All ETDs from UAB

Advisory Committee Chair

Uday K Vaidya

Advisory Committee Members

Brian S Pillay

Haibin Ning

Mark L Weaver

Mohamed F Shehadeh

Document Type

Dissertation

Date of Award

2015

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

Thermoplastic composites have advantages over other traditional materials, such as steel, aluminum and ceramics, in terms of lightweight, specific strength, corrosion resistance and superior damping capacity ability to recycle, shorter processing times, and lack of exothermic reactions or toxic emissions. These advantages allow them to be widely used in various high-end products used in many industrial applications such as marine, offshore, mass-transit and automotive industries. There are several factors that affect the mechanical and thermal properties of thermoplastic composites, such as fiber length, fiber volume fraction and fiber orientation to name a few. Fiber orientation plays a critical role in determining the mechanical and thermal properties of composites, and understanding the fiber orientation will lead to improved part designs and reproducible manufacturing. An extensive review including mathematical modeling, simulation approaches of fiber representation in polymeric composites and various techniques of fiber orientation detection and measurements was conducted. It was found that all the detection techniques have the ability to predict the preferential fiber orientation to some extent; however, a combination of techniques can provide additional information about the fiber orientation. This review was followed by process simulation of the Extrusion Compression Molding (ECM) technique for Long Fiber Thermoplastic (LFT) composites. The effect of the charge size, location and shape on the flow front and mold filling was evaluated. It was found that the charge shape strongly influences the flow front and the mold filling process. Altering the charge location in the mold affects the fiber orientation in the final part. It was found that locating the charge close to one of the mold walls results in higher fiber orientation in a defined direction as the flow has longer time to develop in this specific direction. An experimental investigation was conducted to study the effect of the ECM process on the fiber length distribution, fiber weight fraction distribution and fiber orientation. It was observed that the ECM process resulted in uniform fiber weight fraction distribution in the final fabricated component. The stresses resulting from high back pressure (i.e. 140 psi) extrusion process resulted in an average of 55% fiber attrition. It was found that at high aspect ratio of fiber (i.e. >100), the change in fiber length does not affect the tensile elastic properties. Placing the charge in the mold in a location that has an edge of the charge to edge of the mold ratio of 0.46 results in elastic modulus values that are equivalent to modulus for parts with randomly oriented reinforcement. However, the charge located at a position where the edge of the charge to the edge of the mold ratio less than 0.46 resulted in higher elastic modulus values that indicate that the LFT parts have fibers with high preferential fiber orientation in the flow direction. Different Non Destructive Testing/ Evaluation (NDE/T) methods for characterizing thermoplastic composites were investigated. The techniques were divided into two major groups: radiography methods and ultrasonic techniques. It was observed that the radiography methods had some limitations in identifying the Foreign Object Inclusions (FOIs) with comparable density to the background material. The Ultrasonic Testing (UT) methods were successfully able to capture all of the FOIs in the thermoplastic composite specimen regardless of the material type. Two Dimensional (2-D) fiber texture was captured through X-ray radiography and through transmission UT. However, information about out of plane fiber waviness was not captured. X-ray Computed Tomography (CT) was able to identify the fiber texture through the specimen depth alongside to the out of plane fiber wash. The preferential fiber orientation in ECM composite parts was evaluated with respect to the Frequency Response Function (FRF) of the composite material. The effect of the change of the preferential fiber orientation on the damping characteristics was analyzed. It was found that samples oriented 0° along the squeeze flow direction exhibited the highest natural frequencies and the lowest damping capacity. The results were verified using three point bending test, where it was found that both the vibration and flexure testing were in good agreement. The samples with fibers oriented at 45° to the reference axis exhibited the highest damping capacity due to the shear-coupled nature of fibers aligned off-axis.

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