We investigate global pattern-switching effects in 2D cellular solids in which the voids are arranged in a square lattice. Uniaxial compression of these structures triggers an elastic instability which brings about a period-doubling transformation of the void shapes at a critical strain. Specifically, a square array of circular voids forms a pattern of mutually orthogonal ellipses and a similar effect is observed for diamond-shaped voids. The onset of instability is governed by the void fraction and size-effects are found for the experimental samples. We establish empirical laws which characterise the stiffness, strength and stability of cellular structures comprising square arrays of circular voids. A comparison of these with predictions from a discrete model implies underestimation of the resistance of the lattice to buckling, although the size effects are replicated. We find similar pattern-switching effects in the cubic lattice, which is a three-dimensional porous cube. The effect of buckling in this system is to produce a 2D pattern in one plane of voids. In two-phase granular crystals, rearrangement of a square lattice of particles results in a new, period-doubled, structural pattern. This switch can occur via an intermediate phase depending on the size ratio of the particles as shown in experiments and numerical simulations.

Detonation waves are supersonic combustion waves that have a complex three-dimensional cellular structure. There is growing experimental evidence that the cellular structure of detonations promotes their propagation in the presence of losses. In spite of that, the conventional model for the detonation structure, known as the Zeldovich - Von Neumann - Doring (ZND) model, neglects the existence of cellular structure for detonations and assumes the wave to consist of a strong leading planar shock coupled with trailing chemical reactions. Therefore, the influence of cellular structure on the dynamics and extinction limits of detonation waves has been of particular interest. Previous studies have investigated the influence of cellular structure on the dynamics of detonations with mass divergence in the framework of narrow tubes, porous-walled tubes and weak confinement. However, precise quantification of the loss mechanism in these frameworks has been associated with some difficulties. Complex flow in the boundary layers, inherent in thin tubes, or attenuation of the transverse waves in the porous-walled tubes has made the evaluation of the loss mechanism more difficult in such geometries. In this thesis, a novel well-posed problem is formulated for detonations with mass divergence. It is shown that detonations propagating in a channel with a cross-section area increasing exponentially have a constant mass divergence. The detonations were found to propagate at a quasi-steady speed below the ideal Chapman-Jouguet velocity. This permitted to make meaningful comparison with the theoretical models and simulations. The experiments were performed in two mixtures, one displaying characteristic weakly unstable detonations (2C2H2 + 5O2 + 21Ar), and the other displaying highly unstable detonations (C3H8 + 5O2). The dependence of the velocity deficits and limits on the amount of mass divergence for the two mixtures were compared with the predictions of the quasi-one-dimensional ZND model with lateral mass divergence. Since the ZND model neglects the cellular structure of the detonations, such comparison permitted to asses the influence of cellular structure on the dynamics of detonations with mass divergence. Comparisons were also made with the results of simulations of inviscid cellular detonations. These comparisons showed that the velocity deficits and critical rate of mass divergence in the weakly unstable mixture were reasonably well predicted by the quasi-one-dimensional model. For smaller values of mass divergence rate, a good agreement between the experiments and the predictions of the two-dimensional cellular simulations was observed for the weakly unstable mixture. For the highly unstable detonations, the quasi-one-dimensional model significantly over-predicted the effect of mass divergence.Detonations were observed for rates of mass divergence 93% higher than the critical predicted value, displaying more substantial velocity deficits than predicted. Such observations show conclusively that the ZND model cannot capture the dynamics of highly unstable detonations on large scales.

