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    01 February 2018, Volume 23 Issue 1 Previous Issue    Next Issue

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    Photoacoustic Imaging by Use of Micro-Electro-Mechanical System Scanner
    CHEN Sung-Liang (陈松良)
    2018, 23 (1):  1-10.  doi: 10.1007/s12204-018-1902-4
    Abstract ( 370 )  
    Photoacoustic imaging acquires the absorption contrast of biological tissue with ultrasound resolution. It has been broadly investigated in biomedicine for animal and clinical studies. Recently, a micro-electromechanical system (MEMS) scanner has been utilized in photoacoustic imaging systems to enhance their performance and extend the realm of applications. The review provides a recap of recent developments in photoacoustic imaging using MEMS scanner, from instrumentation to applications. The topics include the design of MEMS scanner, its use in photoacoustic microscopy, miniature imaging probes, development of dual-modality systems, as well as cutting-edge bio-imaging studies.
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    Full Duplex Communications for Next Generation Cellular Networks: The Capacity Gain
    TANG Aimin (唐爱民), HUANG Huaiyu (黄怀玉), WANG Xudong* (王旭东)
    2018, 23 (1):  11-19.  doi: 10.1007/s12204-018-1903-3
    Abstract ( 498 )  
    Full duplex communication highly improves spectrum efficiency of a wireless communication link. However, when it is applied to a cellular network, the capacity gain from this technology remains unknown. The reason is that full duplex communication changes the aggregate interference experienced by each communication link in cellular networks. In this paper, the capacity gain from full duplex communication is studied for cellular networks of 4G and beyond, where the same frequency channel is adopted in each cell. A two-layer Poisson point process (PPP) is adopted to model the network topology, and stochastic geometry is employed to derive the coverage probability and the average capacity of typical link in a cellular network. On the basis of these derived parameters, the capacity gain from full duplex communication is determined. Numerical results reveal that without mutual interference cancellation (MIC), the capacity gain is small under various power levels; with perfect MIC at base stations, the capacity gain can exceed 60%; with imperfect MIC at base stations, the capacity gain decreases quickly even with a slight drop of MIC performance.
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    Link Budget Analysis for Massive-Antenna-Array-Enabled Terahertz Satellite Communications
    ZHEN Ruchen (甄儒辰), HAN Chong* (韩充)
    2018, 23 (1):  20-27.  doi: 10.1007/s12204-018-1904-2
    Abstract ( 373 )  
    Broadband satellite communications can enable a plethora of applications in customer services, global nomadic coverage and disaster prediction and recovery. Terahertz (THz) band is envisioned as a key satellite communication technology due to its very broad bandwidth, astrophysical observation advantages and device maturing in recent years. In this paper, a massive-antenna-array-enabled THz satellite communication system is proposed to be established in Tanggula, Tibet, where the average altitude is 5.068 km and the mean-clear-sky precipitable water vapor (PWV) is as low as 1.31 mm. In particular, a link budget analysis (LBA) framework is developed for THz space communications, considering unique THz channel properties and massive antenna array techniques. Moreover, practical siting conditions are taken into account, including the altitude, PWV, THz spectral windows, rain and cloud factors. On the basis of the developed link budget model, the massive antenna array model, and the practical parameters in Tanggula, the performances of signal-to-noise ratio (SNR) and capacity are evaluated. The results illustrate that 1Tbit/s is attainable in the 0.275—0.37 THz spectral window in Tanggula, by using an antenna array of the size 64.
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    The Acquisition of Sand Vibration Information in Hinterland of Desert Based on Advanced Remote Sensing System and Network Technologies
    MA Xina (马鑫), DENG Shungea (邓顺戈), LI Xinwana,b,c* (李新碗)
    2018, 23 (1):  28-32.  doi: 10.1007/s12204-018-1905-1
    Abstract ( 335 )  
    The deep understanding on sand and sand dunes scale can be useful to reveal the formation mechanism of the sandstorm for early sandstorm forecast. The current sandstorm observation methods are mainly based on conventional meteorological station and satellites remote sensing, which are difficult to acquire sand scale information. A wireless sensing network is implemented in the hinterland of desert, which includes ad hoc network, sensor, global positioning system (GPS) and system integration technology. The wireless network is a three-layer architecture and daisy chain topology network, which consists of control station, master robots and slave robots. Every three robots including one master robot and its two slave robots forms an ad hoc network. Master robots directly communicate with radio base station. Information will be sent to remote information center. Data sensing system including different kinds of sensors and desert robots is developed. A desert robot is designed and implemented as unmanned probing movable nodes and sensors’ carrier. A new optical fiber sensor is exploited to measure vibration of sand in particular. The whole system, which is delivered to the testing field in hinterland of desert (25 km far from base station), has been proved efficient for data acquisition.
