Doctoral Dissertations

Zheng Rong Xu

Date of Award

8-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Ken H. G. Ashbee

Committee Members

W. Becker, C. R. Brooks, M. G. Hanson

Abstract

Composite materials are prone to degradation under a variety of influences. Investigation of single fiber behavior in epoxy resin undergoing water uptake, of the effects on moisture induced degradation of surface treatment of fiber and of post-curing of polymer coatings on electronic packages, together with the development of new experimental techniques, including techniques for measurements of small displacements and for measurement of optical birefringence , have been completed and are reported in this thesis. The mathematical analysis of a new method of optical caustic foci developed here to measure small displacement, of up to 10 nm, is reliable. The new technique has been successfully used to measure dimensional changes accompanying water uptake by model composites and polymer coatings on COB components. The optical interference technique cannot be used for this purpose owing to the transparency of the coating material. The results have shown that resin swelling associated with water uptake can overcome and exceed resin shrinkage resulting from curing, by a margin that is sufficiently large for the axial tension in some short carbon fiber composites to exceed the fiber failure strain. The durability of load transfer between matrix resin and fiber has been enhanced by surface treatments to such an extent that swelling deformations large enough to fracture fibers can be realized. The different surface treatments of fibers employed in experiments reported here have permitted different failure modes to be studied in composites exposed to aqueous environments. These studies concern detailed investigation of stress-induced birefringence around single fibers and of fracture surfaces of test-pieces containing fiber bundles. For proprietary coatings on electronic packages, the results presented here have identified the effects of post-curing treatments on dimensional change rates, and hence on both water uptake rates and saturated water uptake values. Two generic types of degradation intrinsic to the polymer coating technology have been observed, one being electrolytic corrosion of the underlying substrate and the other being swelling and, in some cases, peeling of the coating itself.

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