3D Printing of Hybrid Architectures via Core-Shell Material Extrusion Additive Manufacturing

Biological materials often employ hybrid architectures, such as the core-shell motif present in porcupine quills and plant stems, to achieve unique properties and performance. Drawing inspiration from these natural materials, a new method to fabricate lightweight and stiff core-shell architected filaments is reported. Specifically, a core-shell printhead conducive to printing highly loaded fiber-filled inks, as well as a new low-density syntactic foam ink, are utilized to 3D-print core-shell architectures consisting of a syntactic epoxy foam core surrounded by a stiff carbon fiber-reinforced epoxy composite shell. Effective printing of test specimens and structures with controlled geometry, composition, and architecture is demonstrated with printed core-shell samples exhibiting up to a 25 percent increase in specific stiffness over constituent materials. A detrimental increase in foam density was observed during initial core-shell printing due to failure of glass microballoons (GMBs) during extrusion. To solve this, the second part of the dissertation investigates the relationships between GMB loading, extrusion pressure, nozzle diameter, and flowrate on printed density. These parameters are investigated to gain understanding of the conditions leading to GMB failure, informing selection of process parameters to minimize it. A new syntactic foam ink is formulated with GMBs that exhibit a lower average diameter and higher crush strength, ultimately enabling printing without prominent GMB failure and the ability to achieve near theoretical printed density. The new foam samples are stronger and stiffer than conventional syntactic foams and current DIW-printed foams. Further implementation of the new foam in the C-S architecture enabled a 5 percent increase in specific stiffness over previous values. In the last study, work is done to further expand the capability of C-S printing by demonstrating multimaterial 3D printing using the core-shell nozzle. This approach enables “on-the-fly” switching between materials during fabrication, without the need for two nozzles. Material transition behavior is analyzed, multimaterial components are successfully printed, and flexural testing is conducted. Overall, the new approach enables material switching with a continuous print path, providing greater design flexibility and compositional control, opening new routes to DIW print multimaterial architectures

Bioelectronic Sensor Nodes for Internet of Bodies

Energy-efficient sensing with Physically-secure communication for bio-sensors on, around and within the Human Body is a major area of research today for development of low-cost healthcare, enabling continuous monitoring and/or secure, perpetual operation. These devices, when used as a network of nodes form the Internet of Bodies (IoB), which poses certain challenges including stringent resource constraints (power/area/computation/memory), simultaneous sensing and communication, and security vulnerabilities as evidenced by the DHS and FDA advisories. One other major challenge is to find an efficient on-body energy harvesting method to support the sensing, communication, and security sub-modules. Due to the limitations in the harvested amount of energy, we require reduction of energy consumed per unit information, making the use of in-sensor analytics/processing imperative. In this paper, we review the challenges and opportunities in low-power sensing, processing and communication, with possible powering modalities for future bio-sensor nodes. Specifically, we analyze, compare and contrast (a) different sensing mechanisms such as voltage/current domain vs time-domain, (b) low-power, secure communication modalities including wireless techniques and human-body communication, and (c) different powering techniques for both wearable devices and implants.Comment: 30 pages, 5 Figures. This is a pre-print version of the article which has been accepted for Publication in Volume 25 of the Annual Review of Biomedical Engineering (2023). Only Personal Use is Permitte

Doctor of Philosophy

dissertationExpanded Polystyrene (EPS) geofoam is a superlight weight material used in various geotechnical engineering applications. The goal of this study was to explore the use of EPS embankment to support railways and bridges without being overstressed during extreme events like earthquakes. Static and dynamic deflections that occur on an embankment along a rail line were measured by using numerical, laboratory and field techniques. A numerical method was used to measure static deflection whereas accelerometers were used in case of dynamic deflection. In the laboratory, large scale triaxial and large chamber tests were conducted to determine the resilient modulus of ballast. In the field, accelerometers were placed on sleepers of commuter and light rail line to collect the data for vertical deflection. Monotonic and cyclic triaxial tests, analytical and numerical methods were used to study bridge support embankments. The dynamics of EPS embankment for support of bridge system was studied and possible lateral restrained systems were developed for moderate to higher seismic excitations. Large chamber test is more suitable for the calculation of cyclic nonlinear secant modulus. EPS embankment performed well while considering vertical deflection. The combination of dead and earthquake load can be considered as the stress corresponding to 2 percent axial strain. The critical accelerations for sliding, sway and rocking were 0.6 g, 0.2 g and 0.3 g, respectively. Shear keys, embedment of embankment and cables are required for higher excitations

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Monitoring of the Launched Girder Bridge over the Iowa River on US 20, March 2004

