Monolithic Objects Research Articles

Different build strategies and computer-aided process planning for fabricating a functional component through hybrid-friction stir additive manufacturing

ABSTRACT Hybrid Additive Manufacturing (HAM) technologies leverage the strength of Additive Manufacturing (AM) and Subtractive Manufacturing (SM) in fabricating complex geometries with precision. Friction Stir Welding (FSW) has been acknowledged for joining sheets of similar or dissimilar materials with improved mechanical/metallurgical properties. However, due to the inherent pinhole defect and complicated toolpath planning, this process has not been explored much for fabricating the objects in a layer-by-layer manner. Therefore, this work investigates Computer Aided Process Planning (CAPP) for realizing fully functional metallic parts through a HAM system. This HAM system uses a Sheet Lamination (SL) process, i.e. Friction Stir Additive Manufacturing (FSAM), synergistically coupled with an SM process, i.e. machining. To realize complicated geometries through the proposed HAM system, three different build strategies are developed viz. Near-net block fabrication, Near-net shape fabrication via ‘form-then-bond approach’ and ‘bond-then-form approach.’ Each build strategy consists of a detailed CAPP that includes toolpath planning, tooling aspects, simulated outcome and so on. Further, these build strategies are discussed for realizing the parts with Functionally Graded Materials (FGM) & embedded structures. Finally, a monolithic object of aluminium has been fabricated as a case study to demonstrate one of the aforementioned build strategies.

The Large Superfast Rotators Discovered by the Zwicky Transient Facility

Two high-cadence surveys aiming for rotation period measurements of asteroids have been conducted in 2019 January and October using the Zwicky Transient Facility. From the surveys, 25 large superfast rotators (SFRs) were discovered and they are all main-belt asteroids (MBAs), except for one Mars crosser. These large SFRs have a diameter ranging from 0.43 to 7.87 km and a rotation period between 0.48 and 1.95 hr. Considering their diameters and fast rotations, they cannot be explained by rubble-pile structure unless using extraordinary high bulk densities. Cohesion, if available, can conserve these large SFRs. The estimated cohesion for these large SFRs could be up to thousands of pascals, much higher than the currently estimated cohesion for asteroids and that generated by the regolith of Moon and Mars. Such high-level cohesion can be produced from fine-grain regolith, like clay. However, the availability of such fine-grain regolith for asteroids is still unknown. Although the possibility of these large SFRs being large monolithic objects cannot be ruled out, this scenario is very unlikely given that the timescale of disruptive impact for MBAs in a similar diameter range is 107–108 yr.

Electrohydraulic installation for destruction of boulders and monolithic structures during construction works

This article discusses alternative types of object’s destruction. One of the perspective ways to destroy oversized material is electrohydraulic impact in a water-filled hole. The article’s purpose is to determine plant capacity. The tasks of the work include: substantiation of the mechanism of monolithic object’s destruction by electrohydraulic shock; identification of the main patterns of oversized objectэs destruction; calculation of energy costs associated with the destruction of the object. The article may have practical interest to firms engaged in clearing the territory of stone boulders, or scientific personnel engaged in research in this area. Keywords: electrohydraulic effect, over-sized boulder’s destruction, alternative of drilling-and-blasting method, energy costs calculating

Multi-material direct ink writing of photocurable elastomeric foams

Developments in additive manufacturing have enabled the fabrication of soft machines that can safely interface with humans, creating new applications in soft robotics, wearable technologies, and haptics. However, designing custom inks for the 3D printing of soft materials with Young’s modulus less than 100 kPa remains a challenge due to highly coupled structure-property-process relationship in polymers. Here, we show a three-stage material chemistry process based on interpenetrating silicone double networks and ammonium bicarbonate particles that decouples the transient behavior during processing from the final properties of the material. Evaporation of ammonium bicarbonate particles at the final stage creates gaseous voids to produce foams with a low effective Young’s modulus in the 25 kPa −90 kPa range. Our photoirradiation-assisted direct ink writing system demonstrates the ability to maintain high resolution while enabling controlled loading of ammonium bicarbonate particles. The resultant multi-material possesses programmed porosity and related properties such as density, stiffness, Shore hardness, and ultimate strength in a monolithic object. Our multi-hardness synthetic hand and self-righting buoyant structure highlight these capabilities.

