In this paper, the microcrack evolution in DP590 dual-phase steel was observed by in-situ straining in straining transmission electron microscopy. It was found that a void initiated ahead of a main crack. After the load was applied, a thinned area was nucleated ahead of the void tip, and it grew gradually into shallow nanovoid, penetrating nanovoid, and then new void. Meanwhile, the old void connected with the main crack. The repetitions of the procedures resulted in the continuous propagation of a crack. Interaction between microcracks was observed. The propagation direction of one microcrack may change, affected by other microcracks. Voids were observed between microcracks and have a significant effect on the coalescence of microcracks.
Tapered nanopillars with various cross sections, including cone-shaped, stepwise, and pencil-like structures (300 nm in diameter at the base of the pillars and 1.1 μm in height), are prepared from epoxy resin templated by nanoporous anodic aluminum oxide (AAO) membranes. The effect of pillar geometry on the shear adhesion behavior of these nanopillar arrays is investigated via sliding experiments in a nanoindentation system. In a previous study of arrays with the same geometry, it was shown that cone-shaped nanopillars exhibit the highest adhesion under normal loading while stepwise and pencil-like nanopillars exhibit lower normal adhesion strength due to significant deformation of the pillars that occurs with increasing indentation depth. Contrary to the previous studies, here, we show that pencil-like nanopillars exhibit the highest shear adhesion strength at all indentation depths among three types of nanopillar arrays and that the shear adhesion increases with greater indentation depth due to the higher bending stiffness and closer packing of the pencil-like nanopillar array. Finite element simulations are used to elucidate the deformation of the pillars during the sliding experiments and agree with the nanoindentation-based sliding measurements. The experiments and finite element simulations together demonstrate that the shape of the nanopillars plays a key role in shear adhesion and that the mechanism is quite different from that of adhesion under normal loading.
Abstract Little experimentally explored and understood are the complex dynamics of microstructure formation by ice‐templating when aqueous solutions or slurries are directionally solidified (freeze cast) into cellular solids. With synchrotron‐based, time‐resolved X‐ray tomoscopy it is possible to study in situ under well‐defined conditions the anisotropic, partially faceted growth of ice crystals in aqueous systems. Obtaining one full tomogram per second for ≈270 s with a spatial resolution of 6 µm, it is possible to capture with minimal X‐ray absorption, the freezing front in a 3% weight/volume (w/v) sucrose‐in‐water solution, which typically progresses at 5–30 µm s −1 for applied cooling rates of = 1–10 °C min −1 . These time and length scales render X‐ray tomoscopy ideally suited to quantify in 3D ice crystal growth and templating phenomena that determine the performance‐defining hierarchical architecture of freeze‐cast materials: a complex pore morphology and “ridges”, “jellyfish cap”, and “tentacle”‐like secondary features, which decorate the cell walls.
Presented in this article are systematic microstructural and mechanical property data for anisotropic collagen scaffolds made by freeze casting. Three applied cooling rates (10 °C/min, 1 °C/min, 0.1 °C/min) and two freezing directions (longitudinal and radial) were used during scaffold manufacture. Utilizing a semi-automated image analysis technique applied to confocal micrographs of fully hydrated scaffolds, pore area, long and short pore axes, and pore aspect ratio were determined. Compression testing was performed to determine scaffold modulus, yield strength, and toughness.
Aiming at the serious harmonic pollution and the low power factor in the distribution network of industrial enterprises, this paper develops an integrated method for harmonic suppression and reactive power compensation suitable for the distribution network of industrial enterprises. The integrated method realizes the dual functions of harmonic filtering and reactive power compensation, and filters out the harmonic current to get a symmetrical current waveform while ensuring safe operation of the power compensator. In addition, it solves the problems of high harmonic content, small power factor in the distribution network, and device burnout caused by direct input of reactive power compensator. The main contributions of this paper are as follows: (1) According to the demand for the integration of harmonic suppression and reactive power compensation, the steps of integrated method for harmonic suppression and reactive power compensationare proposed, and then the methods for harmonic filtering and reactive power compensation are investigated; (2) a method for designing the capacity of a filter capacitor and the rated parameter of an electromagnetic coupling reactance converter is proposed, and an optimization simulation system is constructed to design the parameters of the filter; (3) a simulation system is developed, followed by parameter design and simulation analysis of harmonic filtering subsystem (HFSS), reactive power compensation subsystem (RPCSS) and the integrated system of harmonic suppression and reactive power compensation. Simulation results verify that the HFSS is put into operation first and then switched off later to ensure the normal operation of other equipment in the distribution network. After the treatment, the power factor, harmonic current content and total distortion rate all meet the national standards. The integrated method can dynamically track harmonics and reactive power changes, filter out harmonics, improve power factor and the symmetry level of the power source, and ensure the normal operation of other equipment in the distribution network. The research results lay a certain theoretical and technical foundation for the harmonic filtering and reactive power compensation theory, technology and its device innovation to achieve effective suppression of power harmonics and reactive power compensation.
Abstract Despite extensive research, the manufacture in the bulk of high‐performance flake‐based magnetic composites with a highly aligned, nacre‐like structure remains challenging. Many challenges can be overcome by freeze casting in an externally applied, uniform magnetic field, which causes both the flakes and the composite walls of the cellular solid to align parallel to the B‐field lines. When appropriately sized, the flakes experience a second alignment parallel to the freezing direction because of a shear flow that occurs due to both the volumetric expansion of the ice phase and mold contraction during the directional solidification. The resulting orthotropic structure of the freeze‐cast magnetic composite is reflected in orthotropic mechanical and magnetic properties of the material. The magnetic composites manufactured by magnetic‐field assisted freeze casting outperform by a factor of 2–4 in terms of stiffness, strength, and toughness materials that are processed in the absence of a magnetic field and do not exhibit a monodomain architecture. Because of the highly aligned microstructure, it is possible to compact the initially lamellar composite with 90% porosity to at least 80% strain. The results presented in this study illustrate the tremendous potential for magnetic freeze casting of magnetic composites for use in power conversion.
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