Laser net shape manufacturing (LNSM) is a laser cladding/deposition based technology, which can fabricate and repair near-net-shape high-performance components directly from metal powders. ...Characterizing mechanical properties of the laser net shape manufactured components is prerequisite to the applications of LNSM in aircraft engine industrial productions. Nickel-based superalloys such as INCONEL 718 are the most commonly used metal materials in aircraft engine high-performance components. In this study, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct. It is found that the use of plasma rotating electrode processed (PREP) powder and a high energy input level greater than 80 J/mm are necessary conditions to minimize the porosity. Material microstructure and tensile properties of laser-deposited INCONEL 718 are studied and compared under heat treatment conditions of as deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. Tensile test results showed that the direct age heat treatment produces the highest tensile strength equivalent to the wrought material, which is followed by the STA-treated and the homogenization-treated tensile strengths, while the ductility exhibits the reverse trend. Finally, failure modes of the tensile specimens were analyzed with fractography.
Covalently cross-linked network polymers are widely used in engineering applications due to the robust and irreversible nature of the covalent bonds. Recently proposed polyimine-based malleable ...covalent network polymers (MCNPs) show water or temperature induced shape reforming under low temperatures, which enables the green and energy neutral reprocessing and reshaping of the thermoset. The fundamental mechanisms for water/temperature triggered reforming are the water and temperature initiated bond exchange reaction (BER), which releases the stress in the macromolecular chains. In addition, the water-induced BER process is assisted by the configuration entropy change under a water diffusion-induced shift in the glass transition temperature. To set a theoretical foundation for the MCNPs-based structure design and engineering applications, a chemo-thermo-mechanically coupled 3D viscoelastic model is developed under a consistent thermodynamic framework. A field model of mass transportation is developed to couple the chemical potential, temperature, stress, and relaxation time in a polymer system. The material malleability is attributed to two main factors, i.e., water/temperature triggered BERs and the water diffusion induced “plasticization effects”, where BERs-caused stress release is characterized by a phase evolution model, while "plasticization effects" induced material relaxation is captured by the depression of relaxation time which depends on the inverse of the entropy increment. After calibration by a series of experiments, the model is applied to stress relaxation and isothermal uniaxial stretch analysis of MCNPs under humidity condition. Good agreement between experiment and simulation demonstrates the ability of the proposed model in capturing the coupled thermo-chemo-mechanical behaviors of the water/temperature malleable polymer. Moreover, experiments and simulations show the solvent-driven softening mechanism provides an alternative way to recover the fixed temporary configuration. However, for the current BERs featured MCNPs, the water triggered constantly network topology exchange release the stress, not allowing the sample to return to its original shape. This work provides understandings and design guidelines for the hydrothermal-activated malleable covalent network polymer.
Inspired by diverse shape-shifting phenomena in nature, various man-made shape programmable materials have been developed for applications in actuators, deployable devices, and soft robots. However, ...fabricating mechanically robust shape-morphing structures with on-demand, rapid shape-transformation capability, and high load-bearing capacity is still a great challenge. Herein, we report a mechanically robust and rapid shape-shifting material system enabled by the volatilization of a non-fully-reacted, volatile component in a partially cured cross-linking network obtained from photopolymerization. Volume shrinkage induced by the loss of the volatile component is exploited to drive complex shape transformations. After shape transformation, the residual monomers, cross-linkers, and photoinitiators that cannot volatilize still exist in the network, which is ready for a further photopolymerization to significantly stiffen the initial material. Guided by analytic models and finite element analysis, we experimentally demonstrate that a variety of shape transformations can be achieved, including both 2D-to-3D and 3D-to-3D′ transformations, such as a buckyball self-folding from a 2D hexagonal lattice sheet and multiple pop-up structures transforming from their initial compact configurations. Moreover, we show that an ultra-low-weight 3D Miura-ori structure transformed from a 2D sheet can hold more than 1600 times its weight after stiffness improvement via postcuring. This work provides a versatile and low-cost method to fabricate rapid and robust shape-morphing structures for potential applications in soft robots, deployable antennas, and optical devices.
Support for an extremist entity such as Islamic State (ISIS) somehow manages to survive globally online despite considerable external pressure and may ultimately inspire acts by individuals having no ...history of extremism, membership in a terrorist faction, or direct links to leadership. Examining longitudinal records of online activity, we uncovered an ecology evolving on a daily time scale that drives online support, and we provide a mathematical theory that describes it. The ecology features self-organized aggregates (ad hoc groups formed via linkage to a Facebook page or analog) that proliferate preceding the onset of recent real-world campaigns and adopt novel adaptive mechanisms to enhance their survival. One of the predictions is that development of large, potentially potent pro-ISIS aggregates can be thwarted by targeting smaller ones.
