Vacuum Infusion Process Development for Conformal Ablative Thermal Protection System Materials

Conformal ablators are low density composite materials comprised of a flexible carbon felt based fibrous substrate and a high surface area phenolic matrix. These materials are fabricated to near net shape by molding the substrate, placing in a rigid matched mold and infusing with liquid resin through a vacuum assisted process. The open mold process, originally developed for older rigid substrate ablators, such as PICA, wastes a substantial amount of resin. In this work, a vacuum infusion process a type of liquid composite molding where resin is directly injected into a closed mold under vacuum is advanced for conformal ablators. The process reduces waste over the state-of-the-art technique. Small, flat samples of Conformal Phenolic Impregnated Carbon Ablator are infused using the new approach and subjected to a range of curing configurations and conditions. Resulting materials are inspected for quality and compared to material produced using the standard process. Lessons learned inform subsequent plans for process scale up

Optimal Control of UV-Induced Curing Processes for Layer-by-Layer Manufacturing of Composites

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

Recent advances in 3D printing of biomaterials.

3D Printing promises to produce complex biomedical devices according to computer design using patient-specific anatomical data. Since its initial use as pre-surgical visualization models and tooling molds, 3D Printing has slowly evolved to create one-of-a-kind devices, implants, scaffolds for tissue engineering, diagnostic platforms, and drug delivery systems. Fueled by the recent explosion in public interest and access to affordable printers, there is renewed interest to combine stem cells with custom 3D scaffolds for personalized regenerative medicine. Before 3D Printing can be used routinely for the regeneration of complex tissues (e.g. bone, cartilage, muscles, vessels, nerves in the craniomaxillofacial complex), and complex organs with intricate 3D microarchitecture (e.g. liver, lymphoid organs), several technological limitations must be addressed. In this review, the major materials and technology advances within the last five years for each of the common 3D Printing technologies (Three Dimensional Printing, Fused Deposition Modeling, Selective Laser Sintering, Stereolithography, and 3D Plotting/Direct-Write/Bioprinting) are described. Examples are highlighted to illustrate progress of each technology in tissue engineering, and key limitations are identified to motivate future research and advance this fascinating field of advanced manufacturing

Accelerated microwave curing of fibre-reinforced thermoset polymer composites for structural applications: A review of scientific challenges

Accelerated curing of high performance fibre-reinforced polymer (FRP) composites via microwave heating or radiation, which can significantly reduce cure time and increase energy efficiency, has several major challenges (e.g. uneven depth of radiation penetration, reinforcing fibre shielding, uneven curing, introduction of hot spots etc). This article reviews the current scientific challenges with microwave curing of FRP composites considering the underlying physics of microwave radiation absorption in thermoset-matrix composites. The fundamental principles behind efficient accelerated curing of composites using microwave radiation heating are reviewed and presented, especially focusing on the relation between penetration depth, microwave frequency, dielectric properties and cure degree. Based on this review, major factors influencing microwave curing of thermoset-matrix composites are identified, and recommendations for efficient cure cycle design are provided

strategies for modelling and optimization of bi stable composite laminates actuated by embedded shape memory alloy wires

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