PolyU Shenzhen Research Institute

Shenzhen, China

PolyU Shenzhen Research Institute

Shenzhen, China
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Feng W.-Q.,Hong Kong Polytechnic University | Yin J.-H.,Hong Kong Polytechnic University | Yin J.-H.,PolyU Shenzhen Research Institute | Tao X.-M.,Hong Kong Polytechnic University | And 2 more authors.
International Journal of Geomechanics | Year: 2017

A plasticine material exhibits the characterized viscous stress-strain behavior with some similarity to the behavior of clayey soils. This paper presents a series of experimental tests, which include oedometer tests, isotropic creep tests, and triaxial multistrain rate compression tests, on a plasticine material. The test study focuses on effects of time and strain rate on viscous stress-strain behavior of the plasticine material under one-dimensional (1D) straining, isotropic stressing, and triaxial compression conditions. Values of compression index (Ccε), rebounding index (Crε), and creep coefficient (Cαε) are obtained from the 1D straining and 1D stressing test data. The plasticine material has no primary consolidation period, and creep occurs from the beginning. Values of Ccε, Crε, and Cαε are smaller than those of the soft clays. The triaxial multistrain rate compression test data show that the stress-strain behavior of the plasticine depends on the strain rates and the confining pressures. A parameter of ρ0.01 is adopted to evaluate the strain-rate effects. The strain-rate effects on the stress-strain behavior of the plasticine material are obvious and significant. The values of ρ0.01 are larger than those of clays. Both friction angle and cohesion of the plasticine increase with strain rate. This is different from the friction angle and cohesion at the critical state for all soils. The friction angle of the plasticine is from 2.57° at a strain rate of 0.01%/min to 3.21° at a strain rate of 1%/min, which is much smaller than that of all clays. With the help of a scanning electron microscope, the microstructures of this plasticine material before and after oedometer and isotropic creep tests are visualized and compared. The compression of the plasticine material is mainly due to the irrecoverable porosity decrease of the material and the structural compression. © 2016 American Society of Civil Engineers.

Shang X.,Hong Kong Polytechnic University | Cui Z.,Shanghai JiaoTong University | Fu M.W.,Hong Kong Polytechnic University | Fu M.W.,PolyU Shenzhen Research Institute
International Journal of Plasticity | Year: 2017

Ductile fracture is a key factor to the workability of metallic materials undergoing hot deformation. The ductility of materials at elevated temperature is closely related to dynamic recrystallization (DRX). To systematically investigate the DRX based ductile fracture, hot tensile experiments, microscopic observations and modeling of fracture behavior were conducted for 316LN steel. Based on the experimental results, a monotonic increasing relationship between ductility and the percentage of DRX (Xdrx) was figured out and identified to be attributed to the DRX influenced void evolution. With the softening effect caused by DRX, the local stress concentration, which serves as the driving force of void nucleation, void growth as well as void coalescence of the material, is highly relieved and the behaviors of voids thus change. To describe the DRX based void evolution and predict ductile fracture in hot working process, an extended damage model was established by introducing Xdrx into the void-based GTN-Thomason ductile fracture model, which is termed as the extended GTN-Thomason model in this research. In modeling of the ductile fracture considering DRX, the void nucleation strain, which represents the strain with the highest nucleation rate, and the critical void size ratio, which articulates the onset of void coalescence were figured out to increase with Xdrx. In addition, the strain rate sensitivity and the temperature dependency are involved in representing the kinetics of DRX and the flow stress applied in the model. The developed model was then implemented into finite element (FE) simulation and its related parameters were calibrated via a hybrid experiment and simulation method. Finally, the specific validation experiments were designed and conducted and the predicted fractures agree well with experimental results. This research thus offers an in-depth understanding of the DRX based ductile fracture and further facilitate and support the design of hot working process by avoiding ductile fracture occurrence. © 2017 Elsevier Ltd.

