Degree Type

Dissertation

Date of Award

2013

Degree Name

Doctor of Philosophy

Department

Aerospace Engineering

First Advisor

Ashraf Bastawros

Abstract

Adhesively bonding composite structures have many applications in aerospace, automotive and submarine industries. The adhesive bonding joints have substantial advantage over the traditional metallic mechanical bonding joints, such as rivet and welding. However, the adhesive bonding joints require additional steps of surface preparation and cleaning to ensure consistent bond strength. In application, the adhesively bonded joints are exposed to environmental degradation and industrial solvent contaminates. Accordingly, the assurance of reliability of bonded composite structures requires detailed investigation of the role of contaminates on bondline integrity.

This dissertation focuses on assessing the contaminates effect on the adhesive bondline integrity. A combined experimental and numerical framework is developed to study the contamination effect on the adhesive mechanical properties and adhesive joint strength. The bondline integrity were examined for a system of adhesive (EA9394) and the carbon-fiber system (Hexply IM7/8552), after being subjected to different level of exposures to aviation hydraulic fluids and mold cleaning agents. A testing protocol based on nanoindentation for initial screening is used to predict the interfacial fracture characteristics after exposure to contamination. It is found the adhesive modulus and stiffness dropped by up to 10% for the hydraulic fluid contaminates, suggesting increase of the plastic dissipation within the bondline. However, the trend for the cleaning agent was not clear since the modulus drop while its hardness increased.

Detailed measurements of interfacial fracture toughness are carried out via standard tests of double cantilever beam specimens, exposed to varying level of contamination. The tests were carried out in a computer controlled Instron universal testing frame. An optical based crack propagation measurement technique is developed to in situ monitor the crack extensions with micrometer resolution. It is found that even at the trace level of 3 micro-gram/cm2, the interfacial fracture toughness is reduced by more than 35%. The surface topography of the fractured interfaces is further examined by surface profilometer. A clear transition from very rough fractured surface with fiber/matrix pull out, to very smooth fractured surface with interface failure is observed with the increased level of contamination. This transition of fracture surface topography testified the proposed cohesive model.

Finite element analysis with cohesive zone model is used to rationalize the experimental results and understanding the mechanism of contamination degradation. Double cantilever beam model with various adhesive bonding parameters were tested. The interfacial cohesive parameters, the adhesive properties and the thickness of the adhesive layer were examined. The results show the parameters effect on the process zone propagation and the adhesive bonding toughness. The relation of process zone size and the bondline parameters were examined and compared with the existing double cantilever beam results. The finite element work showed that the contamination-induced degradation of the interfacial adhesion strength is the primary effect in mode I fracture.

To fully understand the contaminations effect on adhesively bonded joints, mode II fracture test is conduced. Single shear lap test shows the contaminations has softening effect could strengthen the adhesive bonding initiation force. Further simulation work shows the detailed process zone propagation. It is shown that the contaminates effect on the adhesive matrix hardness becomes the primary effect for the adhesive debonding.

DOI

https://doi.org/10.31274/etd-180810-3357

Copyright Owner

Xu Shang

Language

en

File Format

application/pdf

File Size

142 pages

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