abstract: Honeycomb sandwich panels have been used in structural applications for several decades in various industries. While these panels are lightweight and rigid, their design has not evolved much due to constraints imposed by available manufacturing processes and remain primarily two-dimensional extrusions sandwiched between facings. With the growth in Additive Manufacturing, more complex geometries can now be produced, and advanced design techniques can be implemented into end use parts to obtain further reductions in weight, as well as enable greater multi-functionality. The question therefore is: how best to revisit the design of these honeycomb panels to obtain these benefits? In this work, a Bio-Inspired Design approach was taken to answer this question, primarily since the hexagonal lattice is so commonly found in wasp and bee nests, including the well-known bee’s honeycomb that inspired these panel designs to begin with. Whereas prior honeycomb panel design has primarily focused on the hexagonal shape of the unit cell, in this work we examine the relationship between the various parameters constituting the hexagonal cell itself, specifically the wall thickness and the corner radius, and also examine out-of-plane features that have not been previously translated into panel design. This work reports findings from a study of insect nests across 70 species using 2D and 3D measurements with optical microscopy and X-ray tomography, respectively. Data from these biological nests were used to identify design parameters of interest, which were then translated into design principles. These design principles were implemented in the design of honeycomb panels manufactured with the Selective Laser Sintering process and subjected to experimental testing to study their effects on the mechanical behavior of these panels. / Dissertation/Thesis / Masters Thesis Manufacturing Engineering 2020

Increased interest in and use of additive manufacturing has made it an important component of advanced manufacturing in the last decade. Material Extrusion Additive Manufacturing (MatEx) has seen a shift from a rapid prototyping method harnessed only in parts of industry due to machine costs, to something widely available and employed at the consumer level, for hobbyists and craftspeople, and industrial level, because falling machine costs have simplified investment decisions. At the same time MatEx systems have been scaled up in size from desktop scale Fused Filament Fabrication (FFF) systems to room scale Big Area Additive Manufacturing (BAAM). Today MatEx is still used for rapid prototyping, but it has also found application in molds for fiber layup processes up to the scale of wind turbine blades. Despite this expansion in interest and use, MatEx continues to be held back by poor part performance, relative to more traditional methods such as injection molding, and lack of reliability and user expertise. In this dissertation, a previously unreported phenomenon, irreversible thermal strain (ITε), is described and explored. Understanding ITε improves our understanding of MatEx and allows for tighter dimensional control of parts over time (each of which speaks to extant challenges in MatEx adoption). It was found that ITε occurs in multiple materials: ABS, an amorphous polymer, and PLA, a semi-crystalline one, suggesting a number of polymers may exhibit it. Control over ITε was achieved by tying its magnitude back to part layer thickness and its directionality to the direction of roads within parts. This was explained in a detail by a micromechanical model for MatEx described in this document. The model also allows for better description of stress-strain response in MatEx parts broadly. Expanding MatEx into new areas, one-way shape memory in a commodity thermoplastic, ABS, was shown. Thermal history of polymers heavily influences their performance and MatEx thermal histories are difficult to measure experimentally. To this end, a finite element model of heat transfer in the part during a MatEx build was developed and validated against experimental data for a simple geometry. The application of the model to more complex geometries was also shown. Print speed was predicted to have little impact on bonds within parts, consistent with work in the literature. Thermal diffusivity was also predicted to have a small impact, though larger than print speed. Comparisons of FFF and BAAM demonstrated that, while the processes are similar, the size scale difference changes how they respond to process parameter and material property changes, such as print speed or thermal diffusivity, with FFF having a larger response to thermal diffusivity and a smaller response to print speed. From this experimental and simulation work, understanding of MatEx has been improved. New applications have been shown and rational design of both MatEx processes and materials for MatEx has been enabled.

No description available.

Sandwich panels serve many purposes in engineering applications. Additive manufacturing opened the door for easy fabrication of the sandwich panels with different core structures. In this study, additive manufacturing technique, experiments, and numerical analysis are combined to evaluate the mechanical properties of sandwich panels with different cellular core structures. The sandwich panels having honeycomb, re-entrant honeycomb, diamond, square core topologies are printed with the vat photopolymerization technique. Uniaxial compression testing is performed to determine the compressive modulus, strength, and specific strength of these lightweight panels. Elasto-plastic finite element analysis having good similarities with the experimental results provided a preview of the stress distribution of the sandwich panels under applied loading. The imaging of the tested samples showed the fractured regions of the cellular cores. Dynamic mechanical analysis of the panels provided scope to compare the performance of panels and solid materials with the variation of temperature. Sandwich panels with the diamond structure exhibit better compressive properties and specific strength while the re-entrant structure offers high energy absorption capacity. The sandwich structures provided better thermo-mechanical properties than the solid material. The findings of this study offer insights into the mechanical properties of sandwich panels printed with vat photopolymerization technique which can benefit a wide range of engineering applications.