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    Fuel Property Effects on Liquid and Vapor Penetrations of Evaporating Sprays
    ZHAO Bolin (赵博林), CHEN Chien-Pin* (陈谦斌)
    2018, 23 (1):  33-37.  doi: 10.1007/s12204-018-1906-0
    Abstract ( 504 )  
    The purpose of this study is to investigate the effect of fuel properties on liquid and vapor penetrations in evaporating spray systems. A recently developed model, which can simultaneously account for the finite thermal conductivity, finite mass diffusivity and turbulence effects within atomizing multi-component liquid fuel sprays, is utilized for the numerical predictions. Two different multi-component fuels with different boiling temperatures, densities and other thermal properties are implemented in the KIVA-3V computational fluid dynamics (CFD) code to study the evaporation behaviors. A six-component surrogate fuel is used to emulate the relevant volatility property of the real diesel fuel, and a second bi-component fuel is chosen to represent a low boiling-temperature fuel. The numerical results are compared with the experimental data, and the representative results are obtained. For a lower density and lower boiling temperature fuel, the liquid penetration length is shorter. However, the vapor penetration lengths are not affected by the fuel type in terms of fuel volatility. Available experimental data are used for validation and appraisal of the multi-component evaporation model.
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    Thermal Transport in Nanoporous Yttria-Stabilized Zirconia by Molecular Dynamics Simulation
    ZHAO Shuaishuaia (赵帅帅), SHAO Chenga (邵成), ZAHIRI Saeida,ZHAO Changyingb (赵长颖), BAO Huaa* (鲍华)
    2018, 23 (1):  38-44.  doi: 10.1007/s12204-018-1907-z
    Abstract ( 406 )  
    Yttria-stabilized zirconia (YSZ) is widely used as thermal barrier coatings (TBCs) to reduce heat transfer between hot gases and metallic components in gas-turbine engines. Porous structure can generally reduce the lattice thermal conductivity of bulk material, so porous YSZ can be potentially used as TBCs with better thermal performance. In this work, we investigate the thermal conductivity of nanoporous YSZ using the nonequilibrium molecular dynamics (NEMD) simulation, and comprehensively discuss the effects of cross-sectional area, pore size, structure length, porosity, Y2O3 concentration and temperature on the thermal conductivity. To compare with the results of the NEMD simulation, we solve the heat diffusion equation and the gray Boltzmann transport equation (BTE) to calculate the thermal conductivity of the same porous structure. From the results, we find that the thermal conductivity of YSZ has a weak dependence on the structure length at the length range from 10 to 26 nm, which indicates that the majority of heat carriers have very short mean free path (MFP) but there exists small percentage (about 3%) of phonons with longer MFP (larger than 10 nm) contributing to the thermal conductivity. The thermal conductivity predicted by NEMD simulation is smaller than that of solving heat diffusion equation (diffusive limit) with the same porous structure. It shows that the presence of pores affects phonon scattering and further affects the thermal conductivity of nanoporous YSZ. The results agree well with the solution of gray BTE with a average MFP of 0.6 nm. The thermal conductivity of nanoporous YSZ weakly depends on the Y2O3 concentration and temperature, which shows the phonons with very short MFP play the major contribution to the thermal conductivity. The results help to better understand the heat transfer in porous YSZ structure and develop better TBCs.
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    Enhancing the Efficiencies of Organic Photovoltaic and Organic Light-Emitting Diode Devices by Regular Nano-Wrinkle Patterns
    JI Qinyu1 (冀秦豫), ZHANG Cheng2 (张诚), QI Xiangning1 (祁湘宁), LI Ran1 (李冉),HU Xiao1 (胡晓), GUO L. Jay2 (郭凌杰), YANG Tian1* (杨天)
    2018, 23 (1):  45-51.  doi: 10.1007/s12204-018-1908-y
    Abstract ( 460 )  
    Organic photovoltaic (OPV) cells and organic light-emitting diodes (OLEDs) are energy harvesting and generation devices that have attracted great attention these years because of their low costs, thin film structures, flexibility and environment-friendly manufacturing processes. For such thin film devices, photon management methods that increase the light absorption of OPV cells and the light extraction from OLEDs are highly desirable. Here, we report the experimental efforts to fabricate geometrically tunable and spontaneously formed nano-wrinkle structures with large areas, and the theoretical results on photon management with the nano-wrinkles.
