How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and un...How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SHPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.展开更多
Cellular material under high-velocity impacthas a typical feature oflayer-wise collapse.A cell-based finite element model is employed to simulate the direct impact of closed-cell foam, and one-dimensional velocity fie...Cellular material under high-velocity impacthas a typical feature oflayer-wise collapse.A cell-based finite element model is employed to simulate the direct impact of closed-cell foam, and one-dimensional velocity field distributionsareobtained to characterize thecrushing bandpropagating through a cellular material. An explicit expression of continuous velocity distribution is derivedbased on the features of velocity gradient distribution. The velocity distribution function is adopted to determine the dynamic stress-strain statesof cellular materials under dynamic loading.The local stress-strain history distribution reveals that sectional cells experience a process from the precursor of elastic behavior to the shock stress state, through the dynamic initial crushing state. A power-law relation between the dynamic initial crushing stress andthe strainrate isestablished, which confirms the strain-rate effect of cellular materials. By extracting the critical points immediately before the unloading stage on the local dynamic stress-strain history curves, the dynamic stress-strain statesof cellular materials are determined. They exhibit loading rate-dependence but are independent of the initial impact velocity.Furthermore, with the increase of relative density, the dynamic hardening behaviorof cellular specimen is enhanced and the crushing process event is advanced. The particle velocity-based analytical method is appliedto analyze the dynamic responses of cellular materials.This method is better than continuum-based shock models, since itdoes not require a pre-assumed constitutive relation.Therefore,the particle velocity-based analytical method proposed in this study may provide new ideas to carry out dynamic experimental measurement, which is especially applicable toinhomogeneous materials.展开更多
ZnO nanostructure materials doped with different La contents were synthesized by sonochemical method. The products were characterized by X-ray diffraction（XRD）, scanning electron microscopy（SEM）,transmission elect...ZnO nanostructure materials doped with different La contents were synthesized by sonochemical method. The products were characterized by X-ray diffraction（XRD）, scanning electron microscopy（SEM）,transmission electron microscopy（TEM）, Raman spectroscopy, and Fourier transform infrared spectroscopy（FTIR）. In this research, XRD patterns of pure ZnO and La-doped ZnO are specified as hexagonal wurtzite ZnO structure with no detection of La2O3 phase. SEM and TEM characterization revealed the flower shape of pure ZnO built-up from petals of hexagonal prisms with hexagonal pyramid tips. Upon doping with La, the flower-shaped ZnO is broken into individual 1D prism-like nanorods. Photocatalytic activities of the as-synthesized products were determined by measuring the degradation of methylene blue（MB） under ultraviolet–visible（UV） light irradiation.Among them, the 2.0 mol% La-doped ZnO shows better photocatalytic properties than any other products.展开更多
A virtual Taylor impact of cellular materials is analyzed with a wave propagation technique, i.e. the Lagrangian analysis method, of which the main advantage is that no pre-assumed constitutive relationship is require...A virtual Taylor impact of cellular materials is analyzed with a wave propagation technique, i.e. the Lagrangian analysis method, of which the main advantage is that no pre-assumed constitutive relationship is required. Time histories of particle velocity, local strain, and stress profiles are calculated to present the local stress-strain history curves, from which the dynamic stress-strain states are obtained. The present results reveal that the dynamic-rigid-plastic hardening （D-R-PH） material model introduced in a previous study of our group is in good agreement with the dynamic stress-strain states under high loading rates obtained by the Lagrangian analysis method. It directly reflects the effectiveness and feasibility of the D-R-PH material model for the cellular materials under high loading rates.展开更多
The propagation of shock waves in a cellular bar is systematically studied in the framework of continuum solids by adopting two idealized material models, viz. the dynamic rigid, perfectly plastic, locking （D-R-PP-L...The propagation of shock waves in a cellular bar is systematically studied in the framework of continuum solids by adopting two idealized material models, viz. the dynamic rigid, perfectly plastic, locking （D-R-PP-L） model and the dynamic rigid, linear hardening plastic, locking （D-R-LHP-L） model, both considering the effects of strain-rate on the material properties. The shock wave speed relevant to these two models is derived. Consider the case of a bar made of one of such material with initial length L 0 and initial velocity v i impinging onto a rigid target. The variations of the stress, strain, particle velocity, specific internal energy across the shock wave and the cease distance of shock wave are all determined analytically. In particular the ＂energy conservation condition＂ and the ＂kinematic existence condition＂ as proposed by Tan et al. （2005） is re-examined, showing that the ＂energy conservation condition＂ and the consequent ＂critical velocity＂, i.e. the shock can only be generated and sustained in R-PP-L bars when the impact velocity is above this critical velocity, is incorrect. Instead, with elastic deformation, strain-hardening and strain-rate sensitivity of the cellular materials being considered, it is appropriate to redefine a first and a second critical impact velocity for the existence and propagation of shock waves in cellular solids. Starting from the basic relations for shock wave propagating in D-R-LHP-L cellular materials, a new method for inversely determining the dynamic stress-strain curve for cellular materials is proposed. By using e.g. a combination of Taylor bar and Hopkinson pressure bar impact experimental technique, the dynamic stress-strain curve of aluminum foam could bedetermined. Finally, it is demonstrated that this new formulation of shock theory in this one-dimensional stress state can be generalized to shocks in a one-dimensional strain state, i.e. for the case of plate impact on cellular materials, by simply making proper replacements of the elastic and plastic constants.展开更多