Tissue regeneration using in-vitro scaffold becomes a vital mean to mimic the in-vivo counterpart due to the insufficiency of animal models to predict the applicability of drug and other physiological behavior. Three-dimensional (3D) bio-printing is an emerging technology to reproduce living tissue through controlled allocation of biomaterial and cell. Due to its bio-compatibility, natural hydrogels are commonly considered as the scaffold material in bio-printing process. However, repeatable scaffold structure with good printability and shape fidelity is a challenge with hydrogel material due to weak bonding in polymer chain. Additionally, there are intrinsic limitations for bio-printing of hydrogels due to limited cell proliferation and colonization while cells are immobilized within hydrogels and don’t spread, stretch and migrate to generate new tissue. The goal of this research is to develop a bio-ink suitable for extrusion-based bio-printing process to construct 3D scaffold. In this research, a novel hybrid hydrogel, is designed and systematic quantitative characterization are conducted to validate its printability, shape fidelity and cell viability. The outcomes are measured and quantified which demonstrate the favorable printability and shape fidelity of our proposed material. The research focuses on factors associated with pre-printing, printing and post-printing behavior of bio-ink and their biology. With the proposed hybrid hydrogel, 2 cm tall acellular 3D scaffold is fabricated with proper shape fidelity. Cell viability of the proposed material are tested with multiple cell lines i.e. BxPC3, prostate stem cancer cell, HEK 293, and Porc1 cell and about 90% viability after 15-day incubation have been achieved. The designed hybrid hydrogel demonstrate excellent behavior as bio-ink for bio-printing process which can reproduce scaffold with proper printability, shape fidelity and higher cell survivability. Additionally, the outlined characterization techniques proposed here open-up a novel avenue for quantifiable bio-ink assessment framework in lieu of their qualitative evaluation.

In a push towards more environmentally friendly pyrotechnics, new greener pyrotechnic compositions need to be developed. A primary goal is to replace components such as lead, barium, and chromium in pyrotechnic compositions. Fused Deposition Modelling (FDM) is a 3D printing/additive manufacturing method whereby a thin filament is passed through a heated nozzle, and extruded onto a substrate in successive layers. This method of manufacturing could be used to produce pyrotechnic time delays based on suitable “green” polymer/fuel mixtures. Fluoropolymers are an attractive oxidising system for pyrotechnic use as fluorine is highly reactive and reacts relatively easily with common metallic fuels such as aluminium and magnesium to release a large amount of energy. Fluoropolymers are already in use as oxidisers and binders, especially in infrared decoy flares. PTFE has found wide use in the pyrotechnics industry, but is not melt-processible. A similar fluoropolymer, poly(chloro-trifluoroethylene) (PCTFE) was considered instead. PCTFE differs from PTFE in that one of the fluorine atoms in the TFE monomer has been replaced by a chlorine atom. The larger chlorine atom interferes with the packing of the polymer chains during polymerisation and, as such, may make it easier to process than PTFE. It was found that pure PCTFE degraded heavily during processing and was therefore precluded from any further study. Melt-processible copolymers containing PCTFE are available from industry. These copolymers contain vinylidene fluoride (VDF) in addition to the CTFE i.e. poly(CTFE-co-VDF). Two grades of copolymer were obtained from 3M: FK-800® resin and Dyneon® 31508 resin. These two polymers contain different ratios of CTFE to VDF. FK-800® resin was successfully extruded and showed minimal signs of degradation. Pyrotechnic films, containing aluminium powder as the fuel, were cast with both polymers using solvent techniques. Differential thermal analysis (DTA) was used to determine the ignition points of the compositions. All of the FK-800®-based compositions ignited at approximately 450 °C whilst all the Dyneon® 31508-based compositions ignited at approximately 400 °C. The energy output of the compositions was determined using bomb calorimetry. The experimental energy outputs of the FK-800®-based compositions correlated well with the predictions from the thermodynamic simulations. The maximum energy output, ~7.0 MJ∙kg1, occurred at a fuel loading between 30 – 35 wt.%. Except for one composition, the Dyneon® 31508-based compositions did not ignite in the bomb calorimeter. FK-800® was successfully extruded into a filament and showed minimal signs of degradation. In order to assess the impact of adding a solid filler on the mechanical properties and extrudability of the polymer, magnesium hydroxide was used as inactive model compound in place of aluminium. A filament of FK-800® and Mg(OH)2 was successfully compounded and produced using a filler loading of 30 wt.%. Compounding of the Dyneon 31508® with the magnesium hydroxide was unsuccessful. Addition of LFC-1® liquid fluoroelastomer improved the processibility of the Dyneon 31508® by lowering the melt viscosity. / Dissertation (MEng)--University of Pretoria, 2017. / Chemical Engineering / MEng / Unrestricted

