Comparative Study on Microstructure and Mechanical Properties of Coarse-grained WC-based Cemented Carbides Sintered with Ultrafine WC or (W+C) as Additives

The effects of ultrafine WC(WC_(UF),0.5μm) or W(1μm) and C(0.3μm)(W+C)_(UF) additives on the densification,microstructure and mechanical properties of coarse-grained cemented carbides were compared systematically.Overall,the cemented carbides with WC_(UF)/(W+C)_(UF) additives are almost fully densification to be higher than 99%,and the average grain size is kept above 2.8μm.The WC_(UF) additive assists grains to(truncated)trigonal prism shape by two dimensional(2D) growth,whereas the(W+C)_(UF) additive assists grains to rounded shape by three dimensional(3D) growth,lowers WC contiguity and increases face-centered-cubic Co.The hardness and bending strength of(75WC_(C)-15WC_(UF))-10Co are 86.6 HRA and 2 272 MPa,respectively,both higher than those of(75WC_(C)-15(W+C)_(UF))-10Co,which could be ascribed to the enhanced densification and unblemished grains.However,the fracture toughness of the(75WC_(C)-15(W+C)_(UF))-10Co is 23.5 MPa·m^(1/2),higher than that of the(75WC_(C)-15WC_(UF))-10Co due to the uniform WC-Co structure and flexible binder phase.

Seismic safety assessment with non-Gaussian random processes for train-bridge coupled systems

Extensive high-speed railway(HSR)network resembled the intricate vascular system of the human body,crisscrossing mainlands.Seismic events,known for their unpredictability,pose a significant threat to both trains and bridges,given the HSR’s extended operational duration.Therefore,ensuring the running safety of train-bridge coupled(TBC)system,primarily composed of simply supported beam bridges,is paramount.Traditional methods like the Monte Carlo method fall short in analyzing this intricate system efficiently.Instead,efficient algorithm like the new point estimate method combined with moment expansion approximation(NPEM-MEA)is applied to study random responses of numerical simulation TBC systems.Validation of the NPEM-MEA’s feasibility is conducted using the Monte Carlo method.Comparative analysis confirms the accuracy and efficiency of the method,with a recommended truncation order of four to six for the NPEM-MEA.Additionally,the influences of seismic magnitude and epicentral distance are discussed based on the random dynamic responses in the TBC system.This methodology not only facilitates seismic safety assessments for TBC systems but also contributes to standard-setting for these systems under earthquake conditions.

Preparation and properties of Ni-coated WC powder and highly impact resistant and corrosion resistant WC-Ni cemented carbides

WC powders were uniformly coated by Ni nanoparticles through a combined chemical co-precipitation and subsequent high temperature hydrogen reduction strategy(abbreviated as CM-WCN),and then were consolidated by vacuum sintering at 1450°C for 1 h to obtain WC−Ni cemented carbides.The microstructure and properties of the as-consolidated CM-WCN were investigated.The average grain size of WC in the consolidated CM-WCN was calculated to be in the range of 3.0−3.8μm and only few pores were observed.A relative density of 99.6%,hardness of HRA 86.5 and bending strength of 1860 MPa were obtained for the CM-WCN−10wt.%Ni,and the highest impact toughness of 6.17 J/cm^(2 )was obtained for the CM-WCN−12wt.%Ni,surpassing those of the hand mixed WC−Ni(HM-WCN)cemented carbides examined in this study and the other similar materials in the literature.CM-WCN cemented carbides possess excellent mechanical properties,due to their highly uniform structure and low porosity that could be ascribed to the intergranular-dominated fracture mode accompanied by a large number of plastic deformation tears of the bonding phase.In addition,the corrosion resistance of CM-WCN was superior to that of HM-WCN at the Ni content of 6−12 wt.%.

