates studied. Microbubble bond failure followed by crushing are the failure modes of Eco-Core under a dynamic loading. The SEM studies clearly showed that the amount of crushed microbubbles increases with an increasing strain rate and proved that Eco- Core undergoes a crushing mode of failure at high strain rates. A phenomenological constitutive model was developed for Eco-Core. The dynamic compressive stress-strain responses of Eco-Core coated with polyurea in different configurations was measured at two strain rates near 3,000/s and 3,900/s. All the PU coated Eco-Core samples showed stress-strain responses similar to that of Eco-Core but with a prolonged densification region. The energy absorption capacity of the Eco-Core and its modification were analyzed and compared with each other. Among all the coating arrangements and thickness, 10-PU front-back Eco-Core samples showed about 51% increase of energy absorption density at a strain rate of 2,500/s. Even a very thin coating (0-thickness) of polyurea on the front side of Eco-Core improved the energy absorption by 33%. The high strain rate compressive behavior of the polyurethane impregnated Eco-Core was measured over strain rates 1,000/s - 3,200/s. The dynamic plateau stress of impregnated Eco-Core is 2.5 times higher than that of Eco-Core. The impregnated Eco-Core is not sensitive to strain rates below 1,700/s but at strain rates beyond 1,700/s, the impregnated Eco-Core is mildly strain rate sensitive. At strain rates near 3,100/s, polyurethane impregnated Eco-Core samples showed a very significant improvement of approximately 125% increase of energy absorption density in comparison with Eco-Core with a penalty of increased density.

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