Induction melt thermoforming of multiaxial thermoplastic composites

Campbell, Iain Innes (2024) Induction melt thermoforming of multiaxial thermoplastic composites. PhD thesis, University of Glasgow.

Abstract

Fibre reinforced thermoplastic composites are a widely used material with a growing global market share. This is partially driven by a need to improve energy efficiency through weight reduction. These composites offer high specific strength and stiffness thanks to their mix of a low-density polymer matrix and high-strength fibres. However, during the forming of complex geometries with multiple fibre reinforced plies, these plies must undergo significant deformation. This requires sliding between plies to relieve stress and avoid defects in the final part. Reducing friction coefficients between adjacent plies of fibre reinforcement could allow for more complex geometries to be formed with less wrinkling.

The principle aim of this study was to investigate the feasibility of a blue-sky concept: utilising molten metal as a lubricating interlayer in the thermoforming of thermoplastic composites. The low viscosity of the interlaminar tin is intended to reduce frictional stresses between adjacent sheets, thus reducing compressive stresses resultant wrinkling in the part. Induction heating is used to melt tin sheets in a hybrid composite tin layup and conduction leads to the melting of the matrix phase of adjacent nylon-carbon composites. A novel multi step male tool is then used to form the part while simultaneously removing the majority of the interlaminar tin from the final part. Experiments showed layups with up to three interlayers of tin could be successfully heated and formed via induction heating. Another advantage of the method is the ability to use the metal layers to heat composite parts efficiently and uniformly from the inside.

Building on the development of the novel manufacturing process, mechanical testing, non-destructive testing and some numerical simulation was carried out to further explore the process. Microscope imaging, 2D x-rays and 3D CT scans are used to quantify residual tin. Residual tin volumes as low as 1.5 % were achievable when consolidating flat geometries. The interlaminar shear strength of manufactured components was tested. Despite reductions in yield and flexural stiffness, process refinements produced parts with interlaminar shear stiffness comparable to benchmark samples. Finally, simulation of the multistep forming process using a pantographic beam and membrane model allowed the influence of multi-step forming and friction variation to be studied.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > TA Engineering (General). Civil engineering (General); T Technology > TS Manufactures
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: Engineering and Physical Sciences Research Council (EPSRC), Engineering and Physical Sciences Research Council (EPSRC)
• Supervisor's Name: Mulvihill, Dr. Daniel and Harrison, Dr. Philip
• Date of Award: 2024
• Depositing User: Theses Team
• Unique ID: glathesis:2024-84186
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 02 Apr 2024 15:14
• Last Modified: 02 Apr 2024 15:16
• URI: https://theses.gla.ac.uk/id/eprint/84186

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