Influence of principal stress rotation of unequal tensile and compressive stress amplitudes on characteristics of soft clay

Soil behavior can reflect the characteristics of principal stress rotation under dynamic wave and traffic loads. Unequal amplitudes of tensile and compressive stresses applied to soils have complex effects on foundation soils in comparison with the pure principal stress rotation path. A series of undrained cyclic hollow torsional shear tests were performed on typical remolded soft clay from the Hexi area of Nanjing, China. The main control parameters were the tensile and compressive stress amplitude ratio(α) and the cyclic dynamic stress ratio(η). It was found that the critical η tended to remain constant at 0.13, when the value of the compressive stress amplitude was higher than the tensile stress amplitude. However, the influence of the tensile stress was limited by the dynamic stress level when α= 1.For obvious structural change in the soil, the corresponding numbers of cyclic vibration cycles were found to be independent of α at low stress levels and were only related to η. Finally, a new method for evaluating the failure of remolded soft clay was presented. It considers the influence of the tensile and compressive stresses which caused by complex stress paths of the principal stress rotation. This criterion can distinguish stable, critical, and destructive states based on the pore-water-pressure-strain coupling curve while also providing a range of failure strain and vibration cycles. These results provide the theoretical support for systematic studies of principal stress rotation using constitutive models.

Numerical investigations on mechanical characteristics and failure mechanism of outwash deposits based on random meso-structures using discrete element method

Outwash deposit is a unique type of geological materials, and its features such as heterogeneity, discontinuity and nonlinearity determine the complexity of mechanical characteristics and failure mechanism. In this work, random meso-structure of outwash deposits was constructed by the technique of computer random simulation based on characteristics of its meso-structure in the statistical sense and some simplifications, and a series of large direct shear tests on numerical samples of outwash deposits with stone contents of 15%, 30%, 45% and 60% were conducted using the discrete element method to further investigate its mechanical characteristics and failure mechanism under external load. The results show that the deformation characteristics and shear strength of outwash deposits are to some extent improved with the increase of stone content, and the shear stress–shear displacement curves of outwash deposits show great differences at the post-peak stage due to the random spatial distribution and content of stones. From the mesoscopic view, normal directions of contacts between "soil" and "stone" particles undergo apparent deflection as the shear displacement continues during the shearing process, accompanying redistribution of the magnitude of contact forces during the shearing process. For outwash deposits, the shear zone formed after shear failure is an irregular stripe due to the movements of stones near the shear zone, and it expands gradually with the increase of stone content. In addition, there is an approximately linear relation between the mean increment of internal friction angle and the stone content lying between 30% and 60%, and a concave nonlinear relation between the mean increment of cohesion and stone content, which are in good agreement with the existing research results.

Primary and secondary consolidation compression for saturated soil considering coupling effect of loading and heating

Geotechnical engineering that relates to the energy and environmental problem is receiving more and more attention worldwide.It is of great theoretical and practical value to study the properties of soil under thermal mechanical coupling and its mathematical description.Firstly,based on the general function,a unified primary and secondary consolidation model of saturated soil considering heating temperature is deduced.Combining the existing research achievements,a practical model is obtained which comprehensively reflects the effective stress change,creep and heating effects.After that,a series of thermo-consolidation tests are carried out using a temperature controlled consolidation instrument to study the effects of effective stress,temperature and consolidation duration on saturated soils.The corresponding functional formulas and parameters are obtained thusly.On this basis,the calculation and analysis are carried out to check the reliability and applicability of the newly proposed model.The new model is simple and practical and the parameters are easy to be obtained.And it describes the main law of consolidation compression of saturated soils under the thermal mechanical coupling effect.Therefore,it is suggested for theoretical analysis of thermal geotechnical engineering problems.

Reduction Rate of Dragload and Downdrag of Piles by Taper Angles

Taper angle is one of the effective methods to reduce the dragload and downdrag of piles under the surcharge load. The model tests on the tapered pile and uniform cross-section pile embedded in sand were carried out under the surcharge load. The values of dragload and downdrag of piles versus the surcharge loads were measured. Based on the concentric cylinder shearing theory, a simplified theoretical model for calculating the dragload and downdrag of tapered piles with small taper angles under the surcharge load was proposed considering the angle effect. The correctness of the developed theoretical model was validated through comparing with the model test results obtained in this study and previous literature. Then the parametric studies of the taper angle, surcharge, strength and modulus of soil were discussed. It is shown that the tapered pile with the taper angle of 3° can reduce approximately 65% of the maximum dragload value and 30% of the downdrag value compared with a uniform cross-section pile. The value of downdrag can be decreased by approximately 50% compared with a uniform crosssection pile in the same average pile diameter.