Li W.T.,Hong Kong Polytechnic University | Fu M.W.,Hong Kong Polytechnic University | Fu M.W.,PolyU Shenzhen Research Institute | Shi S.Q.,Hong Kong Polytechnic University
International Journal of Mechanical Sciences | Year: 2017

A constitutive model considering the composition of surface grain, grain boundary and grain interior and their contributions to the flow stress or strength of materials in micro-scale plastic deformation is developed and termed as a combined surface layer and grain boundary strengthening model in this research. To determine the composition of the three interior microstructural parts of materials, optical microscope and digital image processing technologies are employed. A series of micro-tensile experiments using the specimens with three different geometrical shapes and microstructural grain sizes are conducted for study of deformation and ductile fracture behaviors of material. The model is implemented in finite element analysis and validated via physical experiments. The relationship among fracture strain, grain size and stress triaxiality of the deforming material is thus established. It is found both fracture strain and stress triaxiality increase with the decrease of grain size, while the high stress triaxiality leads to small fracture strain for the given grain size. Through observation of the fractographs, it is revealed that the domination of shear fracture in the ‘cup-cone’ fracture increases with grain size. The research thus helps understand the ductile fracture in micro-scale deformation and facilitates deformation based working process determination and application. © 2017 Elsevier Ltd

Chan C.,Hong Kong Polytechnic University | Shi J.,Hong Kong Polytechnic University | Fan Y.,Hong Kong Polytechnic University | Yang M.,Hong Kong Polytechnic University | Yang M.,PolyU Shenzhen Research Institute
Sensors and Actuators, B: Chemical | Year: 2017

Most of the current graphene transistor based deoxyribonucleic acid (DNA) sensors are based on dip-and-dry methods The flow-through approach for graphene transistor based DNA sensors have not been explored yet. Moreover, the effect of probe immobilization strategies on the performance of a graphene transistor biosensor in flowing environment was rarely studied. In this paper, a microfluidic integrated reduced graphene oxide (rGO) transistor was developed for H5N1 influenza virus gene detection with high stability and sensitivity via a flow-through strategy. Different DNA probe immobilization approaches including extended long capture probe via π-π stacking, short capture probe via π-π stacking and covalent immobilization via linker were studied. Both fluorescence measurement and electrical detection were performed to evaluate the performance of rGO transistors in flowing environment for these probe immobilization strategies. The results showed that among these approaches, extended long capture probe could provide both high sensitivity and stability in flowing environment while short capture probe suffered by the low stability in flowing environment and covalent immobilization via linker had relatively low sensitivity. This microfluidic integrated rGO transistor with extended capture probe immobilization approach could provide a promising platform for nucleic acid detection with high sensitivity and stability for potential flow-through chip application. © 2017 Elsevier B.V.

Wang J.L.,Shandong University | Wang J.L.,Hong Kong Polytechnic University | Fu M.W.,Hong Kong Polytechnic University | Fu M.W.,PolyU Shenzhen Research Institute | Shi S.Q.,Hong Kong Polytechnic University
Materials and Design | Year: 2017

In macro-scaled plastic deformation, ductile fracture behaviors have been extensively investigated in terms of formation mechanism, deformation mechanics, influencing factors and fracture criteria. In micro-scaled plastic deformation, however, the fracture behaviors of materials are greatly different from those in macro-scale due to the existence of size effects. To explore the simultaneous interaction of size effect and stress condition on material fracture behavior in meso/micro-scaled plastic deformation, the tensile and compression tests of pure copper with various geometrical sizes and microstructures were conducted. The experiment results show that microvoids exist in compressed samples due to localization of shear band instead of macro fracture. Furthermore, the FE simulation is conducted by using the size dependent surface layer model, which aims to study the interaction of size effect and stress condition on material fracture behavior in multi-scaled plastic deformation. It is found that the stress triaxiality (T) generally increases with the ratio of surface grains η in compression statement. Fracture strain and fracture energy with positive T are much smaller than that with negative T regardless of geometrical and grain sizes. This research provides an in-depth understanding of the influences of size effect and stress condition on ductile fracture behavior in micro-scaled plastic deformation. © 2017 Elsevier Ltd