The work herein covers the complete process for development, production and testing of a melt processable pyrotechnic composition, with the goal of using the composition as a printing material in a fused deposition modelling (FDM) type 3D printer. 3D printing is fast becoming an area of interest for energetic materials research. This is due to the role that geometry can play in combustion performance of a composition and 3D printing’s ability to produce a variety of complex designs. Melt processable fluoropolymers were selected as oxidisers. The polymers selected for the study were FK-800® and Dyneon 31508®. Both are co-polymers of vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE). Aluminium was the choice fuel in this instance as it had better energetic performance than the alternatives investigated. It was also deemed to be a safer fuel when considering the combustion products. Hazardous combustion products like hydrofluoric and hydrochloric acid could be suppressed by increasing the fuel loading to 30 wt.%, thereby reducing the risks associated with burning the composition. Preliminary differential thermal analysis (DTA) analysis indicated that the compositions would only ignite above 400 °C which was well above the suggested processing temperature of 230 °C as determined from thermogravimetric (TGA) analysis. These thermal analysis techniques indicated that the reactions were most likely a gas-solid reactions due to ignition temperatures being significantly lower than those associated with phase changes occurring in the fuels tested, yet above the decomposition temperatures for the oxidisers. ii Extrusion of the compositions proceeded with addition of LFC-1® liquid fluoroelastomer. This addition was made in order to order to lower the melt viscosity, thereby improving the quality of the filament produced. Compositions were extruded with an aluminium loading of 30 wt.%. Oxidiser and LFC-1® made up the rest of the mass with the LFC-1® contributions being either 7 wt.% or 14 wt.%. Burn rates, temperatures and ignition delays were all influenced by the addition of LFC-1® to the system. FK-800® was found to be a better oxidiser in this instance since its burn rates were consistent especially when compared to erratic nature of the Dyneon 31508® burns. Linear burn rates for the FK-800® increased from 15.9 mm·s−1 to 18.9 mm·s−1 with the increase in LFC-1® loading. Combustion temperature also increased by approximately 180 °C from 794 °C. Printing with the material was achieved only after significant alterations were made to the hot end used. Printing proceeded in a staged, start-stop manner. After each new layer of material was deposited the printer was cleared of material and the hot end was allowed to cool. If this procedure was not followed it led to significant preheating of the material within the feeding section of the extruder. This premature heating caused feeding problems due to softening and swelling of the material within the cold side of the hot end which led to blockages, leading to the conclusion that the composition was not compatible with the off-the-shelf hot end used in this study. Low quality printing could be achieved with both FK-800® and Dyneon 31508® compositions. This would suggest that slight compositional changes paired with the alterations made to the hot end could improve the quality of the prints to an extent that would be comparable to that of more commonplace printing materials. / Dissertation (MEng)--University of Pretoria, 2017. / Chemical Engineering / MEng / Unrestricted

