Micro sliding friction model considering periodic variation stress distribution of contact surface and experimental verification

Micro sliding phenomenon widely exists in the operation process of mechanical systems,and the micro sliding friction mechanism is always a research hotspot.In this work,based on the total reflection method,a measuring device for interface contact behavior under two-dimensional(2D)vibration is built.The stress distribution is characterized by the light intensity distribution of the contact image,and the interface contact behavior in the 2D vibration process is studied.It is found that the vibration angle of the normal direction of the contact surface and its fluctuation affect the interface friction coefficient,the tangential stiffness,and the fluctuation amplitude of the stress distribution.Then they will affect the change of friction state and energy dissipation in the process of micro sliding.Further,an improved micro sliding friction model is proposed based on the experimental analysis,with the nonlinear change of contact parameters caused by the normal contact stress distribution fluctuation taken into account.This model considers the interface tangential stiffness fluctuation,friction coefficient hysteresis,and stress distribution fluctuation,whose simulation results are consistent well with the experimental results.It is found that considering the nonlinear effect of a certain contact parameter alone may bring a greater error to the prediction of friction behavior.Only by integrating multiple contact parameters can the accuracy of friction prediction is improved.

基于Micro-CT的BALB/c小鼠肺腺瘤动物模型研究

目的 建立基于Micro-CT动态表征的BALB/c小鼠肺腺瘤动物模型研究方法。方法 取80只SPF级雌性BALB/c小鼠,随机分为4组:模型低剂量组(1 mg/g乌拉坦腹腔注射1次)、模型中剂量组(1 mg/g乌拉坦腹腔注射,每周1次,连续2周)、模型高剂量组(1 mg/g乌拉坦腹腔注射,每周1次,连续4周)和空白组(等体积生理盐水腹腔注射)。采用Micro-CT定期监测小鼠肺结节生长情况,Analyze 12.0分析系统绘制小鼠肺部3D图像,苏木素-伊红(HE)染色观察肺组织病理学变化。结果 与空白组相比,各模型组小鼠第11周时均观测到类圆形高密度影的肺结节。结节形成率随造模周数增加而升高,至第21周时,模型高、中、低剂量组结节形成率分别为93.8%、93.8%、87.5%;结节数分别以2~4个、1个、1~2个为主;低剂量组肺结节最大直径平均值高于中、高剂量组(P<0.05);肺结节体积三组之间差异无统计学意义。HE染色结果显示模型高、中、低剂量组病理类型均为肺腺瘤。结论 模型各剂量组均成功诱导肺腺瘤,Micro-CT可对小鼠肺结节生长情况进行表征,其中模型中剂量组结节形成率高,肺腺瘤数量适中,模型稳定,更适合小鼠肺腺瘤动物模型的研究。

基于Micro-CT评价超声骨焊接技术PDLLA材料对骨愈合性能的影响

目的采用微焦点断层扫描(Micro-CT)评价超声骨焊接技术应用过程中超声震荡产热作用下PDLLA材料对骨愈合性能的影响。方法选择SPF级雄性新西兰大白兔36只,随机分为超声骨焊接技术辅助PDLLA材料组(A组)、拧入技术辅助PDLLA材料组(B组)、假手术组(C组)、空白对照组(D组),各9只。于术后4、8、12周取下颌骨标本,周围骨组织进行HE染色,观察各组植入钉周围情况。对术后4、8、12周的下颌骨大体标本拍摄Micro-CT图,使用VG Studio MAX软件进行三维重建,摆正数据样本,即颌骨颊侧造模处为轴向,动态分析植入钉体积、植入钉周围200μm环状区及骨缺损处的植入材料体积数、相对骨体积分数(BV/TV)、骨小梁厚度(Tb.Th)、骨小梁数(Tb.N)、骨小梁间隙(Tb.Sp)等骨愈合相关指标。结果HE染色结果显示,按3个时间可动态观察到A、B、C组骨缺损处骨性骨痂替代纤维性骨痂,形成类骨质,编织骨的过程符合正常骨损伤愈合的过程。Micro-CT结果显示,A、B组术后3个时间点的材料体积比较差异均无统计学意义(P>0.05);在3个时间点,4组植入钉周围200μm环状区中BV/TV、Tb.Th、Tb.N、Tb.Sp比较差异均无统计学意义(P>0.05);在骨缺损处,A、B、C组三个时间点BV/TV、Tb.Th、Tb.N、Tb.Sp比较差异无统计学意义(P>0.05);术后4周,与D组比较,A、B、C组BV/TV、Tb.Th、Tb.N、Tb.Sp差异有统计学意义(P<0.05);与C组比较,A、B组Tb.N差异有统计学意义(P<0.01)。术后8周,与D组比较,A、B、C组BV/TV、Tb.Th、Tb.N、Tb.Sp差异有统计学意义(P<0.05);术后12周,与D组比较,A、B、C组BV/TV、Tb.Th、Tb.Sp差异有统计学意义(P<0.05)。结论超声骨焊接技术其超声震荡产热作用下PDLLA材料对骨愈合性能无不良影响。