Xu D.-S.,Huazhong University of Science and Technology | Yin J.-H.,PolyU Shenzhen Research Institute | Yin J.-H.,Hong Kong Polytechnic University
Engineering Geology | Year: 2016

In this work, a slope reinforcement system using glass fiber reinforced polymer (GFRP) anchors with pressure grouting was adopted in a field project in Hong Kong. The performance of the GFRP anchor during slope excavation was measured using a novel distributed strain sensing technology, known as Brillouin Optic Time Domain Analysis (BOTDA). The full strain profiles along the GFRP anchor under different excavation stages were obtained using specially protected fiber optic sensors. In addition to fiber optic sensors, traditional strain gauges were installed in the same GFRP anchor. Comparisons show that the BOTDA sensors have good accuracy. In addition, the measured results indicate that the maximum tensile strains and forces occurred at one-third of the GFRP anchor length from the slope surface. The tensile force distribution within the active zone is curvilinear which is confirmed by elastic theory analysis. Shear stress distributions along the GFRP anchors were obtained by differentiating the strain data numerically. The theoretical analysis results were consistent with the measured data at the initial excavation stage. However, the theoretical analysis underestimated the shear stress at the final excavation stage where the slope undergoes plastic deformation. Based on the field measurement results and theory analysis, we conclude that the BOTDA sensing technology provides an alternative and effective approach to identifying distributed strains along anchors and shear zones in reinforced slopes. © 2016 Elsevier B.V.

Yin J.-H.,PolyU Shenzhen Research Institute | Yin J.-H.,Hong Kong Polytechnic University
International Journal of Geomechanics | Year: 2015

In this paper, a number of fundamental concepts are presented and explained. These include (1) differences among an instant compression line, a normal consolidation line, and a true instant compression line; (2) the uniqueness of viscoplastic strain rates with a stress-strain state; (3) whether the creep compression is smaller than the instant compression; (4) the separation of the total strain rates; (5) the relation between elastic-plastic models and elastic viscoplastic (EVP) models, etc. The major conclusions are the following: (1) the elastic compression is the true instant compression; (2) the magnitude of a creep-strain rate at a stress-strain state point is unique, independent of the loading path to reach this point; (3) the true instant (elastic) compression is much smaller than the creep compression; (4) it is more appropriate that strain rates of geomaterials are composed of elastic strain rates and viscoplastic strain rates; (5) the one-dimensional (1D) EVP (1D EVP) is a genuine extension of Maxwell's linear rheological model for considering the nonlinear behavior of soils; (6) the EVP model is more general than an elastic-plastic model; (7) the nonlinear functions proposed by the author are good for fitting the creep compression and the compression under high stress of most soft soils in 1D straining; and (8) the three-dimensional EVP model is rigorously derived using the 1D EVP model approach and the modified Cam-Clay model, but further improvements of this model are still needed. At the end, a number of areas are presented for further study. © 2015 American Society of Civil Engineers.

Fu M.W.,Hong Kong Polytechnic University | Fu M.W.,PolyU Shenzhen Research Institute | Wang J.L.,Hong Kong Polytechnic University | Korsunsky A.M.,University of Oxford
International Journal of Machine Tools and Manufacture | Year: 2016