Increased interest in and use of additive manufacturing has made it an important component of advanced manufacturing in the last decade. Material Extrusion Additive Manufacturing (MatEx) has seen a shift from a rapid prototyping method harnessed only in parts of industry due to machine costs, to something widely available and employed at the consumer level, for hobbyists and craftspeople, and industrial level, because falling machine costs have simplified investment decisions. At the same time MatEx systems have been scaled up in size from desktop scale Fused Filament Fabrication (FFF) systems to room scale Big Area Additive Manufacturing (BAAM). Today MatEx is still used for rapid prototyping, but it has also found application in molds for fiber layup processes up to the scale of wind turbine blades. Despite this expansion in interest and use, MatEx continues to be held back by poor part performance, relative to more traditional methods such as injection molding, and lack of reliability and user expertise. In this dissertation, a previously unreported phenomenon, irreversible thermal strain (ITε), is described and explored. Understanding ITε improves our understanding of MatEx and allows for tighter dimensional control of parts over time (each of which speaks to extant challenges in MatEx adoption). It was found that ITε occurs in multiple materials: ABS, an amorphous polymer, and PLA, a semi-crystalline one, suggesting a number of polymers may exhibit it. Control over ITε was achieved by tying its magnitude back to part layer thickness and its directionality to the direction of roads within parts. This was explained in a detail by a micromechanical model for MatEx described in this document. The model also allows for better description of stress-strain response in MatEx parts broadly. Expanding MatEx into new areas, one-way shape memory in a commodity thermoplastic, ABS, was shown. Thermal history of polymers heavily influences their performance and MatEx thermal histories are difficult to measure experimentally. To this end, a finite element model of heat transfer in the part during a MatEx build was developed and validated against experimental data for a simple geometry. The application of the model to more complex geometries was also shown. Print speed was predicted to have little impact on bonds within parts, consistent with work in the literature. Thermal diffusivity was also predicted to have a small impact, though larger than print speed. Comparisons of FFF and BAAM demonstrated that, while the processes are similar, the size scale difference changes how they respond to process parameter and material property changes, such as print speed or thermal diffusivity, with FFF having a larger response to thermal diffusivity and a smaller response to print speed. From this experimental and simulation work, understanding of MatEx has been improved. New applications have been shown and rational design of both MatEx processes and materials for MatEx has been enabled.

Cyber physical systems (CPS) are an integration of computational and physical processes, where the cyber components monitor and control physical processes. Cyber-attacks largely target the cyber components with the intention of disrupting the functionality of the components in the physical domain. This dissertation explores the role of semantic inference in understanding such attacks and building resilient CPS systems. To that end, we present SIMON, an ontological design and verification framework that captures the intricate relationship(s) between cyber and physical components in CPS by leveraging several standard ontologies and extending the NIST CPS framework for the purpose of eliciting trustworthy requirements, assigning responsibilities and roles to CPS functionalities, and validating that the trustworthy requirements are met by the designed system. We demonstrate the capabilities of SIMON using two case studies – a vehicle to infrastructure (V2I) safety application and an additive manufacturing (AM) printer. In addition, we also present a taxonomy to capture threat feeds specific to the AM domain.

The results from this study, on a few commercial and model metastable beta titanium alloys, indicate that the growth restriction factor (GRF) model fails to interpret the grain growth behavior in the additively manufactured alloys. In lieu of this, an approach based on the classical nucleation theory of solidification incorporating the freezing range has been proposed for the first time to rationalize the experimental observations. Beta titanium alloys with a larger solidification range (liquidus minus solidus temperature) exhibited a more equiaxed grain morphology, while those with smaller solidification ranges exhibited columnar grains. Subsequently, the printability of two candidate beta titanium alloys containing eutectoid elements (Fe) that are prone to beta fleck in conventional casting, i.e., Ti-1Al-8V-5Fe (wt%) or Ti-185, and Ti-10V-2Fe-3Al (wt%) or Ti-10-2-3, is further investigated via two different AM processing routes. These alloys are used for high-strength applications in the aerospace industry, such as landing gears and fasteners. The Laser Engineered Net Shaping and Selective Laser Melting (the two AM techniques) results show that locally higher solidification rates in AM can prevent the problem of beta fleck and potentially produce β-titanium alloys with significantly enhanced mechanical properties over conventionally cast/forged counterparts. Further, the detailed investigation of microstructure-mechanical property relationships indicates that the precipitation or formation of non-equilibrium secondary phases like α or ω in these commercial systems can be advantageous to the mechanical properties. The influence of process parameters on the evolution of such secondary phases within the β matrix grains has also been rationalized using a FEM-based multi-physics thermo-kinetic model that predicts the multiple heating-cooling cycles experienced by the layers during the LENS deposition. Overall, the results indicate that Ti-1-8-5 and Ti-10-2-3 are promising β-Ti alloys for AM processing. Further, the results also demonstrate the ability to tune the microstructure (secondary phase precipitation and grain size) via changes in the process parameters to achieve desirable mechanical properties, obviating the need for any secondary post-processing. The understanding obtained through this work can be coupled with the concept of β-phase stability prediction, via parameters like bond order (Bo), the energy level of metal d-orbital (Md), Mo equivalency, etc., to design novel beta titanium alloys with the desired microstructures tailored via AM for structural applications.