The split Hopkinson pressure bar（SHPB） technique and the wave propagation inverse analysis（WPIA） technique are both extensively used to experimentally investigate the impact behavior of materials, although neither...The split Hopkinson pressure bar（SHPB） technique and the wave propagation inverse analysis（WPIA） technique are both extensively used to experimentally investigate the impact behavior of materials, although neither of them alone provides a fully satisfactory analysis. In the present paper, attention is given to new experimental techniques by incorporating a damagemodified constitutive model into the SHPB technique and combining the Hopkinson pressure bar（HPB） technique with WPIA. First, to distinguish the response due to dynamic constitutive behavior and the response due to dynamic damage evolution, the SHPB method incorporating a damage-modified constitutive model is developed, including an explicit damage-modified Zhu–Wang–Tang model and an implicit damage-modified constitutive model. Second, when the SHPB results become invalid, a method of combining new Lagrange inverse analyses with the HPB technique is developed, including cases of the HPB arranged in front of a long specimen and behind the specimen. As examples of these new methods, typical results are given for nonlinear viscoelastic polymers and concretes considering damage evolution, a super-elastic Ti–Ni alloy with phase transformation and an aluminum foam with shock waves propagating within it.展开更多
Critical impact velocity （CIV） of oxygen-free high-conductivity （OFHC） copper is experimentally measured with a novel facility in a gas gun system. The results are compared with the theoretical predictions using t...Critical impact velocity （CIV） of oxygen-free high-conductivity （OFHC） copper is experimentally measured with a novel facility in a gas gun system. The results are compared with the theoretical predictions using the typical constitutive relations, and the measured CIV value is much lower than the predictions. The difference of physical mechanisms in experiment and in theoretical calculation is discussed. It is suggested that the reduction of CIV in experiment would be related with the damage evolution in tensile copper that needs to be considered in the computation model.展开更多
The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8....The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.展开更多
The wave dispersion due to the lateral inertia in the split Hopkinson pressure bar(SHPB) with large-(diameter) bar is numerically analyzed by means of the LS-DYNA3D code. The results show that, ① the stress distribut...The wave dispersion due to the lateral inertia in the split Hopkinson pressure bar(SHPB) with large-(diameter) bar is numerically analyzed by means of the LS-DYNA3D code. The results show that, ① the stress distribution across the bar section is non-uniform along the radius direction and such non-uniformity depends on the material Poisson ratio and propagation distance; ② with increasing the bar diameter, the high frequency oscillations are notably enhanced and the rise time of wave front becomes longer, meanwhile the amplitude of the stress wave attenuates; ③ with decreasing the rise time of wave front, the wave dispersion markedly enhanced, particularly in the large diameter bar. All of those effects should not be neglected in order to obtain accurate results by the SHPB test..展开更多
The determination of the precise thickness-shear frequency of electroded crystal plates has practical importance in quartz crystal resonator design and fabricatiom especially when the high fundamental thickness-shear ...The determination of the precise thickness-shear frequency of electroded crystal plates has practical importance in quartz crystal resonator design and fabricatiom especially when the high fundamental thickness-shear frequency has reduced the crystal plate thickness to such a degree that proper consideration of the effect of electrodes is very important. The electrodes effect as mass loading in the estimation of the resonance frequency has to be modified to consider the stiffness of electrodes, as the relative strength is increasingly noticeable. By following a known procedure in the determination of the thickness-shear frequency of an infinite AT-cut crystal plate, frequency equations of crystal plate without and with piezoelectric effect are obtained in terms of elastic constants and the electrode material density. After solving these equations for the usual design parameters of crystal resonators, the design process can be optimized to pinpoint the precise configuration to avoid time-consuming trial and reduction steps. Since these equations and solutions are presented for widely used materials and parameters, they can be easily integrated into the existing crystal resonator design and manufacturing processes.展开更多
A facile sonochemical method was developed to synthesize metallic Ag spherical nanoparticles on the surface of ZnWO4 nanorods by forming heterostructure Ag/ZnWO4 nanocomposites.The Ag/ZnWO4 nanocomposites were charact...A facile sonochemical method was developed to synthesize metallic Ag spherical nanoparticles on the surface of ZnWO4 nanorods by forming heterostructure Ag/ZnWO4 nanocomposites.The Ag/ZnWO4 nanocomposites were characterized by X-ray powder diffraction (XRD),Fourier transform infrared spectroscopy (FTIR),field emission scanning electron microscopy (FESEM),transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).The experimental results showed that fcc metallic Ag nanoparticles were supported on surface of monoclinic sanmartinite ZnWO4 nanorods.The Ag 3d3/2 and Ag 3d5/2 peaks have well-separated binding energies of 6.00 eV,certifying the existence of metallic Ag.The Ag/ZnWO4 nanocomposites were evaluated for photodegradation of methylene blue (MB) induced by ultraviolet-visible (UV-Vis) radiation.In this research,heterostructure 10 wt% Ag nanoparticle/ZnWO4-nanorod composites have the highest photocatalytic activity of 99% degradation of MB within 60 min.The increase in photocatalytic activity was the result of photoinduced electrons in conduction band of ZnWO4 that effectively diffused to metallic Ag spherical nanoparticles and the inhibition of electron-hole recombination process.展开更多