Plastic deformation at the macroscopic scale has been widely exploited in industrial practice in order to obtain desired shape and control the requested properties of metallic alloy parts and components. The knowledge of deformation mechanics involved in various forming processes has been systematically advanced over at least two centuries, and is now well established and widely used in manufacturing. However, the situation is different when the physical size of the workpiece is scaled down to the micro-scale (µ-scale). In such cases the data, information and insights from the macro-scale (m-scale) deformation mechanics are no longer entirely valid and fully relevant to µ-scale deformation behavior. One important reason for the observed deviation from m-scale rules is the ubiquitous phenomenon of Size Effect (SE). It has been found that the geometrical size of workpiece, the microstructural length scale of deforming materials and their interaction significantly affect the deformation response of µ-scale objects. This observation gives rise to a great deal of research interest in academia and industry, causing significant recent effort directed at exploring the range of related phenomena. The present paper summarizes the current state-of-the-art in understanding the geometrical and microstructural SEs and their interaction in deformation processing of µ-scale components. The geometrical and grain SEs in µ-scale deformation are identified and articulated, the manifestations of the SE are illustrated and the affected phenomena are enumerated, with particular attention devoted to pointing out the differences from those in the corresponding m-scale domain. We elaborate further the description of the physical mechanisms underlying the phenomena of interest, viz., SE-affected deformation behavior and phenomena, and the currently available explanations and modeling approaches are reviewed and discussed. Not only do the SEs and their interaction affect the deformation-related phenomena, but they also induce considerable scatter in properties and process performance measures, which in turn affects the repeatability and reliability of deformation processing. This important issue has become a bottleneck to the more widespread application of µ-scale deformation processing for mass production of µ-scale parts. What emerges is a panoramic view of the SE and related phenomena in µ-scale deformation processing. Furthermore, thereby the outstanding issues are identified to be addressed to benefit and promote practical applications. © 2016 Elsevier Ltd

Liu X.,Hong Kong Polytechnic University | Liu X.,PolyU Shenzhen Research Institute | Xiao B.,Hong Kong Polytechnic University | Xiao B.,PolyU Shenzhen Research Institute | And 2 more authors.
IEEE Transactions on Parallel and Distributed Systems | Year: 2015

Radio-Frequency Identification (RFID) technology brings revolutionary changes to many fields like retail industry. One important research issue in large RFID systems is the identification of unknown tags, i.e., tags that just entered the system but have not been interrogated by reader(s) covering them yet. Unknown tag identification plays a critical role in automatic inventory management and misplaced tag discovery, but it is far from thoroughly investigated. Existing solutions either trivially interrogate all the tags in the system and thus are highly time inefficient due to re-identification of already identified tags, or use probabilistic approaches that cannot guarantee complete identification of all the unknown tags. In this paper, we propose a series of protocols that can identify all of the unknown tags with high time efficiency. We develop several novel techniques to quickly deactivate already identified tags and prevent them from replying during the interrogation of unknown tags, which avoids re-identification of these tags and consequently improves time efficiency. To our knowledge, our protocols are the first non-trivial solutions that guarantee complete identification of all the unknown tags. We illustrate the effectiveness of our protocols through both rigorous theoretical analysis and extensive simulations. Simulation results show that our protocols can save up to 70 percent time when compared with the best existing solutions. © 2015 IEEE.

Meng B.,Hong Kong Polytechnic University | Fu M.W.,Hong Kong Polytechnic University | Fu M.W.,PolyU Shenzhen Research Institute | Shi S.Q.,Hong Kong Polytechnic University
Materials and Design | Year: 2016

With the increasing demand for meso/micro-scaledmedical products made of biocompatiblematerials, thermalaidedmesoforming is proposed to improvematerial formability and homogenize flow behavior of materials that are difficult to deform at room temperature. However, the unique material deformation behavior and the interactive effects of material microstructure and deformation temperature on forming quality of the fabricated micropart remain unknown. This study thus aims at addressing this issue in thermalmesoforming in terms of deformation load, material flow,microstructural evolution, dimensional accuracy, and defect formation. Accordingly, the fabrication of a titanium dental abutment by one-stroke mesoforming at elevated temperature is conducted and explored. The characteristic and quality of the mesoformed part are extensively examined. The surface grains on the square extrudate undergo severe deformation and generate an equiaxed structure, reflecting that mesoforming at elevated temperature facilitates the homogenization of material flow without coarsening grain size. In addition, the dimensional accuracy, surface quality and the sizes of burr and flash are associated with the initial grain size of pure titanium, and the surface finish is improved by using fine-grained titanium. The fine-grained material is thus desirable for achieving the optimal surface quality in the thermal-aided mesoformed parts. © 2015 Elsevier Ltd.

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