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    Optimal Threshold Policies for Robust Data Center Control
    WENG Paul1,2*, QIU Zeqi3 (邱泽麒), COSTANZO John3, YIN Xiaoqi3 (阴小骐), SINOPOLI Bruno3
    2018, 23 (1):  52-60.  doi: 10.1007/s12204-018-1909-x
    Abstract ( 362 )  
    With the simultaneous rise of energy costs and demand for cloud computing, efficient control of data centers becomes crucial. In the data center control problem, one needs to plan at every time step how many servers to switch on or off in order to meet stochastic job arrivals while trying to minimize electricity consumption. This problem becomes particularly challenging when servers can be of various types and jobs from different classes can only be served by certain types of server, as it is often the case in real data centers. We model this problem as a robust Markov decision process (i.e., the transition function is not assumed to be known precisely). We give sufficient conditions (which seem to be reasonable and satisfied in practice) guaranteeing that an optimal threshold policy exists. This property can then be exploited in the design of an efficient solving method, which we provide. Finally, we present some experimental results demonstrating the practicability of our approach and compare with a previous related approach based on model predictive control.
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    The Analysis of Angle Resolution of Stress Vector Sensor Based on Optical Fiber Sensing Cable for High Speed Railway Traffic
    DENG Shungea (邓顺戈), MA Xina (马鑫), LI Xinwana,b,c* (李新碗)
    2018, 23 (1):  61-65.  doi: 10.1007/s12204-018-1910-4
    Abstract ( 365 )  
    With the development of high speed railway traffic, the structure health monitoring for high-speed rail is necessary due to the safety issue. Optical fiber sensing technology is one of the options to solve it. Stress vector information is the important index to make more reasonable judgments about railway safety. However, information sensed by lots of commercial optical sensors is scalar. According to the stress filed distribution of rail, this paper proposes a new type of stress vector sensor based on optical fiber sensing cable (OFSC) with a symmetrical seven optical fibers structure and analyzes the relations between angle resolution and distance between adjacent of optical fibers through finite-element software (ANSYS) simulation. Through reasonable distance configuration, the angle resolution of the OFSC can be improved, and thus stress vector information, including the stress magnitude and the angle of stress, can be more accurately obtained. The simulation results are helpful to configure OFSC for angle resolution improvement in actual practice, and increase the safety factor in high speed railway structure health monitoring.
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    Detecting Premature Ventricular Contraction in Children with Deep Learning
    LIU Yixiua (刘宜修), HUANG Yujuanb (黄玉娟), WANG Jianyib (王健怡),LIU Lib (刘莉), LUO Jiajiaa* (罗家佳)
    2018, 23 (1):  66-73.  doi: 10.1007/s12204-018-1911-3
    Abstract ( 471 )  
    Premature ventricular contractions (PVCs) are abnormal heart beats that indicate potential heart diseases. Diagnosis of PVCs is made by physicians examining long recordings of electrocardiogram (ECG), which is onerous and time-consuming. In this study, deep learning was applied to develop models that can detect PVCs in children automatically. This computer-aided diagnosis model achieved high accuracy while sustained stable performance. It could save time and repeated efforts for physicians, enabling them to focus on more complicated tasks.This study is a first step toward children’s PVC auto-detection in clinics. Further study will improve the model’s performance with optimized structure and more data in different sources, while facing the challenges of the variety and uncertainty of children’s ECG with heart diseases.
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    Decision Model for Market of Performing Arts with Factorization Machine
    XU Yong1 (徐勇), TANG Qian2 (唐倩), HOU Linzao2 (候林早), LI Mian2* (李冕)
    2018, 23 (1):  74-84.  doi: 10.1007/s12204-018-1912-2
    Abstract ( 305 )  
    Performing arts and movies have become commercial products with high profit and great market potential. Previous research works have developed comprehensive models to forecast the demand for movies. However, they did not pay enough attention to the decision support for performing arts which is a special category unlike movies. For performing arts with high-dimensional categorical attributes and limit samples, determining ticket prices in different levels is still a challenge job faced by the producers and distributors. In terms of these difficulties, factorization machine (FM), which can handle huge sparse categorical attributes, is used in this work first. Adaptive stochastic gradient descent (ASGD) and Markov chain Monte Carlo (MCMC) are both explored to estimate the model parameters of FM. FM with ASGD (FM-ASGD) and FM with MCMC (FM-MCMC) both can achieve a better prediction accuracy, compared with a traditional algorithm. In addition, the multi-output model is proposed to determine the price in multiple price levels simultaneously, which avoids the trouble of the models’ repeating training. The results also confirm the prediction accuracy of the multi-output model, compared with those from the general single-output model.