Constructivist learning theory is based upon the tenets that students come to learning experiences with prior knowledge and experiences that the learner will choose from to make sense of the present situation. This leads to a mixture of understandings among students. This study proposed to reveal students‟ understanding of atomic structure and cell structure as well as the relationships between atoms and cells. High school students from one private school participated in a paper-and-pencil test to uncover conceptual understanding and content knowledge of atoms and cells. The 120 participants were from grades: 9 (13m, 15f), 10 (9m, 20f), 11 (21m, 17f), and 12 (17m, 8f). All 120 students took the paper-and-pencil test and 16 students (4 per grade) participated in a follow-up interview. Drawings were analyzed by individual characteristics then using groups of characteristics models classes were formed. Openended questions were scored holistically by rubric scores and then deconstructed into individual content statements. A limited number of findings follow. Students were more likely to draw a Bohr model. Freshmen were less likely to indicate living materials contained atoms and more likely to indicate forms of energy contained atoms. As students progressed through high school, details included in cells decreased. Students failed to recognize that the sum of the products from cell division will be larger than the original cell due to the two growth periods included in the division cycle. Students were often able to provide the correct yes or no answer to are atoms and cells similar, different, or related but the follow-up answers often included non-scientific conceptions. Recommendations include implementing instructional strategies that promote long-term retention of conceptual understanding and the underlying content knowledge. Design evaluation methods to monitor student understanding throughout a unit of study that go beyond traditional closed-ended questions. Many limitations related to this study suggest that results should not be generalized beyond the targeted population.

This thesis highlights a comprehensive research for the cell nucleation, growth and coarsening mechanisms during plastic foaming processes. Enforced environmental regulations have forced the plastic foam industry to adopt alternative blowing agents (e.g., carbon dioxide, nitrogen, argon and helium). Nevertheless, the low solubilities and high diffusivities of these viable alternatives have made the production of foamed plastics to be non-trivial. Since the controls of the cell nucleation, growth and coarsening phenomena, and ultimately the cellular morphology, involve delicate thermodynamic, kinetic, and rheological mechanisms, the production of plastics foams with customized cell morphology have been challenging. In light of this, the aforementioned phenomena were investigated through a series of theoretical studies, computer simulations, and experimental investigations. Firstly, the effects of processing conditions on the cell nucleation phenomena were studied through the in-situ visualization of various batch foaming experiments. Most importantly, these investigations have led to the identification of a new heterogeneous nucleation mechanism to explain the inorganic fillers-enhanced nucleation dynamics. Secondly, a simulation scheme to precisely simulate the bubble growth behaviors, a modified heterogeneous nucleation theory to estimate the cell nucleation rate, and an integrated model to simultaneously simulate cell nucleation and growth processes were developed. Consequently, through the simulations of the cell nucleation, growth, and coarsening dynamics, this research has advanced the understanding of the underlying sciences that govern these different physical phenomena during plastic foaming. Furthermore, the impacts of various commonly adopted approximations or assumptions were studied. The end results have provided useful guidelines to conduct computer simulation on plastic foaming processes. Finally, an experimental research on foaming with blowing agent blends served as a case example to demonstrate how the elucidation of the mechanisms of various foaming phenomena would aid in the development of novel processing strategies to enhance the control of cellular structures in plastic foams.

This thesis highlights a comprehensive research for the cell nucleation, growth and coarsening mechanisms during plastic foaming processes. Enforced environmental regulations have forced the plastic foam industry to adopt alternative blowing agents (e.g., carbon dioxide, nitrogen, argon and helium). Nevertheless, the low solubilities and high diffusivities of these viable alternatives have made the production of foamed plastics to be non-trivial. Since the controls of the cell nucleation, growth and coarsening phenomena, and ultimately the cellular morphology, involve delicate thermodynamic, kinetic, and rheological mechanisms, the production of plastics foams with customized cell morphology have been challenging. In light of this, the aforementioned phenomena were investigated through a series of theoretical studies, computer simulations, and experimental investigations. Firstly, the effects of processing conditions on the cell nucleation phenomena were studied through the in-situ visualization of various batch foaming experiments. Most importantly, these investigations have led to the identification of a new heterogeneous nucleation mechanism to explain the inorganic fillers-enhanced nucleation dynamics. Secondly, a simulation scheme to precisely simulate the bubble growth behaviors, a modified heterogeneous nucleation theory to estimate the cell nucleation rate, and an integrated model to simultaneously simulate cell nucleation and growth processes were developed. Consequently, through the simulations of the cell nucleation, growth, and coarsening dynamics, this research has advanced the understanding of the underlying sciences that govern these different physical phenomena during plastic foaming. Furthermore, the impacts of various commonly adopted approximations or assumptions were studied. The end results have provided useful guidelines to conduct computer simulation on plastic foaming processes. Finally, an experimental research on foaming with blowing agent blends served as a case example to demonstrate how the elucidation of the mechanisms of various foaming phenomena would aid in the development of novel processing strategies to enhance the control of cellular structures in plastic foams.