The dynamic fracture behaviors of the extruded 2024-T4 and 7075-T6 aluminum alloys are investigated by using an instrumented drop tower machine.The specimens are made from a 25 mm diameter extruded circular rod.The dy...The dynamic fracture behaviors of the extruded 2024-T4 and 7075-T6 aluminum alloys are investigated by using an instrumented drop tower machine.The specimens are made from a 25 mm diameter extruded circular rod.The dynamic three-point bending tests of each alloy are carried out at different impact velocities.The initiation fracture toughness and average propagation fracture toughness of 2024-T4 and 7075-T6 are determined at different loading rates.The results show that both the initiation toughness and the propagation toughness increase with the loading rate.Further,the difference between the fracture toughness behaviors of 2024-T4 and 7075-T6 is found to be dependent on the variation of fracture mechanism.The comprehensive fractographic investigations of the fracture surfaces clearly demonstrate that the fracture mode of 2024-T4 is predominantly transgranular fracture with high density small-sized dimples,and the fracture mode of 7075-T6 is mainly intergranular fracture with many intermetallic particles in the bottom of voids located in the fracture surface.展开更多
When manned spacecraft comes back to the earth, it relies on the impact attenuation seat to protect astronauts from injuries during landing phase. Hence, the seat needs to transfer impact load, as small as possible, t...When manned spacecraft comes back to the earth, it relies on the impact attenuation seat to protect astronauts from injuries during landing phase. Hence, the seat needs to transfer impact load, as small as possible, to the crew. However, there is little room left for traditional seat to improve further. Herein, a new seat system biologically-inspired by felids＇ landing is proposed. Firstly, a series of experiments was carried out on cats and tigers, in which they were trained to jump down voluntarily from different heights. Based on the ground reaction forces combined with kinematics, the experiment indicated that felids＇ landing after self-initial jump was a multi-step impact attenuation process and the new seat was inspired by this. Then the construction and work process of new seat were redesigned to realize the multi-step impact attenuation. The dynamic response of traditional and new seat is analyzed under the identical conditions and the results show that the new concept seat can significantly weaken the occupant overload in two directions compared with that of traditional seat. As a consequence, the risk of injury evaluated for spinal and head is also lowered, meaning a higher level of protection which is especially beneficial to the debilitated astronaut.展开更多
It is necessary to study the validation of strength models under planar shock loading in view of the fact that strength models for metals obtained at moderate strain rates are often used in the numerical simulations o...It is necessary to study the validation of strength models under planar shock loading in view of the fact that strength models for metals obtained at moderate strain rates are often used in the numerical simulations of shock wave phenomena. The variations of longitudinal stress, transverse stress and yield strength of oxygen-free high conductance （OFHC） copper with time under planar shock loading are obtained by using the manganin stress gauges and compared with the predicted results by the constructed seven constitutive models based on Y/G=constant and on G/B=constant （Y the yield strength, G the shear modulus, B the bulk modulus）, respectively. It seems that the pressure, density, temperature and plastic strain dependence of the yield strength for OFHC copper under planar shock loading is essential to the constitutive description.展开更多
In this research, cerium （III） nitrate hexahydrate （Ce（NO3）3·6H2O） and ammonium molybdate tetrahydrate （（NH4）6Mo7O24·4H2O） with Ce3＋-to-Mo6＋ molar ratio of 2：3 were dissolved in 40 ml different ...In this research, cerium （III） nitrate hexahydrate （Ce（NO3）3·6H2O） and ammonium molybdate tetrahydrate （（NH4）6Mo7O24·4H2O） with Ce3＋-to-Mo6＋ molar ratio of 2：3 were dissolved in 40 ml different solvents of deionized （DI） water, polyethylene glycol （PEG） and ethylene glycol （EG） to form different solutions which were followed by adjusting pH from the traditional values to 7.0 and 10.0 with 1 mol.L-1 sodium hydroxide （NaOH）. Subsequently, the solutions were processed by 270-W microwave-hydrother- mal/solvothermal method. Phase, morphology, vibrational modes and photonic properties were fully characterized by X-ray powder diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, Fourier transform infrared spectroscopy （FTIR）, Raman spectrophotometry, ultraviolet-visible （UV-Vis） absorption and photoluminescence （PL） spectroscopy. The as-synthe-sized products were pure cerium molybdenum oxide （Ce2（MoO4）3） of nanoparticles clustered together as nano- plates in DI water and PEG solvents, and of spindle-like nanoparticles in EG solvent, including the presence of Ce-O-H mode and MoO4 units. The results show that direct energy gaps of the first two have the same value of 2.30 eV, and that of the last is 2.80 eV, including their blue emission at the same wavelength of 488 nm.展开更多