The objective of the study presented in this report was to document the launch of the Iowa River Bridge and to monitor and evaluate the structural performance of the bridge superstructure and substructure during the launch. The Iowa Department of Transportation used an incremental launching method, which is relatively unique for steel I-girder bridges, to construct the Iowa River Bridge over an environmentally sensitive river valley in central Iowa. The bridge was designed as two separate roadways consisting of four steel plate girders each that are approximately 11 ft deep and span approximately 301 ft each over five spans. The concrete bridge deck was not placed until after both roadways had been launched. One of the most significant monitoring and evaluation observations related to the superstructure was that the bottom flange (and associated web region) was subjected to extremely large stresses during the crossing of launch rollers. Regarding the substructure performance, the column stresses did not exceed reasonable design limits during the daylong launches. The scope of the study did not allow adequate quantification of the measured applied launch forces at the piers. Future proposed esearch should provide an opportunity to address this. The overall experimental performance of the bridge during the launch was compared with the predicted design performance. In general, the substructure design, girder contact stress, and total launching force assumptions correlated well with the experimental results. The design assumptions for total axial force in crossframe members, on the other hand, differed from the experimental results by as much as 300%

Graph-Segmenter: Graph Transformer with Boundary-aware Attention for Semantic Segmentation

The transformer-based semantic segmentation approaches, which divide the image into different regions by sliding windows and model the relation inside each window, have achieved outstanding success. However, since the relation modeling between windows was not the primary emphasis of previous work, it was not fully utilized. To address this issue, we propose a Graph-Segmenter, including a Graph Transformer and a Boundary-aware Attention module, which is an effective network for simultaneously modeling the more profound relation between windows in a global view and various pixels inside each window as a local one, and for substantial low-cost boundary adjustment. Specifically, we treat every window and pixel inside the window as nodes to construct graphs for both views and devise the Graph Transformer. The introduced boundary-aware attention module optimizes the edge information of the target objects by modeling the relationship between the pixel on the object's edge. Extensive experiments on three widely used semantic segmentation datasets (Cityscapes, ADE-20k and PASCAL Context) demonstrate that our proposed network, a Graph Transformer with Boundary-aware Attention, can achieve state-of-the-art segmentation performance

Dinamičko mehanička svojstva hibridnih nanokompozitnih materijala

Predmet istraživanja ove doktorske disertacije pripada oblasti nanomateijala i nanotehnogija koja je u trendu savremenih istraživanja. Posebno su intenzivna istraživanja u oblasti polimernih nanokompozita gde tradicionalno slabe strane polimera (niske vrednosti parametara mehaničke čvrstoće i loša termostabilnost) se značajno poboljšavaju primenom malog udela nano punioca i ojačanja uz neznatan porast gustine. Razvijena je metoda dizajniranja strukture nanokompozitnih balističkih materijala sa gledišta poboljšanja njihovih svojstava otpornosti pri udarima visoke energije. Proučeni su uslovi dobijanja laminarnih kompozitnih materijala p-aramid/poli (vinil butiral). Poli (vinil butiralni) sloj nanošen je u obliku disperzije poli (vinil butirala) i nano čestica SiOR2R u etil-alkoholu, pri čemu su korišćene modifikovane i nemodifikovane čestice SiOR2 Rsa vezujućim agensom-AMEO silanom. Na taj nači je utvrđen veliki značaj modifikacije nano čestica SiOR2R sa silanima na mehanička, termička i antibalistička svojstva dobijenih hibridnih nanokompozitnih materijala. Savremena istraživanja u ovoj oblasti usmerena su u pronalaženju mehanizama zaustavljanja rasta prsline modifikovanjem strukture na nano nivou što je i predmet ove doktorske disertacije. Proučavanja u okviru ove disertacije bila su usmerena na istraživanja mehanizama apsorpcije energije u nanokompozitima pri udarnim opterećenjima visoke energije i ponašanje nano čestica kao konstituenata u strukturi hibridnih kompozitnih materijala. Sinteza ovih nanokompozitnih materijala izvršiće se primenom koloidnih suspenzija koje se karakterišu ekstremnim porastom viskoznosti pri velikim brzinama smicanja kojima su izloženi pri udarnim naprezanjima. Originalnost ideje se ogleda što je princip hibridizacije primenjen na izradu laminatnih balističkih ploča sa laminama koje su različito nanomodifikovane a samim tim i sa različitim svojstvima. Značaj ove ideje je što različito nanomodifikovane lamine omogućavaju izradu funkcionalno gradijentnih kompozitnih materijla od nano do mikro nivoa. Ciljevi ove disertacije su višestruki: 1) proučavanje mehanizama procesiranja nano prahova različitih oksida u različitim disperzionim sredstvima prema klasičnim metodama i savremenim metodama modifikovanja površine čestica; 2) eksperimentalna istraživanje uticaja procesnih uslova brizganja i toplog presovanja hibridnih nonokompozita sa tkaninama od aramidnih vlakana sa različitim udelom modifikovanih nanočestica na njihova dinamickomehanička svojstva (modul sačuvane i izgubljene energije i tangens gubitaka) u različitom temperaturnom intervalu pri različitim frekvencijama); 3) eksperimentalna istraživanje uticaja procesnih uslova brizganja i toplog presovanja hibridnih laminatnih nonokompozita sa matricom od poli (vinil butirala) sa razlicitim udelom modifikovanih cestica silicijum dioksida na makromehanicka svojstva (Jungov modul elasticnosti, zatezna cvrstoca, prekidno izduženje); 4) eksperimentalna ispitivanja otpornosti na razaranje dobijenih hibridnih nanokompozitnih materijala na udar velikim energijama i brzinama (standardna balisticka ispitivanja sa municijom u realnim uslovima).The purpose of this dissertation is to investigate the effects of lamination and hybrid soft armor systems through ballistic impact. The investigation was carried out by using dynamic mechanical analysis and actual ballistic testing. The most important conclusions derived from this research are that lamination of the systems with very low resin content are superior to multiple non-laminated systems, and this advance could be improved further by hybrid systems using nanomodified fabric layers on the impact side and relatively tighter woven fabrics between the layers. This dissertation reports the preparation of SiOR2R and TiOR2R/poly (vinyl butyral) nanocomposites with enhanced dynamic mechanical properties. Silica and titania nanoparticles were introduced in the matrix as the neat powder and as colloidal sol using the melt mixing process. Composites reinforced with colloidal sol silica and titania showed higher mechanical properties than the ones reinforced with as-received particles. When sol TiOR2R particles are used, the highest increase of storage modulus of about 54% is obtained for 5 wt% loading, while for sol SiOR2R, the storage modulus increases with the addition of nanosilica with the largest increase of about 99% for 7 wt% loading. In addition, nanocomposites were introduced within Kevlar/PVB composites. The addition of 5 wt% silica and titania colloidal sol lead to the remarkable increase of the storage modulus for about 98 and 65%, respectively. Largest contribution of nanoreinforcements in lowering the glass transition temperature is observed for 7 wt% loading of TiOR2R and SiOR2R colloidal sol. This study reports the manufacture of new fabric forms from the preparation of hybrid laminated multi-axial composites with enhanced thermo-mechanical properties. Thermal and dynamic mechanical analysis of polymer matrix films and fabricated hybrid composites were employed in order to determine the optimal material composition and reinforcement content for composites with improved viscoelastic properties. The introduction of 5 wt. % silica nanoparticles in a composite of p-aramid– poly(vinyl butyral) led to significant improvements in the mechanical properties, and the addition of silane coupling agents yielded maximal values of the storage modulus for hybrid nanocomposites. The introduction of silane led to a better dispersion and deagglomeration of SiOR2R particles and to the formation of chemical bonds between organic and inorganic constituents, or p-aramid–poly(vinyl butyral) composites. In this way, the mobility of macromolecules was reduced, which can be seen from the decreasing value of damping factor for the p-aramid–poly(vinyl butyral) composite. Analysis of the glass transition temperature of the composite with amino-functionalized silica nanoparticles revealed improved thermal stability in addition to the aforementioned mechanical properties of the tested materials