Numerical Simulation of Shrinkage Cavity Formation in Spheroidal Graphite Iron Castings Based on Micro Modeling

A micro-modeling method (MM) for the quantitative prediction of the shrinkage cavity formation in SGiron castings is proposed. The mathematical models describing the volume changes during the solidification ofspheroidal graphite cast iron are established based on the models of solidification kinetics. The shrinkage cavityformation of T-shaped SG iron castings is calculated with MM method. The calculated results are compared with theexperimental results. It is shown that the predicted size, shape and distribution of shrinkage cavity by MM methodare in good agreement with the measured results.

CALCULATION OF THE DAMPING OF THE Zn-27Al ALLOY BASED ON THE MICRO INTERFACE SLIDING MODEL

The microstructures of the Zn-27Al alloy after modification, solid-solution treatment, and natural aging were studied. It was clarified why the damping properties of Zn-27Al alloys, after treatment, had advanced most on the basis of analyzing the microstructures. Approximate expressions have been educed, which can be used to quantificationally work out the damping of the Zn-27Al alloy on the basis of the micro interface sliding model. By comparing the testing damping properties of the foundry Zn-27Al alloys and the Zn-27Al alloys after modification, solid solution, and natural aging, it was shown that the expressions were rational.

Crystal Plasticity Finite Element Process Modeling for Hydro-forming Micro-tubular Components

Micro-tubes manufactured by hydro-forming techniques have now been widely used in medical and microelectronics applica- tions. One of the difficulties in forming such parts is the control of localized necking in the initial stages of the deformation/forming process. A lack of microstructural information causes conventional macro-mechanics finite element（FE） tools to break down when used to investigate the localized microstructure evolution and necking encountered in micro-forming. An effort has been made to create an integrated crystal plasticity finite element（CPFE） system that enables micro-forming process simulations to be carried out easily, with the important features in forming micro-parts captured by the model. Based on Voronoi tessellation and probability theory, a virtual GRAIN（VGRAIN） system is created for generating grains and grain boundaries for micro-materials. Numerical procedures are devel- oped to link the physical parameters of a material to the control variables in a Gamma distribution. A script interface is developed so that the virtual microstructure can be input to the commercial FE code, ABAQUS, for mesh generation. A simplified plane strain CPFE modeling technique is developed and used to capture localized thinning and failure features for hydro-forming of micro-tubes. Grains within the tube workpiece, their distributions and orientations are generated automatically by using the VGRAIN system. A set of crystal viscoplasticity constitutive equations are implemented in ABAQUS/Explicit by using the user-defined material subroutine, VUMAT. Lo- calized thinning is analyzed for different microstructures and deformation conditions of the material using the CPFE modeling technique. The research results show that locations of thinning in forming micro-tubes can be random, which are related to microstructure and grain orientations of the material. The proposed CPFE technique can be used to predict the locations of thinning in forming micro-tubes.