Split Hopkinson technique has been developed to test the strength of common concrete and steel fiber reinforced concrete under dynamic tensile stress. Two types of test methods are considered, the splitting tensile te...Split Hopkinson technique has been developed to test the strength of common concrete and steel fiber reinforced concrete under dynamic tensile stress. Two types of test methods are considered, the splitting tensile test and a modified spalling test in which a specimen is loaded under uniaxial stress. The result shows that the dynamic strength enhancement of concrete is remarkable by using the reinforcing fiber. But for the common concrete, the base of compressive strength seems to show little effect on the tensile strength under dynamic loading. The experimental results also show that the resistance to tensile fracture of the steel fiber reinforced concrete for C100-mix is higher than those of C40-mix..展开更多
Monochnic and hexagonal CePO4 nanoparticles and nanorods were successfully synthesized from Ce（NO3）36H2O and Na3PO4 121-120 solu- tions at pH 1-5 by a 180 W microwave radiation for 60 min. The products were characte...Monochnic and hexagonal CePO4 nanoparticles and nanorods were successfully synthesized from Ce（NO3）36H2O and Na3PO4 121-120 solu- tions at pH 1-5 by a 180 W microwave radiation for 60 min. The products were characterized by X-ray diffraction （XRD）, Fourier transform infrared （FFIR） spectroscopy, and scanning electron microscopy （SEM）. XRD patterns revealed that the products are hexagonal CePO4 structures at pH 2-5, and monoclinic CePO4 structtLres at pH 1. SEM characterization shows that these products were nanoparticles, short nanorods, and long nanorods, controlled by the pH of the precursor solutions. Optical properties of the nanorods were also investigated by ultraviolet-visible （UV-vis） and photoluminescence （PL） spectroscopy.展开更多
ZnO particles doped with 0 mol%-5 mol% Cd dopant were successfully synthesized by a precipitation method.The as-synthesized products were characterized by thermogravimetric analysis(TGA),differential scanning calorime...ZnO particles doped with 0 mol%-5 mol% Cd dopant were successfully synthesized by a precipitation method.The as-synthesized products were characterized by thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),X-ray diffraction(XRD),photoluminescence(PL) spectroscopy,Raman spectroscopy,scanning electron microscopy(SEM) coupled with an energydispersive X-ray(EDX) spectroscopy and transmission electron microscopy(TEM).The precursors showed weight loss at 35-700℃ caused by phase transformation and evaporation processes.The as-synthesized products were specified as hexagonal wurtzite ZnO.The crystallite size of ZnO samples doped with 0 mol%-5 mol% Cd gradually increased with the increase in Cd content.The photocatalytic activities of 0 mol%-5 mol% Cd-doped ZnO samples were evaluated through photodecolorization of methylene blue(MB) under ultraviolet(UV) radiation.In this research,ZnO doped with 3 mol% Cd shows the best photocatalytic activity in degrading of MB molecules as much as 89% within 240 min.展开更多
Numerous studies highlight advantages of electrospun scaffolds in bone tissue engineering,in which cellular behavior is tightly affected by fiber topographical cues of scaffolds.However,the classic electrospinning set...Numerous studies highlight advantages of electrospun scaffolds in bone tissue engineering,in which cellular behavior is tightly affected by fiber topographical cues of scaffolds.However,the classic electrospinning setup limits a desired presentation of biomimetic fibrous microenvironments that sense mechanosignaling and regulate stem cell behavior.The aims of this study were to fabricate advanced asspun scaffolds presenting tree-like microfiber/nanonet networks and to evaluate their regulatory potentials on behavior of human mesenchymal stem cells(h MSCs)and bone regeneration.Here we developed a novel electrospinning setup that allowed the presentation of patterned Trunk microfibers(TMF)and/or branched nanonet fibers(BNn Fs)in biomimetic fibrous scaffolds.As the cellular mechanisms,anisotropichierarchical topography of TMF controlled behavior of h MSCs through focal adhesion formation and Yesassociated protein(YAP)induction,whereas BNn F disturbed such mechanosensing responses in the cells.The fiber microenvironment-related expression and nuclear localization of YAP were.also correlated with the potentials of as-spun scaffolds to enhance osteogenic differentiation of the h MSCs and alveolar bone defect healing in an animal model.Collectively,this study provides an advanced approach of the modified electrospinning setup for presentation of biomimetic fibrillar microenvironments in as-spun scaffolds along with their application in stem cell behavior regulation and regenerative tissue engineering.展开更多
Following the original approach of Bowden and Tabor and introducing state variables, an effective friction coefficient μ_e for solid particle erosion is defined as a combination of shearing term and ploughing term. I...Following the original approach of Bowden and Tabor and introducing state variables, an effective friction coefficient μ_e for solid particle erosion is defined as a combination of shearing term and ploughing term. In the case of continuous sliding, based on considering the interaction between asperities under certain condition, it is indicated that during the oblique impact of a hardened steel sphere against a mild steel target, a possible value of μ_e is 0.05, which was chosen in all of the calculations by Hutchings for consistency with both experiments and calculations. In the case of continuous ploughing, it is shown that the value of μ_e is a function of the impact process and the initial impact angle and is greater than 0.05 on an average for Hutchings' experiments. It is suggested that the variation of sliding, rolling and ploughing state at each instant in the impact process makes “the coefficient of friction” equal to 0.05 for Hutchings' experiments, and in general, makes the effective friction coefficient during particle impact on metal far less than the friction coefficient during simple continuous sliding on an average.展开更多
文摘How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SHPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.
