Generally, data is available abundantly in unlabeled form, and its annotation requires some cost. The labeling, as well as learning cost, can be minimized by learning with the minimum labeled data ...instances. Active learning (AL), learns from a few labeled data instances with the additional facility of querying the labels of instances from an expert annotator or oracle. The active learner uses an instance selection strategy for selecting those critical query instances, which reduce the generalization error as fast as possible. This process results in a refined training dataset, which helps in minimizing the overall cost. The key to the success of AL is query strategies that select the candidate query instances and help the learner in learning a valid hypothesis. This survey reviews AL query strategies for classification, regression, and clustering under the pool-based AL scenario. The query strategies under classification are further divided into: informative-based, representative-based, informative- and representative-based, and others. Also, more advanced query strategies based on reinforcement learning and deep learning, along with query strategies under the realistic environment setting, are presented. After a rigorous mathematical analysis of AL strategies, this work presents a comparative analysis of these strategies. Finally, implementation guide, applications, and challenges of AL are discussed.
In this article, three switched-capacitor-based Z-source equivalent dc-dc boost converter (SCZEBC) topologies are proposed. The basis for these topological evolutions is the shifted L-C-D cell of the ...fourth-order boost converter on the front-end side, whereas a charge-pump cell on the upstream side. This hybrid combination results in quasi-Z-source equivalent dc-dc boost conversion along with enhanced voltage boosting capability. The operating duty ratio of the proposed topologies is less than 0.5 like in the conventional Z-source dc-dc converter (ZSC) topologies. In comparison to the existing Z-source topologies, remarkable features of the proposed SCZEBC topologies are: low source current ripple, common ground, and low voltage stress on capacitors and active devices. A detailed steady-state analysis is presented to identify the salient features of the proposed topologies and is thereafter compared with the existing ZSCs. Small-signal analysis is established and a voltage-mode controller is designed. In the controller design stage, a modulus margin 0.5-0.75 is adopted to ensure tradeoff between robustness and performance of the proposed converter topologies. A 48 to 220 V, 100-W prototype is built to demonstrate the effectiveness of the SCZEBC. The steady-state and closed-loop response measurements validate the theoretical studies.
Background: The heart is the central organ of the circulatory system, which maintains the flow of blood along with the transport of nutrients to different cells and tissues. A well-functioning ...cardiac state is a complicated mode of changeability. A healthy heart is not only about oscillation, as the rhythmometer is not the same in every circumstance. Heart rate shows variations so that it can be regulated according to psychophysiological conditions to maintain the effect of the internal-external stimulus.
Objective: The main objective of this review is to provide a piece of all-inclusive information about heart rate variability (HRV) and different variables affecting HRV. The direct interconnection so that HRV can be used in clinical practices.
Methods: This review article contains a detailed survey of literature about HRV available in different online sources such as; Google Scholar, Science Direct, PubMed, and Web of Science, etc. In this review, the authors have focused on the role of the autonomic nervous system in the regulation of HRV and the role of various factors affecting HRV.
Results: The variation in the time between two heartbeats is termed as HRV. It is one of the indicators of many pathological conditions related to cardiovascular health. It provided reliable information about the interaction of the sympathetic and parasympathetic nervous systems. The analysis of the variation of heart rate is a well-known non-invasive technique to identify the functioning of the autonomic nervous system. The autonomic nervous system (ANS) depends on the sympathetic and parasympathetic nervous system for transferring information. The cardio-accelerating center, lungs, and non-striated muscles are innervated by cardiac sympathetic nerves. This division of ANS latches upon the heart accordingly via the cervicothoracic ganglion and vagus nerve. It is found that cardiac normal variability depends upon this stimulation towards the sinoatrial node (pacemaker), which can be evaluated by analyzing the HRV. In human-based studies, it has been found that a low level of HRV is one of the main causes of death rate among adults. Hence, HRV helps in identifying the risk of cardiac diseases and the state of ANS.
Conclusion: The heart plays a vital role in the human body and the well-functioning of the cardiac system is the need for a healthy life. The heart contains its nervous system termed as neurocardio system in which ANS plays a key role in which the sympathetic and parasympathetic systems interplay to regulate HRV. High HRV is associated with healthy condition, while low HRV is associated with pathological conditions. The HRV is influenced by various variables such as; pathological, physiological, psychological, environmental factors, lifestyle factors, and genetic factors, etc.
