Date of Award

Summer 8-15-2022

Level of Access Assigned by Author

Open-Access Thesis

Language

English

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Justin Lapp

Second Committee Member

Zhihe Jin

Third Committee Member

William Davids

Abstract

The manufacturing of continuous fiber-reinforced thermoplastic (CFRTP) laminates requires the application of heat and pressure. Standard CFRTP manufacturing methods like thermoforming take a global approach to manufacturing, where the whole part is heated and undergoes a forming process. There is an opportunity to develop advanced manufacturing methods based on localized heating and deformations of consolidated stock. This thesis provides a localized heating method via networks of resistive heating elements embedded within the laminate and a means to evaluate that method.

Typical heating methods for CFRTP laminates include infrared and convective ovens or surface contact heaters. They have the same drawbacks of slow heating times and overheating surfaces. These are not significant issues in a normal thermoforming process. However, they would hinder an advanced manufacturing process because the laminate will not be under full compression to prevent deconsolidation outside of the bend area. The proposed method provides rapid and even through-thickness heating of a composite laminate within the desired region.

The methodology for validation of the local heating was experimental and numerical. The experimental validation consisted of heating tests with PETg/E-glass laminates ranging from 2.81 cm to 3.23 cm in thickness. Two heating element materials, nichrome, and 304 stainless steel, were tested. The temperature field within the specimen was monitored via embedded thermocouples. The criterion for validating the embedded heating elements was quantitative for heating time and maximum temperature and qualitative for part quality. Seven heating tests were conducted using different designs to evaluate the embedded heating element method. Heating times ranged from 13 to 56 minutes to reach a maximum temperature of 168.33 C. None of the laminates showed deconsolidation from the heating.

Heating is a critical component in composite CFRTP design and manufacturing, yet it is often overlooked. A literature survey reveals that assumptions of isothermal conditions are often made where a heat transfer analysis would show that to be false. That assumption might be adequate in a global processing method but not for localized methods. Therefore, this thesis presents a numerical heat transfer simulation and a methodology for modeling local heat transfer in CFRTP laminates that accounts for fiber orientation in the thermal conductivity. The purpose of this simulation is to assist in designing, improving, and evaluating heating methods in CFRTPs including locally heating with resistive heating elements.

The simulation, coded in MATLAB, models two-dimensional transient heat transfer with heat generation. The domain material is considered a continuum, but the thermal conductivity is modeled as an orthotropic, fiber angle-dependent, effective property within the lamina. Six effective thermal conductivity models from the literature were evaluated for accuracy in the simulation. The simulation uses the Crank-Nicolson finite-difference discretization and Newton’s method to solve the non-linear transient heat transfer over a rectangular Cartesian coordinate system in a rectangular domain. A domain temperature field is considered the initial condition; boundary conditions can be Dirichlet, Neumann, and Robins.

A training set of four heating tests validated the simulations, which was evaluated with a test set of three heating tests. The simulations were used to numerically evaluate the heating element design for temperature profiles and efficiency. Efficiencies of 60 – 77% were observed, with faster heating rates producing greater efficiencies. Temperature differences through the thickness ranged from 7.49 C to 33.96 C, with more heating elements creating greater uniformity.

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