基金This work was supported by the National Natural Science Foundation of China (Grants 11802002, 11772330, and 11372308)the Fundamental Research Funds for the Central Universities (Grant WK2480000003)the Youth Foundation of Anhui University of Technology (Grant RD 17100204).
文摘Cellular material under high-velocity impacthas a typical feature oflayer-wise collapse.A cell-based finite element model is employed to simulate the direct impact of closed-cell foam, and one-dimensional velocity field distributionsareobtained to characterize thecrushing bandpropagating through a cellular material. An explicit expression of continuous velocity distribution is derivedbased on the features of velocity gradient distribution. The velocity distribution function is adopted to determine the dynamic stress-strain statesof cellular materials under dynamic loading.The local stress-strain history distribution reveals that sectional cells experience a process from the precursor of elastic behavior to the shock stress state, through the dynamic initial crushing state. A power-law relation between the dynamic initial crushing stress andthe strainrate isestablished, which confirms the strain-rate effect of cellular materials. By extracting the critical points immediately before the unloading stage on the local dynamic stress-strain history curves, the dynamic stress-strain statesof cellular materials are determined. They exhibit loading rate-dependence but are independent of the initial impact velocity.Furthermore, with the increase of relative density, the dynamic hardening behaviorof cellular specimen is enhanced and the crushing process event is advanced. The particle velocity-based analytical method is appliedto analyze the dynamic responses of cellular materials.This method is better than continuum-based shock models, since itdoes not require a pre-assumed constitutive relation.Therefore,the particle velocity-based analytical method proposed in this study may provide new ideas to carry out dynamic experimental measurement, which is especially applicable toinhomogeneous materials.
基金financially supported by the National Research University Project for Chiang Mai University (CMU) from the Thailand’s Office of the Higher Education Commission, Thailand
文摘ZnO nanostructure materials doped with different La contents were synthesized by sonochemical method. The products were characterized by X-ray diffraction（XRD）, scanning electron microscopy（SEM）,transmission electron microscopy（TEM）, Raman spectroscopy, and Fourier transform infrared spectroscopy（FTIR）. In this research, XRD patterns of pure ZnO and La-doped ZnO are specified as hexagonal wurtzite ZnO structure with no detection of La2O3 phase. SEM and TEM characterization revealed the flower shape of pure ZnO built-up from petals of hexagonal prisms with hexagonal pyramid tips. Upon doping with La, the flower-shaped ZnO is broken into individual 1D prism-like nanorods. Photocatalytic activities of the as-synthesized products were determined by measuring the degradation of methylene blue（MB） under ultraviolet–visible（UV） light irradiation.Among them, the 2.0 mol% La-doped ZnO shows better photocatalytic properties than any other products.
基金supported by the National Natural Science Foundation of China(11372308 and 11372307)the Fundamental Research Funds for the Central Universities(WK2480000001)
文摘A virtual Taylor impact of cellular materials is analyzed with a wave propagation technique, i.e. the Lagrangian analysis method, of which the main advantage is that no pre-assumed constitutive relationship is required. Time histories of particle velocity, local strain, and stress profiles are calculated to present the local stress-strain history curves, from which the dynamic stress-strain states are obtained. The present results reveal that the dynamic-rigid-plastic hardening （D-R-PH） material model introduced in a previous study of our group is in good agreement with the dynamic stress-strain states under high loading rates obtained by the Lagrangian analysis method. It directly reflects the effectiveness and feasibility of the D-R-PH material model for the cellular materials under high loading rates.
基金supported by the National Natural Science Foundation of China (11032001)the K.C.Wong Magna Fund in Ningbo University
文摘The propagation of shock waves in a cellular bar is systematically studied in the framework of continuum solids by adopting two idealized material models, viz. the dynamic rigid, perfectly plastic, locking （D-R-PP-L） model and the dynamic rigid, linear hardening plastic, locking （D-R-LHP-L） model, both considering the effects of strain-rate on the material properties. The shock wave speed relevant to these two models is derived. Consider the case of a bar made of one of such material with initial length L 0 and initial velocity v i impinging onto a rigid target. The variations of the stress, strain, particle velocity, specific internal energy across the shock wave and the cease distance of shock wave are all determined analytically. In particular the ＂energy conservation condition＂ and the ＂kinematic existence condition＂ as proposed by Tan et al. （2005） is re-examined, showing that the ＂energy conservation condition＂ and the consequent ＂critical velocity＂, i.e. the shock can only be generated and sustained in R-PP-L bars when the impact velocity is above this critical velocity, is incorrect. Instead, with elastic deformation, strain-hardening and strain-rate sensitivity of the cellular materials being considered, it is appropriate to redefine a first and a second critical impact velocity for the existence and propagation of shock waves in cellular solids. Starting from the basic relations for shock wave propagating in D-R-LHP-L cellular materials, a new method for inversely determining the dynamic stress-strain curve for cellular materials is proposed. By using e.g. a combination of Taylor bar and Hopkinson pressure bar impact experimental technique, the dynamic stress-strain curve of aluminum foam could bedetermined. Finally, it is demonstrated that this new formulation of shock theory in this one-dimensional stress state can be generalized to shocks in a one-dimensional strain state, i.e. for the case of plate impact on cellular materials, by simply making proper replacements of the elastic and plastic constants.