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    Novel Electrically Stimulated Catalytic Converter Prototype for Replacement of Conventional Auto Exhaust Emission Converters
    HAMADE Thomas A.
    2018, 23 (1):  85-96.  doi: 10.1007/s12204-018-1913-1
    Abstract ( 313 )  
    High voltage electrostatics and corona discharge are utilized for various applications in pollution and environmental control. The traditional applications have many flaws due to improper construction of electrode design and assembly that cause system failure, in particular when electrically stimulated devices are exposed to high humidity. A new innovative-patented design by Hamade, electrically stimulated catalytic converter (ESCC), eliminates such flaws and shows the wide practical applications of the new design. The new design utilized previous patented designs and work of the same inventor but retrofitted for catalytic auto exhaust emission control. The current and previous patents include: employing electrically stimulated filtration (ESF) to replace high efficiency particulate air (HEPA) filters, treatment of biological and infectious diseases, electret fabrication, and, most notably, the invention of a new electrically stimulated catalytic converter (ESCC). The electrically stimulated catalytic converter invention includes an exhaust conduit fed from the engine exhaust port with a housed corona charger apparatus. The opposite end is opened to the atmosphere outside of the vehicle or connected to a reduced-size catalytic converter. The corona charger is intrusively or non-intrusively associated with a main flow path defined by the exhaust conduit. The corona charger includes at least one electrode, which may be recessed away from, the main flow path. A plurality of corona chargers may be used in various combinations, optimally a two dimensional grid. The electrically stimulated catalytic converter is adapted to treat and eliminate auto exhaust pollution emission to air.
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    Understanding Spray Coating Process: Visual Observation of Impingement of Multiple Droplets on a Substrate
    HUANG Jianchi (黄渐持), YUAN Zhihao (袁志豪), GAO Siyi (高思易),LIAO Jianshan (廖健杉), ESLAMIAN Morteza*
    2018, 23 (1):  97-105.  doi: 10.1007/s12204-018-1914-0
    Abstract ( 421 )  
    Spray coating is a facile deposition process with numerous existing and emerging applications. However, spray coating is a stochastic process comprising impingement of many droplets which upon impact on a heated substrate may dry or solidify individually or coalesce first to form a thin liquid film and then dry to yield a thin solid film. There is very limited knowledge on how this process occurs; therefore in this work, high speed imaging is used to visualize the spray coating process. Two model solutions including food-dye with properties like those of water, and poly (3, 4-ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT:PSS), a polymeric solution, are sprayed onto permeable glossy paper and regular impermeable glass substrates. Substrates are kept at room temperature and 80 ?C elevated temperature. In some cases, a vertical ultrasonic vibration is imposed on the substrate to study its effect on the coating process. It is observed that the spray coating process is highly random and stochastic. A higher substrate temperature results in a better coating process in that a more uniform and defect-free coating forms. Imposed vibration in the case of glossy paper substrates results in better droplet spreading and more uniform coating. The results also show that under the conditions of these experiments, impinged droplets dry individually or as islands of multiple coalesced droplets to form a coating. In other words, at used spray flow rate and spray droplet size, a continuous thin liquid film does not form prior to drying even at room temperature. Further systematic studies and high magnification lenses are required to visualize and understand the details of the process.