Low thermal conductivity of the superfast rotator (499998) 2011 PT

Context.Asteroids with a diameter of up to a few dozen meters may spin very fast and complete an entire rotation within a few minutes. These small and fast-rotating bodies are thought to be monolithic objects because the gravitational force due to their small size is not strong enough to counteract the strong centripetal force caused by the fast rotation. This argument means that the rubble-pile structure is not feasible for these objects. Additionally, it is not clear whether the fast spin prevents dust and small particles (regolith) from being kept on their surface.Aims.We develop a model for constraining the thermal conductivity of the surface of the small, fast-rotating near-Earth asteroids. This model may suggest whether regolith is likely present on these objects.Methods.Our approach is based on the comparison of the measured Yarkovsky drift and a predicted value using a theoretical model that depends on the orbital, physical and thermal parameters of the object. The necessary parameters are either deduced from statistical distribution derived for near-Earth asteroids population or determined from observations with associated uncertainty. With this information, we performed Monte Carlo simulations and produced a probability density distribution for the thermal conductivity.Results.Applying our model to the superfast rotator asteroid (499998) 2011 PT, we find that the measured Yarkovsky drift can only be achieved when the thermal conductivityKof the surface is low. The resulting probability density function for the conductivity is bimodal, with two most likely values being around 0.0001 and 0.005 W m−1K−1. Based on this, we find that the probability thatKis lower than 0.1 W m−1K−1is at least 95%. This low thermal conductivity might indicate that the surface of 2011 PT is covered with a thermal insulating layer, composed of a regolith-like material similar to lunar dust.

The 3dp – three-dimensional printing – is it a real prospect of automating the building processes

The construction industry is particularly well-suited to take the advantages of the 3DP technology however it still seems to be a distant perspective. The method itself, allows to manufacture prefabricated elements, small landscape facilities and detached houses, pedestrian bridges (of relatively little span lengths) etc. On that field, the considered technology is already prospering. Furthermore, the technology is a great solution for scale models used by architects. The 3-Dimensional print turned out to be a quick and an almost unbeatable way of presenting a concept to the potential investors. The visualization of the project became easier, giving an exact mapping of the surroundings and the structure itself, which helps to eliminate the risk of possible architectural or constructional mistakes. The technology facilitates the implementation of any needed amendments, whereas the scale models became one of the verification ways of the project’s documentation and the building’s proportions. However, more research and law regulations are needed to state the final applicability and limitations of the presented technology. Ensuring the mechanical properties along with the quality control work together to provide us with safe elements for the use of large-scale engineering. The article presents the application of 3D printing technology in construction and bridge engineering. Available technologies and methods of printing objects were presented. Examples of structures made of various materials are presented: concrete, steel and synthetics. The features of an emerging 3-dimensional monolithic objects are conditioned upon the used software advancement, printing technology, the printer used and the material applied. In the mentioned construction industry, where the labour workload becomes an increasingly significant component in the final cost of the construction project, the 3DP could revolutionise the whole process of the execution methodology. The „complexity and universality” of a facility and its execution method is being understood as an increased number of openings, irregular and complex shapes, the precision of the formed elements, fragments or segments and efficiency of the technology used. (Review article)

The COVID-19 Pandemic, Part 1: Can Antimicrobial Copper-Based Alloys Help Suppress Infectious Transmission of Viruses Originating from Human Contact with High-Touch Surfaces?

The COVID-19 Pandemic, Part 1: Can Antimicrobial Copper-Based Alloys Help Suppress Infectious Transmission of Viruses Originating from Human Contact with High-Touch Surfaces?

