All ETDs from UAB

Advisory Committee Chair

Haibin Ning

Advisory Committee Members

Selvum Pillay

Robin Foley

Peter Walsh

Ruigang Wang

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

Composite materials are on the verge of becoming primary materials choices for many applications due to their promising features of achieving high strength and stiffness and lightweighting of many components. Thermoplastic matrix composites have been gaining favor due to their superior toughness and energy dissipation properties coupled with corrosion resistance and ease of processing. Long fiber thermoplastics (LFTs) are a class of composites, which contain discontinuous reinforcements at a length above the critical fiber length for efficient load transfer to the reinforcement. These materials are gaining steady favor due to their positive aspects of tailorable properties, intrinsic recyclability, high specific strength and stiffness, low-cost, and ease of processing into complex geometries. LFTs can be processed by injection molding (IM) or extrusion-compression molding (ECM). Both processes are known to produce parts at a high production rate. In the current study, through thickness property variations of a compression molded LFT composite system, which consist of polypropylene/glass fiber (PPGF) at 40 weight %, is investigated. All molded parts are known to have through thickness microstructure differences; the effect is more significant for thicker parts. This study is performed to evaluate through thickness processing induced structure differences, which ultimately affects properties of the final part. Firstly, through thickness microstructure and thermal property variations are characterized for PPGF LFT plaques with dimensions of 152.4 x 152.4 mm (6 x 6 inches) at different thicknesses of 5, 8, 14, and 21 mm. The study showed that the thickest variation had a higher percentage of voids in the core region, which is seen by microscopy at a certain distance below the surface of the part. The thermal properties of crystallinity showed an increasing trend from the skin to the core, this also increased the thermal stability of the thermoplastic matrix. Static and dynamic flexural property variations were studied for the different thickness plaques. The z-direction properties of the thickest plaques were also studied. The properties of the thickest plaque from skin to core showed a significant decrease of 70% in strength and 36% of modulus; z-direction properties were not as high as the in-plane properties. The dynamic properties showed a significant decline in properties above 130°C. Fiber length after processing of the different thickness plaques were determined; the thinner plaques had lower residual fiber length with an increasing trend as thickness of the plaques increased. Lastly, process simulation of the compression molding process was performed using software packages by Moldex 3D. Filling, fiber orientation, and through thickness density results of the different thickness plaques were analyzed. Fiber orientation results showed a high percentage of fiber orientation in the direction of flow for thinner plaques. The 21 mm plaque showed more random fiber orientation. The qualitative fiber orientation analysis correlated more for the 4 and 8 mm plaques. Density variation through the thickness from simulation and experimentally determined values were in agreement for different depth to thickness ratios of the 14 and 21 mm plaque. The 4 and 8 mm plaque density did not show variation through the thickness which is also seen experimentally.

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