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    A Solution Processable Flexible Transparent Conductive Graphene/PEDOT : PSS Film Fabricated by Spin and Blade Coating
    FANG Xiaozhu1 (房霄竹), FAN Zhijun1 (范志君), GU Yichen1 (顾宜宸),SHI Jiawen1 (史佳雯), CHEN Min2 (陈敏), CHEN Xuewen2 (陈学文),QIU Shaohua2 (邱韶华), ZABIHI Fatemeh3, ESLAMIAN Morteza1,4, CHEN Qianli1,4* (陈倩栎)
    2018, 23 (1):  106-111.  doi: 10.1007/s12204-018-1915-z
    Abstract ( 436 )  
    Flexible transparent conductive films were made on PET substrates by spin and blade coating, using graphene sheets dispersed in PEDOT: PSS solution. Ultrasonic substrate vibration was used to improve microstructure and properties of the films. Comparing to the pristine PEDOT: PSS film, the sheet resistance is 3 to 4 orders of magnitude lower with the addition of graphene. The conductivity and reproducibility of the film are improved for two-layer films comparing to one-layer films, with a reduction in transparency. Films prepared with substrate vibration showed lower sheet resistance for one-layer films, as the size of dewetting areas is reduced. In addition, large-area flexible films with desirable conductivity and transmittance were successfully fabricated by blade coating, which is promising, as the process is low-cost, scalable and compatible with roll-to-roll manufacturing.
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    Novel Application of High Voltage Electrostatics Corona Ions Discharge Related to Treatment, Sanitization and Disinfection of Biological Matter Such HIV-AIDS Infected Blood
    HAMADE Thomas A.
    2018, 23 (1):  112-121.  doi: 10.1007/s12204-018-1916-y
    Abstract ( 324 )  
    A novel high-voltage electrostatics corona ions pre-charger apparatus and methods were invented earlier by Hamade related to treat various types of receptors such as but not limited to electret polymer, air filters, particulates, catalytic converters, bioaerosols, fluids, pollutants, virus, and bacteria. It is shown in this article that his work led to the construction of various prototype chargers, customized differently for each type of a receptor. In particular his recent development of biological matter corona charger (BMCC) prototype related to expose, treat, sanitize, and disinfect bioaerosols, virus, bacteria, and contaminated fluids and blood such as human immunodeficiency virus (HIV) - acquired immune deficiency syndrome (AIDS). It is shown in this paper that each previous investigated research contemplated ionized corona charger attendant to a charging process and the corona, imparts and provides enough treatment charges to receptors including the aforementioned receptors. Researchers often relied on adopting prior corona charger methods that do not necessarily and effectively solve the problems associated with them or utilize them for optimum treatment effect. The inventor exhaustively studied the characteristics of corona discharge, and has found that the greatest difficulty in corona discharge has to do with the maintenance of the corona, particularly when the receptor is being charged. This is due to variations in either the dielectric value between the corona electrode and a grounded base or flaws in the design as the receptor passes there between suppressing or hindering corona and its effectiveness. What is needed in-the-art is an apparatus and method to achieve maximum possible charge on a receptor, a charge order of magnitude greater than that used by other investigators. This often requires customizing each apparatus and method and does not just merely use one type of a charger to satisfy all applications. To satisfy this need, we build a low cost prototype BMCC that generates self-sustaining charge corona, eliminates many previous design flaws such as spark over, and make it ready for testing remotely or with apparatus.
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    Estimation of Load-Induced Damage and Repair Cost in Post-Tensioned Concrete Rocking Walls
    JAFARI Abouzar, DUGNANI Roberto*
    2018, 23 (1):  122-131.  doi: 10.1007/s12204-018-1917-x
    Abstract ( 390 )  
    Post-tensioned concrete rocking walls might be used to avoid severe seismic damage at the base of structural walls, decrease residual drift, and lessen post-earthquake repair costs. The prediction of load-induced damage to the rocking wall resulting from seismic loading can provide an extremely valuable tool to evaluate the status and safety of structural concrete walls following earthquakes. In this study, the behavior and the damage state of monolithic, self-centering, rocking walls, as a new type of concrete rocking wall, are investigated. The nonlinear mechanical behavior of the wall is first modeled numerically, and subsequently the mechanical parameters from the numerical simulation are used to generate the local damage index. The results from the damage index model are compared with the full-scale test results, confirming the viability of the numerically based damage index method for estimating the seismically induced damage in concrete walls. Moreover, the estimated damage can be utilized as a qualitative and quantitative scale to assess the status of the wall following seismic loading events. Finally, an equation is proposed to estimate the repair cost based on the predicted damage state for the studied structural system.