Hybrid layered manufacturing of a bimetallic injection mold of P20 tool steel and mild steel with conformal cooling channels

Hybrid layered manufacturing (HLM) is one of the most efficient rapid tooling processes that uses a metal inert gas, tungsten inert gas or laser cladding processes for producing near-net shape and machining operations for finishing on the same platform. Materials saving, elimination of rough machining, reduction of programming time, ability to make conformal cooling channels (CCCs), and bimetallic objects are some of the significant advantages of HLM in tooling applications. CCCs, as against the straight cooling channels, reduce the cycle time and distortion and improve the matrix integrity. Bimetallic objects, as compared to monolithic objects, reduce the overall cost by replacing the interior with cheaper material. In this work, the HLM has been investigated for realizing bimetallic structures of P20 tool steel and mild steel. A complex bimetallic plastic injection mold with hard casing (P20 tool steel) and soft core (mild steel) along with CCCs have been built as a case study. The mechanical and microstructural characterization leads to the fine-tuning of the process parameters. The plastic injection mold with CCCs is designed through heat flow analysis using a commercial software.

Dyson spheres

I review the origins and development of the idea of Dyson spheres, their purpose, their engineering, and their detectability. I explicate the ways in which the popular imagining of them as monolithic objects would make them dynamically unstable under gravity and radiation pressure, and mechanically unstable to buckling. I develop a model for the radiative coupling between a star and large amounts of material orbiting it, and connect the observational features of a star plus Dyson sphere system to the gross radiative properties of the sphere itself. I discuss the still-unexplored problem of the effects of radiative feedback on the central star's structure and luminosity. Finally, I discuss the optimal sizes of Dyson spheres under various assumptions about their purpose as sources of low-entropy emission, dissipative work, or computation.

Fully 3D-Printed, Monolithic, Mini Magnetic Actuators for Low-Cost, Compact Systems

We report the design, fabrication, and experimental characterization of the first fully 3D-printed, multi-material miniature magnetic actuators for compact systems in the literature. The actuator design integrates a bonded hard magnet made of NdFeB microparticles embedded in a Nylon 12 matrix (55% by volume) with structural and support elements made of pure Nylon 12. The device is a 10 mm-diameter, 1.2 mm wall-thick, and 9 mm tall cylindrical frame that mounts on an off-the-shelf solenoid and a 10 mm diameter, 100 $\mu \text{m}$ -thick, leak-tight membrane connected at its center to a 4 mm diameter, 4.95 mm tall hard magnet. The actuators are monolithically printed in layers as thin as $100~\mu \text{m}$ using $600~\mu \text{m}$ -wide strokes via fused filament fabrication (FFF) –a low-cost 3D printing technology capable of processing high-performance thermoplastics to create monolithic objects made of a plurality of distinctive feedstock. The average surface roughness, Young’s modulus, and hardness of the FFF-printable hard-magnetic filament were estimated at $58.55~\mu \text{m}$ , 3.59 GPa, and Shore D 71.5, respectively, while the average surface roughness, Young’s modulus, and yield strength of FFF-printed magnetic material were estimated at $5.79~\mu \text{m}$ , 2.02 GPa, and 55.99 MPa ± 4.59 MPa, respectively. Magnetic characterization of the FFF-printed NdFeB-embedded Nylon 12 feedstock demonstrates the fabrication of isotropic hard magnets with an intrinsic coercivity of ~700 kA/m, remanence of ~395 mT, and a maximum energy product of 27 kJ/ $\text{m}^{3}$ . Simulations of the stray magnetic field produced by a printed sample made of NdFeB-embedded Nylon 12 were validated using a scanning Hall probe. The vertical displacement of a miniature 3D-printed magnetic actuator was characterized with a solenoid for various coil bias voltages; a maximum displacement equal to 50 $\mu \text{m}$ was obtained with 3.1 V DC applied to the driving coil. Finite element simulations of the actuator design estimate at 2.38 MPa the maximum stress on the membrane at $50~\mu \text{m}$ actuation (i.e., below the fatigue limit of Nylon 12), and at 592.61 Hz the natural frequency of the device, which was corroborated via experiment. [2018-0288]

Nanothermite with Meringue-like Morphology: From Loose Powder to Ultra-porous Objects.