Customized cellular material is a relatively new area made possible by advancements in rapid manufacturing technologies. Rapid manufacturing is ideal for the production of customized cellular structure, especially on the meso scale, due to the size and complexity of the design. The means to produce this type of structure now exist, but the processes to design the structure are not well developed. The manual design of customized cellular material is not realistic due to the large number of features. Currently there are few tools available that aid in the design of this type of material. In this thesis, an automated tool to design customized cellular structure is presented.

No description available.

The advent of detonation-based propulsion systems represents an opportunity for more sustainable combustion processes and hypersonic travel. In regular detonations, some yet to be resolved instabilities are attributed to the propagation and collision of triple points, formed at the intersection of a Mach stem, an incident shock and a transverse wave. Over time, the tracks observed by these points form a structure made of diamond-shaped cells. Ultimately, The ability to sustain these instabilities plays a key role in the propagation of detonations. The present work unveils the dynamics of gaseous detonations at a sub-cellular level. The experiments are conducted with hydrogen fuel which is of great potential for detonation engine applications. The hydrogen-oxygen mixtures are held at stoichiometry and the nitrogen dilution in oxygen is varied from 30% to 70%. This allows to observe the effects of activation energy through the dilution on the sub-cellular wave dynamics. Measurements of cell sizes and wave velocity are reported through shadowgraph imaging. A new methodology is developed for the simultaneous resolution of the velocity field and cellular structure. The statistical analysis is made possible due to the design of a fully automated detonation facility. The experiments are conducted in a thin channel to minimize gradients in the third direction and confine the detonation cells to a plane. The results in cell sizes are in good agreement with the literature and expand the conditions reported thus far. Local observations of the velocity within the cells are used to explain the regularity of the overall wave speed, found to increase at lower dilution. Lastly, high fidelity simulations are conducted to model the cellular structure in hydrogen-air oblique detonation waves. Similarly to the experiments, the velocity field is extracted along the detonation cells and reveals the effects of wave curvature on triple point dynamics.

The overall goal of this work is to improve the structural performance of cellular structures in bending applications by incorporating tensegrity behavior using long continuous fibers. The designs are inspired by the hierarchical cellular structure composition present in pomelo fruit and the structural behavior of tensegrity structures. A design method for analyzing and predicting the behavior of the structures is presented. A novel manufacturing method is developed to produce the cellular structures with tensegrity behavior through the combination additive manufacturing and metal casting techniques. Tensegrity structures provide high stiffness to mass ratio with all the comprising elements experiencing either tension or compression. This research investigates the possibility of integrating tensegrity behavior with cellular structure mechanics and provides a design procedure in this process. The placement of fibers in an octet cellular structure was determined such that tensegrity behavior was achieved. Furthermore, using finite element analysis the bending performance was evaluated and the influence of fibers was measured using the models. The overall decrease in bending stress was 66.6 %. Extending this analysis, a design strategy was established to help designers in selecting fiber diameter based on the dimensions and material properties such that the deflection of the overall structure can be controlled. This research looks to Additive Manufacturing (AM) as a means to introduce tensegrity behavior in cellular structures. By combining Binder Jetting and metal casting a controlled reliable process is shown to produce aluminum octet-cellular structures with embedded fibers. 3D-printed sand molds embedded with long continuous fibers were used for metal casting. The fabricated structures were then subjected to 4 point bending tests to evaluate the effects of tensegrity behavior on the cellular mechanics. Through this fabrication and testing process, this work addresses the gap of evaluating the performance of tensegrity behavior. The overall strength increase by 30%. The simulation and experimental results were then compared to show the predictability of this process with errors of 2% for octet structures without fibers and 6% for octet structures with fibers. / Master of Science / Cellular materials are a class of lightweight structures composed by a network of cells comprising inter-connected struts, which help in reducing the material present in the structure. These structures provide high stiffness for low mass, better shock-absorption, thermal and acoustic insulation. Best known examples in nature include honeycomb, bamboo and cedar. There is a constant desire to improve strength of the cellular structures while wanting low mass. This research aims to provide a new approach towards the enhancing structural performance of cellular structures for bending applications through designs featuring long continuous fibers to impose tensegrity behavior. The designs in this research are inspired by the structural composition of pomelo fruit and tensegrity arrangements, where continuous long fibers are observed to enhance structural performance. Tensegrity structures are another class of lightweight structures composed of compressive bars and pre-stressed strings/fibers such that the structural elements undergo either tension or compression. The absence of bending stress makes these structures more efficient. A design method for analyzing and predicting the behavior of the structures is presented. To address the imposing manufacturing challenges, a novel manufacturing method is developed, producing cellular structures with tensegrity behavior through the combination of Binder Jetting and metal casting techniques. Binder Jetting is an additive manufacturing process, which selectively binds sand, layer by layer to create molds of desired designs and metal can be cast into the printed molds to realize parts. The bending performance was evaluated and the influence of fibers was measured using the models. The overall decrease in bending stress was 66.6 %. The fabricated structures were then subjected to 4 point bending tests. The overall strength increased by 30%. The simulation and experimental results were then compared to show the predictability of this process with errors of 2% for octet structures without fibers and 6% for octet structures with fibers. This research takes another step towards creating efficient lightweight structures and adds to the efforts taken to build multifunctional hierarchical cellular materials, which can provide better performance while saving material. Potential applications of these structures include earthquake resistant wall panels, aircraft fuselage/interior supports, automotive chassis structure, beams for supporting roof loads, armor panels in battle tanks, ship building and packaging (electromechanical systems).