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    Effect of Skin on Finite Element Modeling of Foot and Ankle During Balanced Standing
    OU Haihuaa (欧海华), QAISER Zeeshana, KANG Lipinga (康利平), JOHNSON Shanea,b*
    2018, 23 (1):  132-137.  doi: 10.1007/s12204-018-1918-9
    Abstract ( 444 )  
    Experimental measurements of stresses and strains for lower extremity injuries (LEI) are invasive, and therefore, predictions require physiologically accurate 3D finite element (FE) models of the foot and ankle. In previous models, skin is typically neglected. However, experimental studies have shown that skin is much stiffer than soft tissue. In this study, the material sensitivity of skin on foot arch deformation is investigated. A finite element model of the foot is developed, incorporating bones, soft tissue, ligament, articulating surfaces, plantar aponeurosis, skin and plantar plate. Balanced standing is simulated without skin or with three different skin mechanical properties. By including different skin models, the foot static vertical stiffness, navicular displacement and plantar aponeurosis strain change significantly, when compared with the model without skin. The study shows that skin, showing a much stiffer behaviour than soft tissue, should not be neglected in the foot modelling. Further, the plantar plate considered in this model can give merit to modelling injuries such as plantar plate tearing.
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    A Note on the Behaviour of the Number Field Sieve in the Medium Prime Case: Smoothness of Norms
    BENGER Naomi1, CHARLEMAGNE Manuel2*, CHEN Kefei3 (陈克非)
    2018, 23 (1):  138-145.  doi: 10.1007/s12204-018-1919-8
    Abstract ( 393 )  
    As we examine the behaviour of the number field sieve (NFS) in the medium prime case, we notice various patterns that can be exploited to improve the running time of the sieving stage. The contributions of these observations to the computational mathematics community are twofold. Firstly, we clarify the understanding of the true practical effectiveness of the algorithm. Secondly, we propose a test for a better choice of the polynomials used in the NFS. These results are of particular interest to cryptographers as the run-time of the NFS directly determines the security level of some discrete logarithm problem based protocols.
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    Focal Points at Infinity for Short-Range Scattering Trajectories
    HOHBERGER Horst1*, KLEIN Markus2*
    2018, 23 (1):  146-157.  doi: 10.1007/s12204-018-1920-2
    Abstract ( 328 )  
    Classical scattering trajectories are known to form a Lagrangian manifold in euclidean phase space, which allows the classification of local focal points for sufficiently small dimensions. For the case of a short-range potential, we show that the natural description of focal points at infinity is a Lagrangian manifold in the cotangent bundle of the sphere and establish the relationship between focal points at infinity and the projection singularities of that manifold.
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    Mapped Displacement Discontinuity Method: Numerical Implementation and Analysis for Crack Problems
    JIANG Feng (姜锋), SHEN Yongxing* (沈泳星)
    2018, 23 (1):  158-165.  doi: 10.1007/s12204-018-1921-1
    Abstract ( 339 )  
    The displacement discontinuity method (DDM) is a kind of boundary element method aiming at modeling two-dimensional linear elastic crack problems. The singularity around the crack tip prevents the DDM from optimally converging when the basis functions are polynomials of first order or higher. To overcome this issue, enlightened by the mapped finite element method (FEM) proposed in Ref. [13], we present an optimally convergent mapped DDM in this work, called the mapped DDM (MDDM). It is essentially based on approximating a much smoother function obtained by reformulating the problem with an appropriate auxiliary map. Two numerical examples of crack problems are presented in comparison with the conventional DDM. The results show that the proposed method improves the accuracy of the DDM; moreover, it yields an optimal convergence rate for quadratic interpolating polynomials.
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    Implementation Details for the Phase Field Approaches to Fracture
    SHEN Yongxing1,2* (沈泳星), MOLLAALI Mostafa1,2, LI Yihuan1,2 (李毅环),MA Weixin1,2 (马维馨), JIANG Jiahao3 (蒋家皓)
    2018, 23 (1):  166-174.  doi: 10.1007/s12204-018-1922-0
    Abstract ( 510 )  
    Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete descriptions of cracks. In this work, we provide an overview of phase field models for quasistatic and dynamic cases. Afterward, we present useful vectors and matrices for the implementation of this method in two and three dimensions.
