Date of Award
Doctor of Philosophy (PhD)
Dr. Beshah Ayalew, Committee Chair
Dr. Srikanth Pilla, Co-Chair
Dr. Yunyi Jia
Dr. Matthias Schmid
Composite materials are becoming viable solutions for making safe, yet lighter and more fuel-efficient vehicles in the automotive industry. However, conventional thermal-based composite manufacturing methods are energy intensive. Potential alternatives are radiation-based curing processes which lend themselves to layer-by-layer additive processes that are suitable for making thick structural parts. This dissertation documents an investigation into ultraviolet (UV) induced curing and layering processes including schemes for their optimization and control. First, a curing process model is developed that is comprised of the coupled cure-kinetics and thermal evolution for a cationic polymerization of a single layer of material. This model is then extended to the process of concurrent layering and curing of multiple layers. The model for processing multiple layers is characterized as a multi-mode hybrid system that switches modes both when the UV source is turned off and when a new layer is added. A computational framework is outlined for determining the optimal sequence of switching times that gives a minimal cure level deviation across all layers subjected to the multi-mode hybrid system model of the process. For validation purposes, a one layer material with two mode has been considered. Comparison of the hardness of a sample cured with optimal switching time versus another sample cured for a longer time showed similar hardness values while using energy/total time.
To improve the interlaminar shear strength, the effect of in-situ consolidation pressure on the inter-laminar shear strength of the final product is assessed experimentally. Using the optimal time sequence, a fiber-reinforced composite is made with in-situ consolidation and curing. The results showed that thick composite parts fabricated with in-situ consolidation and UV curing process, with the optimal sequence, showed increased inter-laminar shear strength with increases of the consolidation pressure up to a certain point. An increase in consolidation pressure beyond this point decreased the interlaminar-shear strength. Scanning electron microscopy (SEM) is used to investigate the effect of compaction on the microstructure of the final cured product.
For online control, first, a nonlinear model predictive control (NMPC) scheme is outlined for UV-induced acrylate-based curing of a single layer thick composite part. Then, the model is extended for switching nonlinear model predictive control (SNMPC) for layer-by-layer curing process. The key characteristic is that the processes model switches when a new layer is added to the existing layer. Open loop optimal control is used to determine the optimal layering time and temperature profile which give a nearly uniform cure distribution of a thick composite material. Once the temperature trajectory and optimal time sequences are found, the SNMPC is implemented for online control. The objective is to determine theoretical optimal behavior which is then used for online SNMPC for tracking the reference temperature distribution. To demonstrate the effectiveness of the proposed approach a three-layer fiber-reinforced resin is considered and results show a very good agreement between the reference temperature distribution and SNMPC.
Beyene, Shiferaw Damtie, "Optimal Control of UV-Induced Curing Processes for Layer-by-Layer Manufacturing of Composites" (2019). All Dissertations. 2398.