Electroless Plating of Ni Nanoparticles on WC to Assist Its Pressureless Sintering of WC-Ni Cemented Carbide with Enhanced Mechanical and Corrosion-resistant Performance

Ni nanoparticles were coated uniformly on the surface of WC powder via a facile electroless plating method(abbreviated as WCN-EP),and then consolidated for mechanical and corrosion resistance performance characterization,in comparison with hand mixed WC-Ni(WCN-H).Under the optimized electroless plating parameters,Ni particles,less than 1μm in average diameter,were found to be uniformly and densely wrapped on the surface of the tungsten carbide matrix of WCN-EP.In comparison,in WCN-H,the Ni particles about 1.8μm in average diameter,were randomly distributed together with irregular WC particles.The uniform coating of Ni was found to assist the densification process of WCN-EP effectively,with higher densities and less pores than those of WCN-H at the Ni content of 10.6wt%,25.5wt%,and 30.3 wt%.However,at the Ni content of 18.8wt%,the relative densities of WCN-EP and WCN-H both increased to the maximum value of 98%.The maximum hardness of the consolidated WCN-EP was 82.6 HRA,about 1.2 HRA higher than that of WCN-H.In addition,the consolidated WCN-EP also exhibits a superior corrosion resistance by the polarization curve analysis at an electrochemical workstation.

Scattering of SH waves induced by a symmetrical V-shaped canyon: a unified analytical solution

This paper reports a series solution of wave functions for two-dimensional scattering and diffraction of plane SH waves induced by a symmetrical V-shaped canyon with different shape ratios. A half-space with a symmetrical V-shaped canyon is divided into two sub-regions by using a circular-arc auxiliary boundary. The two sub-regions are represented by global and local cylindrical coordinate systems, respectively. In each coordinate system, the wave field satisfying the Helmholtz equation is represented by the separation of variables method, in terms of the series of both Bessel functions and Hankel functions with unknown complex coefficients. Then, the two wave fields are described in the local coordinate system using the Graf addition theorem. Finally, the unknown coefficients are sought by satisfying the continuity conditions of the auxiliary boundary. To consider the phase characteristics of the wave scattering, a parametric analysis is carried out in the time domain by assuming an incident signal of the Ricker type. Surface and subsurface transient responses demonstrate the characteristics and mechanisms of wave propagating and scattering.

Role of equal-strain assumption in unit-cell theory for consolidation with vertical drains

In the development of unit-cell theory for the analytical analysis of consolidation with vertical drains, the equal-strain assumption is often made with the intention of modelling consolidation under uniform settlement conditions. In contrast, the free-strain assumption for modelling consolidation under uniform load conditions is seldom employed, mainly because of the complexities involved in the analysis. This study derives a rigorous analytical solution to the generalised governing equations of free-strain consolidation with a vertical drain subjected to an instantaneous load. Calculated results from the newly proposed solution are compared with those from three available solutions derived based on the equal-strain assumption. Surprisingly good agreement is obtained in terms of excess pore-water pressure, degree of consolidation, and settlement. Horizontal profiles of settlement were not uniform before the end of consolidation. This indicates that the uniform settlement condition is not actually reproduced by the analytical solutions derived based on the equal-strain assumption. The equal-strain assumption is a sufficient but not necessary condition for deriving an analytical solution to unit-cell consolidation theory. The assumption plays no role in modelling consolidation under uniform settlement conditions but simplifies the analytical analysis of free-strain consolidation and results in an approximate solution of high accuracy for consolidation under uniform load conditions. Moreover, drain resistance and smear effects not only retard the consolidation rate, but also importantly shape the vertical and horizontal profiles of excess pore-water pressure, respectively.