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    Numerical Investigation of Ultrasonic Guided Wave Dynamics in Piezoelectric Composite Plates for Establishing Structural Self-Sensing
    WANG Junzhen (王军振), SHEN Yanfeng* (申岩峰)
    2018, 23 (1):  175-181.  doi: 10.1007/s12204-018-1923-z
    Abstract ( 373 )  
    This article presents a numerical investigation of guided wave generation, propagation, interaction with damage, and reception in piezoelectric composite plates for the purpose of establishing structural self-awareness. This approach employs piezoelectric composite materials as both load bearing structure and sensing elements. Finite element modal analysis of a plate cell with Bloch-Floquet boundary condition (BFBC) is performed to understand the wave propagation characteristics in piezoelectric composite plates. A comparative study is carried out between a standard composite plate and a piezoelectric composite plate to highlight the influence of piezoelectricity on guided wave dispersion relations. Subsequently, a transient dynamic coupled-field finite element model is constructed to simulate the procedure of guided wave generation, propagation, interaction with damage, and reception in a piezoelectric composite plate. Active sensing array is designed to capture the structural response containing the damage information. Three engineering scenarios, including a pristine case, a one-damagelocation case and a two-damage-location case, are considered to demonstrate the ultrasonic sensing capability of the piezoelectric composite system. Finally, time-reversal method is utilized to locate and image the damage zones. This research shows that piezoelectric composite material possesses great potential to establish structural self-awareness, if it serves both as the load bearing and structural sensing components.
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    Non-Linearity of the Mirror Constant for Glasses Fractured in Flexure
    MA Lingyue1 (马凌越), DUGNANI Roberto1*, MOULINS Anthony2
    2018, 23 (1):  182-189.  doi: 10.1007/s12204-018-1924-y
    Abstract ( 439 )  
    Fractography relies on topographic features on the fracture surfaces to deduce the stress state of the failed component. Traditionally, fractographers assumed that the fracture strength of brittle materials was related to the reciprocal of the square root of the mirror radius linearly, in accordance with the empirical equation formally postulated by Orr in 1972. In this manuscript a novel analytical model to describe the dynamic behavior of elliptical cracks was derived on the basis of thermodynamic principles. Unlike Orr’s equation, the proposed model considered the effects of the plate thickness and it could be used to improve the strength prediction based on the plate’s geometry and material properties. To verify the validity of the model, 39 annealed, 2mm thick, soda-lime silicate glass plates were fractured by four-points bending test and the fracture surfaces were imaged by confocal laser microscope and carefully studied. Excellent agreement was observed between the experimental data and the proposed theoretical framework.
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    Revised Three-Dimensional Navier-Stokes Characteristic Boundary Conditions for Intense Reactive Turbulence
    ZHAO Peipei (赵培培), WANG Lipo* (王利坡)
    2018, 23 (1):  190-201.  doi: 10.1007/s12204-018-1925-x
    Abstract ( 336 )  
    The three-dimensional Navier-Stokes characteristic boundary conditions (3D-NSCBC), although physically reasonable and popular in many applications, may encounter the instability problem in simulating complex flows, especially for large Reynolds number reactive turbulence where locally the strong reversed flow appears at the outflow boundary surfaces. In the present work, a revised 3D-NSCBC strategy is proposed based on the kinematic relation in different moving coordinate systems. Following this strategy, a systematic formulation is presented for the outflow surface with local reversed flow and can be easily extended to the coupled edge and corner boundaries. Direct numerical simulation (DNS) tests of flow with different turbulence intensities are carried out. Compared with the conventional 3D-NSCBC, the newly proposed method exhibits satisfactory performance to confine numerical instability in the strong reversed flow region. The results confirm the robustness and effectiveness of this newly proposed algorithm.
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    Electronic Structure and Stability of Lead-free Hybrid Halide Perovskites: A Density Functional Theory Study
    WU Jiayia (邬嘉义), QI Wena (戚文), LUO Zheb (罗哲), LIU Kea (刘科), ZHU Honga,c* (朱虹)
    2018, 23 (1):  202-208.  doi: 10.1007/s12204-018-1926-9
    Abstract ( 400 )  
    The most commonly used and studied hybrid halide perovskite is ABX3, where A usually stands for CH3NH3, B for Pb, and X for I. A lead-free perovskite with high stability and ideal electronic band structure would be of essence, especially considering the toxicity of lead. In this work, we have considered 11 metal elements for the B site and three halide elements (Cl, Br, and I) including various combinations among the three halides for the X site. A total number of 99 hybrid perovskites are studied to understand how the crystal structure, band gap and stability can be tuned by the chemistry modification, i.e., the replacement of toxic element, Pb in the original MAPbX3, with non-toxic metal elements. We find that the favorable substitutes for Pb in MAPbI3 are Ge and Sn.
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