基金supported by the National Natural Science Foundation of China(No.11032001)the K.C.Wong Magna Fund in Ningbo University
文摘The split Hopkinson pressure bar（SHPB） technique and the wave propagation inverse analysis（WPIA） technique are both extensively used to experimentally investigate the impact behavior of materials, although neither of them alone provides a fully satisfactory analysis. In the present paper, attention is given to new experimental techniques by incorporating a damagemodified constitutive model into the SHPB technique and combining the Hopkinson pressure bar（HPB） technique with WPIA. First, to distinguish the response due to dynamic constitutive behavior and the response due to dynamic damage evolution, the SHPB method incorporating a damage-modified constitutive model is developed, including an explicit damage-modified Zhu–Wang–Tang model and an implicit damage-modified constitutive model. Second, when the SHPB results become invalid, a method of combining new Lagrange inverse analyses with the HPB technique is developed, including cases of the HPB arranged in front of a long specimen and behind the specimen. As examples of these new methods, typical results are given for nonlinear viscoelastic polymers and concretes considering damage evolution, a super-elastic Ti–Ni alloy with phase transformation and an aluminum foam with shock waves propagating within it.
基金Supported by the National Natural Science Foundation of China under Grant No 10672082, the Scientific Research Fund of Zhejiang Provincial Education Department under Grant No 20061673, and K. C. Wong Magna Fund in Ningbo University.
文摘Critical impact velocity （CIV） of oxygen-free high-conductivity （OFHC） copper is experimentally measured with a novel facility in a gas gun system. The results are compared with the theoretical predictions using the typical constitutive relations, and the measured CIV value is much lower than the predictions. The difference of physical mechanisms in experiment and in theoretical calculation is discussed. It is suggested that the reduction of CIV in experiment would be related with the damage evolution in tensile copper that needs to be considered in the computation model.
基金supported by the National Natural Science Foundation of China(No.10572064)the National Basic Research Program of China(973 Program,2009CD623203)+1 种基金K.C.Wong Magna Fund in Ningbo Universitythe Natural Science Foundation of Zhejiang Province(No.Y107780).
文摘The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.
文摘The wave dispersion due to the lateral inertia in the split Hopkinson pressure bar(SHPB) with large-(diameter) bar is numerically analyzed by means of the LS-DYNA3D code. The results show that, ① the stress distribution across the bar section is non-uniform along the radius direction and such non-uniformity depends on the material Poisson ratio and propagation distance; ② with increasing the bar diameter, the high frequency oscillations are notably enhanced and the rise time of wave front becomes longer, meanwhile the amplitude of the stress wave attenuates; ③ with decreasing the rise time of wave front, the wave dispersion markedly enhanced, particularly in the large diameter bar. All of those effects should not be neglected in order to obtain accurate results by the SHPB test..
基金Project supported by the Qianjiang River Fellow Fund of ZhejiangProvince, and Bureau of Personnel and Human Resource, Ningbo,China
文摘The determination of the precise thickness-shear frequency of electroded crystal plates has practical importance in quartz crystal resonator design and fabricatiom especially when the high fundamental thickness-shear frequency has reduced the crystal plate thickness to such a degree that proper consideration of the effect of electrodes is very important. The electrodes effect as mass loading in the estimation of the resonance frequency has to be modified to consider the stiffness of electrodes, as the relative strength is increasingly noticeable. By following a known procedure in the determination of the thickness-shear frequency of an infinite AT-cut crystal plate, frequency equations of crystal plate without and with piezoelectric effect are obtained in terms of elastic constants and the electrode material density. After solving these equations for the usual design parameters of crystal resonators, the design process can be optimized to pinpoint the precise configuration to avoid time-consuming trial and reduction steps. Since these equations and solutions are presented for widely used materials and parameters, they can be easily integrated into the existing crystal resonator design and manufacturing processes.
基金financial support through the contact No. SCI610022S, and Center of Excellence in Materials Science and Technology, Chiang Mai University under the administration of Materials Science Research Center, Faculty of Science, Chiang Mai University, Thailand
文摘A facile sonochemical method was developed to synthesize metallic Ag spherical nanoparticles on the surface of ZnWO4 nanorods by forming heterostructure Ag/ZnWO4 nanocomposites.The Ag/ZnWO4 nanocomposites were characterized by X-ray powder diffraction (XRD),Fourier transform infrared spectroscopy (FTIR),field emission scanning electron microscopy (FESEM),transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).The experimental results showed that fcc metallic Ag nanoparticles were supported on surface of monoclinic sanmartinite ZnWO4 nanorods.The Ag 3d3/2 and Ag 3d5/2 peaks have well-separated binding energies of 6.00 eV,certifying the existence of metallic Ag.The Ag/ZnWO4 nanocomposites were evaluated for photodegradation of methylene blue (MB) induced by ultraviolet-visible (UV-Vis) radiation.In this research,heterostructure 10 wt% Ag nanoparticle/ZnWO4-nanorod composites have the highest photocatalytic activity of 99% degradation of MB within 60 min.The increase in photocatalytic activity was the result of photoinduced electrons in conduction band of ZnWO4 that effectively diffused to metallic Ag spherical nanoparticles and the inhibition of electron-hole recombination process.