A general solution for vertical-drain consolidation with impeded drainage boundaries

An analytical solution is derived from the generalized governing equations of equal-strain consolidation with vertical drains under multi-ramp surcharge preloading. The hydraulic boundary conditions at both top and bottom of the consolidating soil are modelled as impeded drainage. The impeded drainage is described by using the third type boundary condition with a characteristic factor of drainage efficiency. Fully drained and undrained boundary conditions can also be modelled by applying an infinite and a zero characteristic factor, respectively. Simultaneous radial and vertical flow conditions are considered, together with the effects of drain resistance and smear. An increase in total stress due to multi-ramp loading is reasonably modelled as a function of both time and depth. A solution to calculate excess pore-water pressure at any arbitrary point in soil is derived, and the overall average degree of consolidation is obtained. It shows that the proposed solution can be used to analyze not only vertical-drain consolidation but also one-dimensional consolidation under either one-way or two-way vertical drainage conditions. The characteristic factors of drainage efficiency of top and bottom boundaries have a potentially important influence on consolidation. The boundary may be considered fully drained when the characteristic factor is greater than 100 and fully undrained when the characteristic factor is less than 0.1. The stress distribution along depth induced by the surcharge loading has a limited effect on the overall average degree of consolidation.

Is Terzaghi's effective stress a stress variable under seepage conditions?

From the continuum mechanics perspective, an attempt was made to clarify the role of Terzaghi's effective stress in the theoretical analysis of saturated soil subjected to seepage. The necessity of performing a coupled hydromechanical analysis to solve the seepage-deformation interaction problem was illustrated by examining the equations of static equilibrium among the effective stress, seepage force, pore-water pressure and total stress. The conceptual definition of stress variable that satisfies the principles of continuum mechanics is applied in the coupled hydromechanical analysis. It is shown that Terzaghi's effective stress is in fact not a stress variable under seepage conditions, and the seepage force acting on the soil skeleton cannot be viewed as a body force. This offers a clue to the underlying cause of a paradox between the real Pascal's hydrostatic state and the hydrostatic state predicted by a class of continuum hydromechanical theories.

Centrifuge modeling of tunneling-induced ground surface settlement in sand

Stress changes in the soil induced by tunnel excavation may cause excessive ground settlement.However,high-quality experimental data on ground settlement due to tunnel excavation are limited.In this study,centrifuge tests are conducted to investigate the threedimensional ground surface settlement,considering different intersection angles and cover-to-tunnel diameter ratios.The results indicate that the major influence zone along the longitudinal direction on the ground surface settlement is±1.25D,where D is the tunnel diameter.When the monitoring section is perpendicular to the tunneling direction,the transverse ground settlement due to the tunnel excavation is symmetrical with respect to the tunnel centerline.In contrast,an asymmetric ground settlement profile is observed when the monitoring section intersects the tunneling direction at an angle of 60.Applying a Gaussian curve to fit the ground surface settlement curve,the width parameter,K(i.e.,the distance between the tunnel centerline and the inflection point of the settlement trough to the tunnel burial depth),varies from 0.33 to 0.39.The ground surface settlement induced by twin tunnel excavation can be captured reasonably by superimposing two identical Gaussian curves.When the cover to tunnel diameter ratios(C/D)are 1.5 and 2.7,the maximum ground surface settlements are 0.67%of D and 0.35%of D,respectively.It is clear that the maximum ground surface settlement decreases with an increase in the C/D ratio.

Facile fabrication of dolomite-doped biochar/bentonite for effective removal of phosphate from complex wastewaters

The removal of phosphate from wastewater using traditional biological or precipitation methods is a huge challenge.The use of high-performance adsorbents has been shown to address this problem.In this study,a novel composite adsorbent,composed of dolomite-doped biochar and bentonite(DO/BB),was first synthesized via co-pyrolysis.The combination of initial phosphate concentration of 100 mg/L and 1.6 g/L of DO/BB exhibited a high phosphate-adsorption capacity of 62 mg/g with a removal efficiency of 99.8%.It was also stable in complex water environments with various levels of solution pH,coexisting anions,high salinity,and humic acid.With this new composite,the phosphate concentration of the actual domestic sewage decreased from 9 mg/L to less than 1 mg/L,and the total nitrogen and chemical oxygen demand also decreased effectively.Further,the cross-flow treatment using a PVC membrane loaded with DO/BB(PVC-DO/BB),decreased the phosphate concentration from 1 to 0.08 mg/L,suggesting outstanding separation of phosphate pollutants via a combination of adsorption and separation.In addition,the removal of phosphate by the PVC-DO/BB membrane using NaOH solution as an eluent was almost 90%after 5 cycles.The kinetic,isotherm and XPS analysis before and after adsorption suggested that adsorption via a combination of electrostatic interaction,complexation and precipitation contributed to the excellent separation by the as-obtained membranes.