The goal of the protocol described in this article is to prepare aluminothermic compositions (nanothermites) in the form of porous, monolithic objects. Nanothermites are combustible materials made up of inorganic fuel and an oxidizer. In nanothermite foams, aluminum is the fuel and aluminum phosphate and tungsten trioxide are the oxidizing moieties. The highest flame propagation velocities (FPVs) in nanothermites are observed in loose powders and FPVs are strongly decreased by pelletizing nanothermite powders. From a physical standpoint, nanothermite loose powders are metastable systems. Their properties can be altered by unintentional compaction induced by shocks or vibrations or by the segregation of particles over time by settling phenomena, which originates from the density differences of their components. Moving from a powder to an object is the challenge that must be overcome to integrate nanothermites in pyrotechnic systems. Nanothermite objects must have both a high open porosity and good mechanical strength. Nanothermite foams meet both of these criteria, and they are prepared by dispersing a nano-sized aluminothermic mixture (Al/WO3) in orthophosphoric acid. The reaction of aluminum with the acid solution gives the AlPO4 "cement" in which Al and WO3 nanoparticles are embedded. In nanothermite foams, aluminum phosphate plays the dual role of binder and oxidizer. This method can be used with tungsten trioxide, which is not altered by the preparation process. It could probably be extended to some oxides, which are commonly used for the preparation of high performance nanothermites. The WO3-based nanothermite foams described in this article are particularly insensitive to impact and friction, which makes them far safer to handle than loose Al/WO3 powder. The fast combustion of these materials has interesting applications in pyrotechnic igniters. Their use in detonators as primers would require the incorporation of a secondary explosive in their composition.

On Safety Assessment and Base Isolation of Heavy Non-structural Monolithic Objects

Under seismic actions heavy non-structural objects, which are usually placed at the top of existing constructions, may constitute a danger to human lives and a considerable loss for world heritage. In this contribution, safety assessment of non-structural monolithic objects is discussed through the illustration of a case study, which concerns seismic protection of eleven ancient marble decorative pinnacles placed at the top of a three-arched masonry city gate in Ferrara (ITALY). A method for assessing the safety of the underlying masonry structure under the action of seismic excitations is outlined and the amplification of the ground motions due to the presence of such structure is evaluated.

Base isolation of heavy non-structural monolithic objects at the top of a masonry monumental construction

The present paper deals with the relevant topic of seismic protection of heavy non-structural monolithic objects, which are usually placed at the top of masonry monumental constructions for mainly decorative purposes, like pinnacles and heavy artwork. Even if, after seismic events, most of the losses are due to structural collapse of buildings and other structural systems, heavy non-structural objects of the kind considered in the present work represent a serious potential hazard for both human lives and cultural heritage. During earthquakes, such objects undergo large base accelerations, which may eventually cause their collapse by rocking and overturning. In the present contribution, the seismic protection of eleven ancient marble decorative pinnacles placed at the top of the three-arched masonry city gate in Ferrara (Italy) is illustrated as a case study. In particular, a method for assessing the safety level of these systems under the action of seismic excitations is outlined and base isolation is proposed as a very promising technique for the seismic retrofit of heavy non-structural monolithic objects. The dynamical response to seismic actions of the underlying masonry construction is assessed through time-history dynamic analyses and the amplification of the ground accelerations at the base of the pinnacles is evaluated. Furthermore, the pinnacles are modeled as rigid bodies and their rocking behavior under base excitations is discussed. Finally, the effectiveness of the proposed base isolation system is assessed through non-linear dynamic numerical simulations.