基金supported by the NatiS100onal Science Foundation of China under Grant No.11072119the Defense Industrial Technology Development Program under Grant No.B1520110003+2 种基金the K.C.Wong Magna Foundation of Ningbo University,Chinaa grant from the Department of Education of Zhejiang Province through the Impact and Safety of Costal Engineering Initiativea COE Program at Ningbo University
文摘The dynamic fracture behaviors of the extruded 2024-T4 and 7075-T6 aluminum alloys are investigated by using an instrumented drop tower machine.The specimens are made from a 25 mm diameter extruded circular rod.The dynamic three-point bending tests of each alloy are carried out at different impact velocities.The initiation fracture toughness and average propagation fracture toughness of 2024-T4 and 7075-T6 are determined at different loading rates.The results show that both the initiation toughness and the propagation toughness increase with the loading rate.Further,the difference between the fracture toughness behaviors of 2024-T4 and 7075-T6 is found to be dependent on the variation of fracture mechanism.The comprehensive fractographic investigations of the fracture surfaces clearly demonstrate that the fracture mode of 2024-T4 is predominantly transgranular fracture with high density small-sized dimples,and the fracture mode of 7075-T6 is mainly intergranular fracture with many intermetallic particles in the bottom of voids located in the fracture surface.
基金financially supported by the National Natural Science Foundation of China (No. 11032001)
文摘When manned spacecraft comes back to the earth, it relies on the impact attenuation seat to protect astronauts from injuries during landing phase. Hence, the seat needs to transfer impact load, as small as possible, to the crew. However, there is little room left for traditional seat to improve further. Herein, a new seat system biologically-inspired by felids＇ landing is proposed. Firstly, a series of experiments was carried out on cats and tigers, in which they were trained to jump down voluntarily from different heights. Based on the ground reaction forces combined with kinematics, the experiment indicated that felids＇ landing after self-initial jump was a multi-step impact attenuation process and the new seat was inspired by this. Then the construction and work process of new seat were redesigned to realize the multi-step impact attenuation. The dynamic response of traditional and new seat is analyzed under the identical conditions and the results show that the new concept seat can significantly weaken the occupant overload in two directions compared with that of traditional seat. As a consequence, the risk of injury evaluated for spinal and head is also lowered, meaning a higher level of protection which is especially beneficial to the debilitated astronaut.
基金Supported by the National Natural Science Foundation of China under Grant No 10472048, and the Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, CAEP, under Grant No 9140C6702020603.
文摘It is necessary to study the validation of strength models under planar shock loading in view of the fact that strength models for metals obtained at moderate strain rates are often used in the numerical simulations of shock wave phenomena. The variations of longitudinal stress, transverse stress and yield strength of oxygen-free high conductance （OFHC） copper with time under planar shock loading are obtained by using the manganin stress gauges and compared with the predicted results by the constructed seven constitutive models based on Y/G=constant and on G/B=constant （Y the yield strength, G the shear modulus, B the bulk modulus）, respectively. It seems that the pressure, density, temperature and plastic strain dependence of the yield strength for OFHC copper under planar shock loading is essential to the constitutive description.
基金financially supported by Thailand's Office of the Higher Education Commission through the National Research University Project for Chiang Mai University
文摘In this research, cerium （III） nitrate hexahydrate （Ce（NO3）3·6H2O） and ammonium molybdate tetrahydrate （（NH4）6Mo7O24·4H2O） with Ce3＋-to-Mo6＋ molar ratio of 2：3 were dissolved in 40 ml different solvents of deionized （DI） water, polyethylene glycol （PEG） and ethylene glycol （EG） to form different solutions which were followed by adjusting pH from the traditional values to 7.0 and 10.0 with 1 mol.L-1 sodium hydroxide （NaOH）. Subsequently, the solutions were processed by 270-W microwave-hydrother- mal/solvothermal method. Phase, morphology, vibrational modes and photonic properties were fully characterized by X-ray powder diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, Fourier transform infrared spectroscopy （FTIR）, Raman spectrophotometry, ultraviolet-visible （UV-Vis） absorption and photoluminescence （PL） spectroscopy. The as-synthe-sized products were pure cerium molybdenum oxide （Ce2（MoO4）3） of nanoparticles clustered together as nano- plates in DI water and PEG solvents, and of spindle-like nanoparticles in EG solvent, including the presence of Ce-O-H mode and MoO4 units. The results show that direct energy gaps of the first two have the same value of 2.30 eV, and that of the last is 2.80 eV, including their blue emission at the same wavelength of 488 nm.