Selective Laser Melting (SLM), a laser-based Additive Manufacturing technique has appealed to the bio-medical, automotive, and aerospace industries due to its ability to fabricate geometrically complex parts with tailored properties and high-precision end-use products. The SLM processing parameters highly influence the part quality, microstructure, and mechanical properties. The process-structure-property relationship of the SLM process is not well-understood. In the process-structure study, a quasi-2D model of Micro-Selective Laser Melting process using molecular dynamics is developed to investigate the localized melting and solidification of a randomly-distributed Aluminum nano-powder bed. The rapid solidification in the meltpool reveals the cooling rate dependent homogeneous nucleation of equiaxed grains at the center of the meltpool. Long columnar grains that spread across three layers, equiaxed grains, nano-pores, twin boundaries, and stacking faults are observed in the final solidified nanostructure obtained after ten passes of the laser beam on three layers of Aluminum nano-powder particles. In the structure-property study, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Nickel concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. Additionally, the subgranular cell size has an inverse relationship with mechanical strength, and the nano-twinned cells exhibit higher strength in comparison with twin-free cells. / Master of Science / Additive Manufacturing's (AM) rise as a modern manufacturing paradigm has led to the proliferation in the number of materials that can be processed, reduction in the cost and time of manufacturing, and realization of complicated part geometries that were beyond the capabilities of conventional manufacturing. Selective Laser Melting (SLM) is a laser-based AM technique which can produce metallic parts from the fusion of a powder-bed. The SLM processing parameters greatly influence the part's quality, microstructure, and properties. The process-structure-property relationship of the SLM process is not well-understood. In-situ experimental investigation of the physical phenomena taking place during the SLM process is limited because of the very small length and time scales. Computational methods are cost-effective alternatives to the challenging experimental techniques. But, the continuum-based computational models are ineffective in modeling some of the important physical processes such as melting, nucleation and growth of grains during solidification, and the deformation mechanisms at the atomistic scale. Atomistic simulation is a powerful method that can offset the limitations of the continuum models in elucidating the underlying physics of the SLM process. In this work, the influence of the SLM process parameters on the microstructure of the Aluminum nano-powder particles undergoing μ-SLM processing and the mechanical deformation characteristics of the unique cellular structures observed in the SLM-fabricated 316L stainless steel are studied using molecular dynamics simulations. Ten passes of the laser beam on three layers of Aluminum nano-powder particles have unfolded the formation mechanisms of a complex microstructure associated with the SLM process. The study on the deformation mechanisms of 316L stainless steel has revealed the contribution of the cellular structures to its superior mechanical properties.