A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a ...two-stage thermal-photo curing reaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.
Background
Although stratifying individuals with respect to nasopharyngeal carcinoma (NPC) risk with Epstein‐Barr virus–based markers is possible, the performance of diagnostic methods for detecting ...lesions among screen‐positive individuals is poorly understood.
Methods
The authors prospectively evaluated 882 participants aged 30 to 70 years who were enrolled between October 2014 and November 2018 in an ongoing, population‐based NPC screening program and had an elevated NPC risk. Participants were offered endoscopy and magnetic resonance imaging (MRI), and lesions were identified either by biopsy at a follow‐up endoscopy or further contact and linkage to the local cancer registry through December 31, 2019. The diagnostic performance characteristics of endoscopy and MRI for NPC detection were investigated.
Results
Eighteen of 28 identified NPC cases were detected by both methods, 1 was detected by endoscopy alone, and 9 were detected by MRI alone. MRI had significantly higher sensitivity than endoscopy for NPC detection overall (96.4% vs 67.9%; Pdifference = .021) and for early‐stage NPC (95.2% vs 57.1%; P = .021). The sensitivity of endoscopy was suggestively lower among participants who had previously been screened in comparison with those undergoing an initial screening (50.0% vs 81.2%; P = .11). The authors observed a higher overall referral rate by MRI versus endoscopy (17.3% vs 9.1%; P < .001). Cases missed by endoscopy had early‐stage disease and were more commonly observed for tumors originating from the pharyngeal recess.
Conclusions
MRI was more sensitive than endoscopy for NPC detection in the context of population screening but required the referral of a higher proportion of screen‐positive individuals. The sensitivity of endoscopy was particularly low for individuals who had previously been screened.
This prospective study of people at high risk of nasopharyngeal carcinoma (NPC) shows that magnetic resonance imaging (MRI) screening is significantly more sensitive than endoscopy for detecting NPC and early‐stage NPC. There is also a higher overall rate of referral by MRI versus endoscopy, and this leads to a slightly lower positive predictive value for MRI versus endoscopy.
Digital light processing (DLP) is a widely used additive manufacturing technique for functional applications due to its high accuracy and print speeds. However, a variety of factors such as pixel ...size, motion stage resolution, optical focus, and chemical properties of the resin limit DLP's minimum resolution. Recently, research into locally varying light intensities has led to the emergence of grayscale DLP printing, which offers new capabilities including sub‐pixel manipulation of the printed shape. Here, a methodology is developed to enhance accuracy beyond what is typically capable for a given projector resolution by using pixel‐level grayscale control to create round features from sharp pixels. A numerical representation of the DLP pixel shape is developed to account for the effects of the incident light patterns. A reaction‐diffusion model is then used to predict the printed shapes before and after grayscale enhancement. This model is used to determine the optimal pixel intensities to match a target shape. Finally, the minimum feature size allowed by the proposed method is explored. The promising results represent an important step forward in raising DLP printing to higher accuracy, which will allow the fabrication of functional and structural components with smaller features or smoother faces.
Digital light processing is a widely used additive manufacturing technique due to its high accuracy and print speed. However, a variety of factors including pixel size limit the maximum accuracy of the technique. Here, a methodology is presented to enhance accuracy beyond what is typically capable for a given projector resolution by using pixel‐level grayscale control to create round features from sharp pixels.
Liquid crystal elastomers (LCE) are appealing candidates among active materials for 4D printing, due to their reversible, programmable and rapid actuation capabilities. Recent progress has been made ...on direct ink writing (DIW) or Digital Light Processing (DLP) to print LCEs with certain actuation. However, it remains a challenge to achieve complicated structures, such as spatial lattices with large actuation, due to the limitation of printing LCEs on the build platform or the previous layer. Herein, a novel method to 4D print freestanding LCEs on‐the‐fly by using laser‐assisted DIW with an actuation strain up to −40% is proposed. This process is further hybridized with the DLP method for optional structural or removable supports to create active 3D architectures in a one‐step additive process. Various objects, including hybrid active lattices, active tensegrity, an actuator with tunable stability, and 3D spatial LCE lattices, can be additively fabricated. The combination of DIW‐printed functionally freestanding LCEs with the DLP‐printed supporting structures thus provides new design freedom and fabrication capability for applications including soft robotics, smart structures, active metamaterials, and smart wearable devices.
A novel hybrid 4D printing method, which integrates laser‐assisted direct ink writing and Digital Light Processing, is developed to fabricate freestanding liquid crystal elastomers (LCEs) “on‐the‐fly” with optional structural or removable materials. Various architectures, including hybrid active lattices, active tensegrities, loading‐bearing actuators, and spatial LCE lattices, can be successfully fabricated.
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