Date of Award

August 2016

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Rani El-Hajjar

Committee Members

Anoop Dhingra, Llya Avdeev

Abstract

Inspection of kissing bond defects in composite structures is a challenge for traditional nondestructive testing (NDT) methods. A kissing bond (KB) is a bondline defect where two surfaces are in intimate contact with each other but with little to no bond strength. New nondestructive testing methods need to be developed that can detect adhesive kissing bonds, and areas within a composite joint where the bondline is compromised. The primary goal of this thesis is to explore if a NDT technique based on Dynamic Mechanical Analysis (DMA) can be established which is capable of detecting kissing bonds within bondlines in composite laminates that have been bonded with epoxy film and paste adhesives. DMA as a test method has the benefit that it can be conducted quickly and is able to test the material at various temperature and frequency ranges. Thirty specimens were prepared and attempts were made to create kissing bonds by introducing different types of contaminants at the bond location. Contamination used within the adhesive joint consisted of introduction of mold release and grease lubricant. Ultrasonic inspection is first performed to demonstrate that the surfaces were in contact in the bondline and no attenuation from porosity or delamination is present. Dynamic testing was performed on each sample in an attempt to detect these kissing bonds using the DMA method. Information on changes in the sample’s stiffness, storage modulus and loss modulus over a range of temperatures is recorded and analyzed. The specimens were subsequently tested in tension to determine the maximum strength of the materials. A geometrically nonlinear, 3D finite element (FE) analysis was performed to determine the stress and strain distributions within the composite single-lap joint (SLJ). The results obtained from the study showed artificial KBs appear to have been successfully fabricated based on tension testing and optical scans of fractured surfaces. Visual inspections of the fracture surfaces showed that contaminated bonds predominately failed via adhesion failure, while the control samples failed via cohesion. The production of KBs within the joints were more successful among the paste adhesive rather than the film adhesive. Analysis of DMA parameters shows a reasonable correlation for some of the parameters and the failure load. The FE results on the tension test showed that the stresses became increasingly higher within the single-lap joint for paste adhesive compared to the film adhesive. It was seen from the FEA models that the maximum shear stress and elastic strain occur near the overlap joint corners ends, suggesting that cohesive crack initiation is most likely to occur at the corners for defect-free samples. The stiffness results obtained from the DMA showed that all the paste samples had stiffness values much larger than the film samples; this elevated stiffness could contribute to the increased stress evident in the FE models. As the stresses were higher in the paste adhesive specimens, it is interpreted to be a contributing factor in the reductions of shear strength within the SLJs when defects are present.

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