Multiscale characterization of carbon fiber-reinforced epoxy composites

Transverse cracking is an early form of damage in fiber-reinforced composite materials that can nucleate delamination, fiber breakage, and ultimate failure. Although many variables influence transverse cracking, little work has been done to characterize local and complex microstructural features which affect this damage formation. By relating experiments at the micro-, meso-, and macroscales this research provides additional evidence to support the significance of fiber packing, interphase formation, and residual stresses on the behavior of carbon fiber reinforced epoxy composite materials. For this systematic study across length scales, carbon fibers with three different commercially available surface treatments in a Aradur/Araldite epoxy system were selected. The surface functional groups on the carbon fibers were investigated with x-ray photoelectron spectroscopy, interfacial adhesion was characterized using a single fiber microbond test procedure, and the onset and density of transverse cracking events in cross-ply composites were measured using acoustic emission. Additional characterizations of the epoxy mechanical properties, thermal properties, and degree of cure were also performed. The presence of oxygen and nitrogen on the fiber surface significantly increased adhesion to the matrix material, while the specific functional groups on the surface did not significantly alter the adhesion for the surface treatments tested. Additionally, higher matrix glass transition temperatures resulted in stronger adhesion. Interestingly, this higher adhesion did not always translate to greater transverse crack resistance in cross-ply composites. The variation in epoxy stoichiometry was investigated near the fiber surface using a new atomic force microscopy-infrared spectroscopy based instrument. The IR absorption peak corresponding to the chemical cross-linking reaction increased in intensity relative to neighboring peaks in interphase regions near the carbon fiber surface. Additionally, a broadening of the hydroxyl peak consistent with increased amine concentration in control samples was also observed near the fiber surface. This increased cross-linking and amine concentration near the fiber surface supports the hypothesis of a chemical gradient interphase region which forms during the curing process due to preferential amine adsorption on the fiber surface. Full-field high resolution strain maps of composites under tension were generated using digital image correlation (DIC) in a scanning electron microscope (SEM). A newly developed procedure for reconfiguring sputtered thin films was optimized to produce appropriate surface speckle patterns for SEM-DIC. The patterns were tailored for a wide range of magnifications and are compatible with many materials systems. Deformations local to the fiber/matrix interface were measured with a noise floor of 0.24 pixels (less than 3 nm at 6000X magnification). The capabilities of SEM-DIC using the newly developed thin film reconfiguration procedure were demonstrated through measurements of deformation around single fibers in model composite systems, and in the measurement of highly localized deformations in high volume fraction composite systems.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2019

Author dc:creator

• Montgomery, Christopher B.

Contributors dc:contributor

• Sottos, Nancy R.
• Geubelle, Philippe H.
• Krogstad, Jessica A.
• Evans, Christopher M.

• Carbon fiber
• epoxy
• digital image correlation
• scanning electron microscopy
• atomic force microscopy
• infrared spectroscopy
• AFM-IR
• microbond
• x-ray photoelectron spectroscopy
• acoustic emission
• dynamic mechanical testing
• tapered double cantilever beam

Statement dc:rights

• Copyright 2018 Christopher B. Montgomery. All rights reserved.

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/102809

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/102809