Structure-Property Relationships in Sea Urchin Spines and Implications for Technical Materials

Sea urchin spines have been studied for numerous reasons including their crystallographic and chemical composition, their aesthetic appearance and their enigmatic growth at ambient conditions. Depending on the species, sea urchins use their spines for protection against predators, for burial in the substrate, for locomotion and for withstanding wave energy by wedging into reef cervices. Hence, sea urchin spines are in most cases optimized for bearing load. This study deals with the mechanical properties of the unique spines of Heterocentrotus mamillatus, a large Indo-Pacific Echinoid. They consist as all skeletal elements of Echinoids of Mg-calcite arranged in a porous meshwork (stereom) with very little organic material incorporated (<0.5 wt%). By the overall porosity of 0.6-0.7 their density is similar to sea water and the large and thick spines are not a burden to carry. These properties make the spines of H. mamillatus a promising biomimetic role model for high performance, intelligently structured, lightweight ceramics. Since biological role models are usually a lot smaller than the technical application they inspire, the question of how properties change with an increase in size, is intimately linked to biomimetic research. In contrast to man-made materials, biological materials gain much of their mechanical performance from the elaborate structuring on many hierarchical levels. Therefore, the relation between structure and property was analysed in depth before addressing the question of scaling. Mechanical properties were tested with uniaxial compression, 3-point bending and resonance frequency damping analysis. The structure was visualized by optical microscopy, secondary scanning microscopy and computer tomography. X-ray diffraction, infrared spectroscopy, thermogravimetry and dilatometry gave insight into the crystallography and chemical composition. For scaling analyses theories of Weibull and Bažant were applied. The spines generally derive their high strength, high stiffness and exceptional damage tolerance from their construction out of >107 struts/cm3. The µm sized struts can be bent elastically, demonstrating that they are practically free of surface flaws. The struts are separated by pores which restrict crack growth and keep damage localised. The porous meshwork is covered irregularly by dense layers, the “growth layers” marking earlier growth stages. They provide the spines with additional stiffness and strength. Spines with many growth layers have a significantly higher strength and stiffness. The strength of the spines seems not to decrease significantly with increasing size, contradicting scaling theories. To test this unexpected finding, compression tests on samples with and without growth layers were conducted. A novel micro-compression test, the pin indentation was also applied. Despite the uncertainties induced by natural heterogeneities, it seems that spines of H. mamillatus counteract the size effect by adding more and denser growth layers to larger (older) spines. By this they work against the decrease in strength with increasing size. This hypothesis was confirmed by segments lacking growth layers that show a size effect