Local chemical ordering (LCO) in the CrCoNi medium-entropy alloy was investigated by transmission electron microscopy (TEM) after different annealing treatments and their corresponding mechanical ...properties by bulk tensile tests and nanoindentation. A cold-rolled alloy was annealed at 1000°C for 0.5 h followed by ice water quenching and then aged at a number of different temperatures (600°C, 700°C, 800°C, 900°C, and 1000°C) under vacuum for 240 h to generate different degrees of chemical ordering. A splat-quenched sample rapidly cooled from the liquid phase was also examined. While bulk mechanical properties did not vary among samples with equivalent grain sizes, nanoindentation tests revealed notable differences. As indicated by the load at first pop-in using a Berkovich tip or the indentation yield strength via continuous stiffness measurements using a 10 μm spherical tip, the nanoindentation tests revealed that the stress for onset of plasticity during indentation varied with heat treatment and peaked in the 900°C aged sample. Energy-filtered TEM characterization indicated the presence of ordering in all specimens, with a higher degree of LCO in the aged samples relative to the splat-quenched and 1000°C-quenched samples. The evolution of LCO during aging was determined to occur on the time scale similar to those of bulk diffusion. The difference in nanoindentation strength was attributed to the difference in dislocation nucleation barriers imposed by different degrees of LCO.
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•Microfissures in directed energy deposited PH15-5 stainless steel are visually elusive and poses quality control issues.•The highly aligned microfissures introduces significant ...anisotropy in fatigue crack growth and tensile behavior.•Remarkably, the material demonstrates a high fracture initiation toughness, irrespective of the crack propagation direction.
The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.
Strengthening materials via conventional "top-down" processes generally involves restricting dislocation movement by precipitation or grain refinement, which invariably restricts the movement of ...dislocations away from, or towards, a crack tip, thereby severely compromising their fracture resistance. In the present study, a high-entropy alloy Al
CrCoFeNi is produced by the laser powder-bed fusion process, a "bottom-up" additive manufacturing process similar to how nature builds structures, with the microstructure resembling a nano-bridged honeycomb structure consisting of a face-centered cubic (fcc) matrix and an interwoven hexagonal net of an ordered body-centered cubic B2 phase. While the B2 phase, combined with high-dislocation density and solid-solution strengthening, provides strength to the material, the nano-bridges of dislocations connecting the fcc cells, i.e., the channels between the B2 phase on the cell boundaries, provide highways for dislocation movement away from the crack tip. Consequently, the nature-inspired microstructure imparts the material with an excellent combination of strength and toughness.
The fatigue crack growth (FCG) behavior and fatigue strength of 304L stainless steel (SS) manufactured by the laser powder bed fusion (LB-PBF) process were investigated. Effect of build orientation, ...microstructure, and temperature--considering that the alloy undergoes temperature-dependent stress-induced martensitic transformation (SIMT)--were determined. FCG rates were found to be broadly independent of the build orientation and microstructure, although it is reduced in shorter builds due to the presence of higher compressive residual stress in them. Microstructural investigations of the fatigue crack reveal that SIMT occurs at the crack tip of alloys tested at room temperature, whereas the same was negligible at 150 °C. SIMT induces dilatation and shear, which enhances crack closure and retards FCG rates at RT compared to that at 150 °C. Using the microstructural observations of the transformed zone, an estimate of the crack closure due to SIMT is provided. Finally, failure envelopes or the Kitagawa-Takahashi diagram was prepared for different temperatures to facilitate a damage tolerant design approach.
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The paper presents a new methodology for hydrodynamic-based flood forecast that focuses on scenario generation and database queries to select appropriate flood inundation maps in real-time. In ...operational flood forecasting, only discharges are forecasted at specific gauges using hydrological models. Hydrodynamic models, which are required to produce inundation maps, are computationally expensive, hence not feasible for real-time inundation forecasting. In this study, we have used a substantial number of pre-calculated inundation maps that are stored in a database and a methodology to extract the most likely maps in real-time. The method uses real-time discharge forecast at upstream gauge as an input and compares it with the pre-recorded scenarios. The results show satisfactory agreements between offline inundation maps that are retrieved from a pre-recorded database and online maps, which are hindcasted using historical events. Furthermore, this allows an efficient early warning system, thanks to the fast run-time of the proposed offline selection of inundation maps. The framework is validated in the city of Kulmbach in Germany.
Uniaxial tensile response and J integral–resistance behavior of 304L austenitic stainless steel manufactured by laser powder bed fusion process (LPBF) were investigated. The steel undergoes ...transformation induced plasticity (TRIP) at room temperature, resulting in high fracture toughness (JQ) values combined with high strength (σY) and ductility. Upon increasing the temperature to 75°C, TRIP becomes inactive, and the deformation mechanism is dominated by dislocation glide and twinning. This leads to a substantial reduction in JQ and marked anisotropy, while σY remains unchanged. Significance of these results in terms of fracture properties of steels manufactured via LPBF processes is discussed.
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