Introduction: En-gendering Islamic Authority in West Africa

Islamic Africa, vol. 5, no. 2, 2014. issn 2154-0993. Copyright © 2014 by Northwestern University Press. All rights reserved. www.islamicafricajournal.org Women and gender have very often been marginal or absent in the academic literature on Islam in West Africa. Where the literature has not itself marginalized or completely ignored women, it has often defined women as essentially marginal in relation to Islam. Contributors to this special issue of Islamic Africa seek not merely to bring more attention to Muslim women in West Africa but to examine the mutually constitutive relationships between Islamic authority and gendered discourses and practices. This introduction begins by reviewing the emergence of literature on women and gender in Muslim West Africa, pointing out some of the assumptions that have often limited this literature as well as recent attempts to remedy these assumptions. These include the assumption that women are categorically defined as marginal in relation to Islam; that men are to women as the public is to the private; and that “women” and “Islam” can be approached as monolithic objects of analysis. Then we outline several themes that we consider key to an engendered study of Islamic authority in West Africa. First, we propose performativity as an organizing concept for understanding how authority is exercised, recognized, reconfigured, and challenged in relation to gender and Islam. For instance, we ask how women present new leadership roles as “felicitous” in relation to gendered norms of piety. Second, we suggest reexamining Islamic knowledge, a key component of performances of Islamic authority, moving beyond binaries such as “scriptural” and “oral” knowledge and examining forms and practices of Islamic knowledge that Introduction: Engendering Islamic Authority in West Africa

Call for Papers—Special Issue on Routine Dynamics: Exploring Sources of Stability and Change in Organizations Submission Deadline: September 1, 2013

<b>Call for Papers</b>—Special Issue on Routine Dynamics: Exploring Sources of Stability and Change in Organizations <b>Submission Deadline: September 1, 2013</b>

Imaging Fractional Incompressible Stripes in Integer Quantum Hall Systems

Transport experiments provide conflicting evidence on the possible existence of fractional order within integer quantum Hall systems. In fact, integer edge states sometimes behave as monolithic objects with no inner structure, while other experiments clearly highlight the role of fractional substructures. Recently developed low-temperature scanning probe techniques offer today an opportunity for a deeper-than-ever investigation of spatial features of such edge systems. Here we use scanning-gate microscopy and demonstrate that fractional features were unambiguously observed in every integer quantum Hall constriction studied. We present also an experimental estimate of the width of the fractional incompressible stripes corresponding to filling factors 1/3, 2/5, 3/5, and 2/3. Our results compare well with predictions of the edge-reconstruction theory.

".. . the binding altered not only my feet but my whole character": Footbinding and First-World Feminism in Chinese American Literature

Footbinding and First-World Feminism in Chinese American Literature This article traces the trope of footbinding in Chinese American literature and argues that it primarily appears framed within a first-world feminist perspective that largely condemns it as oppressive, exotic, patriarchal, and/or sexualized. While not condoning this cruel and inhumane practice, this article suggests that a first-world critique of footbinding does an injustice to Chinese women by making them appear to be homogenous and monolithic objects who, through powerlessness and oppression, perpetrated this custom on themselves for a thousand years. The author calls for an activist Asian American literature that gives a better understanding of why reasonable women would have capitulated to such maiming, one that thoroughly illustrates the complex relations of dignity and power symbolized by such a practice. Interdisciplinary cultural studies approach used, including literary criticism, post-colonial theory, psychoanalysis, and historical analysis.

