Date of Award

August 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Rani Elhajjar

Committee Members

Adeeb Rahman, Hani Titi, Anoop Dhingra, Benjamin Church, Anastasia Muliana

Keywords

Defects in Composites, Digital Image Correlation Technique, Fracture Mechanics, Laminated Composites, Near Infrared Hyperspectral Imaging, Progressive Modeling

Abstract

Fiber waviness is one of the most common defects observed in reinforced laminated composites which can occur during manufacturing. The effects of waviness on the mechanical responses of laminated composites under the standard mechanical testing are investigated with proposed experiments, analytical and numerical approaches. The damage consequences including kink band formation, crack onset, delamination and fiber fractures are characterized by means of full field digital imaging and acoustic emission techniques. The notch and waviness size effects on the notched composite compressive and tensile strength are studied using a progressive damage approach using the finite element method. A numerical approach based on a combined continuum damage analysis and cohesive zone interlaminar behavior is proposed to simulate the failure initiation and propagation responses. The proposed FE modeling approach will attempt to predict the response of the laminate structure inferring distinctive failure mechanisms and their interactions with the defects. Hybrid near infrared hyperspectral imaging surfaces with a bottom-up design discretization approach have been developed to build the finite element model. The proposed method overcomes the limitations of current wrinkle assessment methods by connecting the high sensitivity near infrared hyperspectral measurements to direct structural models. Temporal evaluations of the load-deformation response, acoustic emissions, and optical microscopy are used to study and verify the failure modes and damage progression models in the tension and compression specimens. An analytical model based on the orthotropic stress concentration factor and a generalized expression using traction continuity through the kink band is developed to predict failure strength of the Open Hole Compression (OHC) specimens. In this thesis, a new methodology to determine the limit point is also proposed based on the out-of-plane displacement tracking using an image correlation method. The method can be used to determine the start of incipient interlaminar delamination in continuous fiber reinforced composite materials.

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