香港科技大学土工离心机先进模拟技术在岩土工程中的应用(英文)

研究目的:采用香港科技大学的先进土工离心模拟技术来研究和解决复杂的岩土工程问题。创新要点:1.验证竖向钻孔开挖技术(应力释放)对建筑纠偏的有效性;2.研究隧道坍塌对其邻近既有隧道的影响;3.研究基坑开挖对坑中既有桩基承载力的影响;4.揭示不同颗粒级配形成的土坡在水位上升和动力荷载作用下的破坏模式。研究方法:1.用香港科技大学全球首台离心机中的双向震动台(见图5)模拟地震荷载对土坡的影响;2.用香港科技大学全球第二台四轴机械手(见图6)模拟不停机情况下的钻孔开挖。重要结论:1.竖向钻孔开挖技术能有效地对建筑物进行纠偏;2.隧道坍塌会对其邻近既有隧道产生很大的附加弯矩,尤其是拱脚处的弯矩可增加多达228%;3.基坑开挖后坑中桩基的承载力取决于桩土接触面的粗糙程度,粗糙的桩-土接触面在剪切过程中倾向于发生剪胀,这会增大桩周围土的水平土压力,从而使桩基承载力增加;4.当水位上升时,颗粒均匀,级配差的松散砂土坡容易发生静态液化;相反地,颗粒级配好的松散砂土坡(风化土)不太可能发生静态液化,而只是发生整体滑动破坏;5.离心机双向震动台实验显示松散的风化岩土坡在0.3g的地震加速度作用下不会发生液化,可以抵御香港地区的地震荷载(0.08g到0.11g)。

砂土地基中摩擦型能量桩群桩热力学特性研究（英文）

目的:能量桩在工作状态下的热力学响应十分复杂,同时受到桩顶荷载、桩侧摩擦以及温度等多重因素的影响。当群桩中出现部分能量桩不工作时,将造成上部结构的额外应力与变形。因此,本文重点探讨摩擦型能量桩群桩中部分能量桩在加热制冷作用下的热力学响应,并与单桩的热力学效应进行对比分析。创新点:1.通过建立摩擦型能量桩群桩模型试验,探讨桩侧摩擦对能量桩群桩的影响规律;2.利用能量桩群桩与单桩对比,揭示能量桩群桩与单桩热力响应特性的区别;3.揭示部分能量桩加热制冷作用对能量桩群桩的影响机理。方法:1.建立摩擦型能量桩群桩及单桩的模型试验;2.将能量桩群桩与单桩进行对比,研究能量桩群桩与单桩热力响应特性的区别;3.进行能量桩群桩部分加热制冷试验。结论:1.对于长期工作的能量群桩,可以将其视为一个长宽高与整个群桩相同的热交换体,其表面温度与群桩的平均表面温度一致。2.能量桩单桩在加热过程中,由于桩底受到的限制较大,所以桩顶位移大于桩底位移。3.能量桩单桩在制冷过程中,由于土体及桩体收缩,会出现明显的下沉。4.能量桩群桩桩帽在加热过程中,桩帽的位移与群桩的上半部分长度相关;在本文的试验中,由于群桩上半部分受土的限制较小,因此其位移与桩自由膨胀的位移一样。5.能量桩群桩在制冷期间,群桩的下沉量级要比单桩的大。6.在制冷过程中,能量桩群桩在群桩效应作用下,内部桩的桩底热位移较大。7.能量桩群桩在部分加热的情况下,会出现不均匀沉降,且在加热期间,沉降主要受到不工作桩的牵制影响;而在制冷期间,沉降主要受工作桩的下沉影响。8.摩擦型能量桩的热引起的桩身轴力是与桩侧的土压力大小相关的;由于群桩在群桩效应作用下,桩侧土压力要小于单桩,因此群桩的热引起的桩身轴力要大于单桩。