文摘Split Hopkinson technique has been developed to test the strength of common concrete and steel fiber reinforced concrete under dynamic tensile stress. Two types of test methods are considered, the splitting tensile test and a modified spalling test in which a specimen is loaded under uniaxial stress. The result shows that the dynamic strength enhancement of concrete is remarkable by using the reinforcing fiber. But for the common concrete, the base of compressive strength seems to show little effect on the tensile strength under dynamic loading. The experimental results also show that the resistance to tensile fracture of the steel fiber reinforced concrete for C100-mix is higher than those of C40-mix..
基金the National Nanotechnology Center (NANOTEC)National Science and Technology Development Agency, Thailand, for providing financial support through the project code: P-10-11345+1 种基金the Thailand's Office of the Higher Education Commission through the National Research University Projectthe Thailand Research Fund (TPF) through the TPF Basic Research Grant,includ-ing the Graduate School of Chiang Mai University through the general support
文摘Monochnic and hexagonal CePO4 nanoparticles and nanorods were successfully synthesized from Ce（NO3）36H2O and Na3PO4 121-120 solu- tions at pH 1-5 by a 180 W microwave radiation for 60 min. The products were characterized by X-ray diffraction （XRD）, Fourier transform infrared （FFIR） spectroscopy, and scanning electron microscopy （SEM）. XRD patterns revealed that the products are hexagonal CePO4 structures at pH 2-5, and monoclinic CePO4 structtLres at pH 1. SEM characterization shows that these products were nanoparticles, short nanorods, and long nanorods, controlled by the pH of the precursor solutions. Optical properties of the nanorods were also investigated by ultraviolet-visible （UV-vis） and photoluminescence （PL） spectroscopy.
基金financially supported by the Chiang Mai University Research Fundthe administration of Materials Science Research Center,Faculty of Science,Chiang Mai University,Thailand。
文摘ZnO particles doped with 0 mol%-5 mol% Cd dopant were successfully synthesized by a precipitation method.The as-synthesized products were characterized by thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),X-ray diffraction(XRD),photoluminescence(PL) spectroscopy,Raman spectroscopy,scanning electron microscopy(SEM) coupled with an energydispersive X-ray(EDX) spectroscopy and transmission electron microscopy(TEM).The precursors showed weight loss at 35-700℃ caused by phase transformation and evaporation processes.The as-synthesized products were specified as hexagonal wurtzite ZnO.The crystallite size of ZnO samples doped with 0 mol%-5 mol% Cd gradually increased with the increase in Cd content.The photocatalytic activities of 0 mol%-5 mol% Cd-doped ZnO samples were evaluated through photodecolorization of methylene blue(MB) under ultraviolet(UV) radiation.In this research,ZnO doped with 3 mol% Cd shows the best photocatalytic activity in degrading of MB molecules as much as 89% within 240 min.
基金the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science,Information and Communications Technology and Future Planning(Nos.2019R1A2C2084453,2021R1A2C2006032,and 2021R1C1C2011469),Republic of Korea。
文摘Numerous studies highlight advantages of electrospun scaffolds in bone tissue engineering,in which cellular behavior is tightly affected by fiber topographical cues of scaffolds.However,the classic electrospinning setup limits a desired presentation of biomimetic fibrous microenvironments that sense mechanosignaling and regulate stem cell behavior.The aims of this study were to fabricate advanced asspun scaffolds presenting tree-like microfiber/nanonet networks and to evaluate their regulatory potentials on behavior of human mesenchymal stem cells(h MSCs)and bone regeneration.Here we developed a novel electrospinning setup that allowed the presentation of patterned Trunk microfibers(TMF)and/or branched nanonet fibers(BNn Fs)in biomimetic fibrous scaffolds.As the cellular mechanisms,anisotropichierarchical topography of TMF controlled behavior of h MSCs through focal adhesion formation and Yesassociated protein(YAP)induction,whereas BNn F disturbed such mechanosensing responses in the cells.The fiber microenvironment-related expression and nuclear localization of YAP were.also correlated with the potentials of as-spun scaffolds to enhance osteogenic differentiation of the h MSCs and alveolar bone defect healing in an animal model.Collectively,this study provides an advanced approach of the modified electrospinning setup for presentation of biomimetic fibrillar microenvironments in as-spun scaffolds along with their application in stem cell behavior regulation and regenerative tissue engineering.
文摘Following the original approach of Bowden and Tabor and introducing state variables, an effective friction coefficient μ_e for solid particle erosion is defined as a combination of shearing term and ploughing term. In the case of continuous sliding, based on considering the interaction between asperities under certain condition, it is indicated that during the oblique impact of a hardened steel sphere against a mild steel target, a possible value of μ_e is 0.05, which was chosen in all of the calculations by Hutchings for consistency with both experiments and calculations. In the case of continuous ploughing, it is shown that the value of μ_e is a function of the impact process and the initial impact angle and is greater than 0.05 on an average for Hutchings' experiments. It is suggested that the variation of sliding, rolling and ploughing state at each instant in the impact process makes “the coefficient of friction” equal to 0.05 for Hutchings' experiments, and in general, makes the effective friction coefficient during particle impact on metal far less than the friction coefficient during simple continuous sliding on an average.