Karin cluster formation by asteroid impact

Insights into collisional physics may be obtained by studying the asteroid belt, where large-scale collisions produced groups of asteroid fragments with similar orbits and spectra known as the asteroid families. Here we describe our initial study of the Karin cluster, a small asteroid family that formed 5.8 ± 0.2 Myr ago in the outer main belt. The Karin cluster is an ideal ‘natural laboratory’ for testing the codes used to simulate large-scale collisions because the observed fragments produced by the 5.8-Ma collision suffered apparently only limited dynamical and collisional erosion. To date, we have performed more than 100 hydrocode simulations of impacts with non-rotating monolithic parent bodies. We found good fits to the size–frequency distribution of the observed fragments in the Karin cluster and to the ejection speeds inferred from their orbits. These results suggest that the Karin cluster was formed by a disruption of an ≈33-km-diameter asteroid, which represents a much larger parent body mass than previously estimated. The mass ratio between the parent body and the largest surviving fragment, (832) Karin, is ≈0.15–0.2, corresponding to a highly catastrophic event. Most of the parent body material was ejected as fragments ranging in size from yet-to-be-discovered sub-km members of the Karin cluster to dust grains. The impactor was ≈ 5.8 km across. We found that the ejections speeds of smaller fragments produced by the collision were larger than those of the larger fragments. The mean ejection speeds of >3-km-diameter fragments were ≈ 10 m s −1 . The model and observed ejection velocity fields have different morphologies perhaps pointing to a problem with our modeling and/or assumptions. We estimate that ∼ 5 % of the large asteroid fragments created by the collision should have satellites detectable by direct imaging (separations larger than 0.1 arcsec). We also predict a large number of ejecta binary systems with tight orbits. These binaries, located in the outer main belt, could potentially be detected by lightcurve observations. Hydrocode modeling provides important constraints on the interior structure of asteroids. Our current work suggests that the parent asteroid of the Karin cluster may have been an unfractured (or perhaps only lightly fractured) monolithic object. Simulations of impacts into fractured/rubble pile targets were so far unable to produce the observed large gap between the first and second largest fragment in the Karin cluster, and the steep slope at small sizes (≈6.3 differential index). On the other hand, the parent asteroid of the Karin cluster was produced by an earlier disruptive collision that created the much larger, Koronis family some 2 – 3 Gyr ago. Standard interpretation of hydrocode modeling then suggests that the parent asteroid of the Karin cluster should have been formed as a rubble pile from Koronis family debris. We discuss several solutions to this apparent paradox.

Alignment of Liquid-Crystalline Polymers by Field-Oriented, Carbon Nanotube Directors

Bulk liquid-crystalline polymer (LCP) material can be macroscopically aligned by seeding the growth of LCP domains with preoriented, single-wall carbon nanotubes (SWNTs). SWNT seeds dispersed in a columnar polyoxazoline melt were first oriented by applying an ac electric field across the molten material. Then, cooling the material below the liquid-crystalline transition temperature yielded domains that were oriented in the direction of the applied field. The orientation of these domains was characterized by temperature-controlled polarized light microscopy, static birefringence, and wide-angle X-ray scattering. These experiments demonstrate that domains can be oriented at field strengths that are orders of magnitude lower than those used to pole anisotropic polymers alone, and that the seeded domains can grow together to form homogeneously oriented bulk material. Because the nanotubes act only as nucleants, very little SWNT material is required. We anticipate that this general concept could aid the fabrication of monolithic objects with tailored anisotropic properties from SWNT−polymer composites.

Interior structure and surface properties of NEOs: What is known and what should be understood to mitigate potential impacts

Indirect information about interior structure and surface properties of near-Earth asteroids and cometary nuclei is provided by observations, together with numerical and laboratory simulations. Such studies already indicate a huge diversity amongst the whole population, in bulk density (from 0.2 to 3 g cm −3), in macro-porosity (tentatively from 18% to 60%), and thus in structure (small monolithic objects, bigger fractured objects or gravitational aggregates, as well as porous and nevertheless robust objects formed of cohesive material). The surface physical properties of asteroids are consistent with the existence of regolithic layers, built up of irregular particles with a size greater than the wavelength, and corresponding to a porosity higher than that of the whole object. A precise knowledge of the interior structure and of the regolith properties – of major importance to assess the consequences of an impact and develop efficient mitigation strategies – can only be provided by in situ rendezvous missions, aimed at significantly different objects, representative of different categories of physical properties.