Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Aerospace Engineering

Major

Engineering Mechanics

First Advisor

Ashraf F. Bastawros

Abstract

Structural interface property controls a wide spectrum of applications, ranging from the hybrid integration of dissimilar structural materials, to the enhanced physical properties of high performance engineering alloys. Practically, interfaces can be perceived as the weakest part of a heterogonous material or a structure. They are the most prone to environmental disturbances such as chemical or thermal, with significant deterioration of the structural integrity. Despite the large volume of literature on the topic, there remains a lack of proper understanding of degradation mechanisms of these interfaces. The challenge remains in the early detection of interfacial damage, and assessment of the remaining safe service life. This work is an attempt to provide some physical and mechanical understanding of these interfacial degradation mechanisms in two problem sets; (1) contamination-induced interfacial degradation in polymer matrix composites, and (2) early stage degradation of microstructure and mechanical properties in pipeline steels. The ultimate goal is to provide metrics for the physical and mechanical changes associated with these degradation mechanisms, and provide the bases for further development of new nondestructive evaluation techniques.

For the first topic, the contamination-induced degradation mechanisms in polymer-matrix interfaces were examined experimentally and the observed trends were rationalized analytically and numerically. Two complementary frameworks were developed for the prediction of residual interfacial fracture energy, based on the changes in the adhesive hardness, and the level of the surface contaminants. Both of these quantities can be measured nondestructively by nanoindentation and IR-spectroscopy, respectively. Additionally, role of bond line contamination on mode-I failure and fracture surface evolution is quantified by performing statistical fracture surface analysis. With additional examination of other material systems, the proposed correlations may provide the basis for nondestructive evaluation of bond line integrity.

For the second topic, initial microstructure surface evolution was examined in high strength steel under corrosive environment. The environmentally occurring corrosion process is mimicked by laboratory scale electrochemical experiments. Nanoindentation technique was utilized to locally characterize the mechanical property degradation in the sub-surface layer of a pipeline steel undergoing intergranular corrosion at active dissolution potentials at pH 8.2. Additionally, atomistic computational analysis was conducted to rationalize the experimentally observed trends. The present study reports two key observations: (1) mechanically degraded grains in the subsurface layer with 10% reduction in hardness, (2) 1 μm-thick mechanically degraded layers adjacent to corroded grain boundaries with 25% lower hardness relative to the grain interior. The observed degradation is attributed to weakening of lattice resistance arising from the clusters of non-equilibrium atomic vacancies generated by the preferential dissolution of silicon atoms by intergranular corrosion activity. The findings might provide a new insight for the corrosion process and might suggest a mitigation strategy.

Copyright Owner

Denizhan Yavas

Language

en

File Format

application/pdf

File Size

237 pages

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