Micro-bubble Drag Reduction on a High Speed Vessel Model

轮船壳形式在水下区域强烈影响轮船的抵抗。在轮船抵抗的主要因素是皮肤磨擦抵抗。球状的弓，聚合物油漆，水驱虫剂油漆(高度防水的墙) ，空气注射，和特定的粗糙被研究人员用作一次尝试获得抵抗减小和轮船的操作效率。微水泡的注射是为阴沉的摩擦抵抗的一种有希望的技术。注射空气水泡应当以某种方式在狂暴的边界层内修改精力并且从而降低皮肤磨擦。这研究的目的是在一支海军上识别注射微水泡的效果快的巡逻队船(FPB ) 有下列主要尺寸的 57 m 类型模型：L=2 450 公里， B=400 公里，和 T=190 公里。微水泡注射和水泡速度的地点的影响也被调查。轮船模型被其速度能被改变并且调整的一台电动机拉。轮船模型抵抗被一个负担房间变换器精确测量。有或没有微水泡的注射的轮船抵抗的比较作为 drag 系数和弗鲁德数字的功能在一张图上被显示出。在船体中央部后面的微水泡注射是最好的地点完成，这被显示出最有效拖减小，和微水泡的罐头活动范围 6%9% 引起的 drag 减小。

A Micro-layer Model for the Lubrication Effect Near Collisions of Immersed Particles

A micro-layer model is proposed to account for the lubrication effect of liquid layer near collisions of immersed particles at moderate particle Reynolds number.This new model is to allow determination of the pressure profile within the micro-layer including the fluid inertia and viscosity.Then a correction based on the micro-layer model is applied to unsteady 3-D direct simulation of a particle approaching another one.The simulation is based on a modified immersed boundary method with direct force scheme.The quantitative agreement between numerical and experimental results validates the model presented in the study.The simulation results show that the fluid is squeezed prior to contact.When a particle approaches a flat wall or another particle,the lubrication force,obtained by integrating the pressure profile over the particle surface,is increased and prevents the particle from approaching.The model predicts that the velocity of approaching particle starts to decrease when separation distance of particles is less than 0.1dp,where dp is the particle diameter.

Micro Model of Carbon Fiber/Cyanate Ester Composites and Analysis of Machining Damage Mechanism

Machining damage occurs on the surface of carbon fiber reinforced polymer (CFRP) composites during processing. In the current simulation model of CFRP, the initial defects on the carbon fiber and the periodic random distribution of the reinforcement phase in the matrix are not considered in detail, which makes the characteristics of the cutting model significantly different from the actual processing conditions. In this paper, a novel three-phase model of carbon fiber/cyanate ester composites is proposed to simulate the machining damage of the composites. The periodic random distribution of the carbon fiber reinforced phase in the matrix was realized using a double perturbation algorithm. To achieve the stochastic distribution of the strength of a single carbon fiber, a novel method that combines the Weibull intensity distribution theory with the Monte Carlo method is presented. The mechanical properties of the cyanate matrix were characterized by fitting the stress-strain curves, and the cohesive zone model was employed to simulate the interface. Based on the model, the machining damage mechanism of the composites was revealed using finite element simulations and by conducting a theoretical analysis. Furthermore, the milling surfaces of the composites were observed using a scanning electron microscope, to verify the accuracy of the simulation results. In this study, the simulations and theoretical analysis of the carbon fiber/cyanate ester composite processing were carried out based on a novel three-phase model, which revealed the material failure and machining damage mechanism more accurately.

A composite material model for investigation of micro-fracture mechanism of brittle rock subjected to uniaxial compression

A two phase model of rock was proposed in order to investigate the mechanism of brittle fracture due to uniaxial compression, in which rock was considered to be a composite material consisting of hard grains and colloids. The stress state in colloid region near grains was calculated using Finite Element Method (FEM). The influence of the tensile stresses on the crack initiation and failure process of brittle rock subjected to uniaxial compression was investigated by numerical experiments. The FE results show that tensile stresses are induced easily in the neighboring area of hard grains with the maximum value near grain boundaries. The distribution of tensile stresses depends on the relative position of hard grains. The cracks initiated just near the boundary area of hard grains, which was governed by tensile stress. These results clearly reveal the micro fracture mechanism of brittle rock loaded by uniaxial compression. It can be concluded that the failure mode of brittle rock under uniaxial compression is still tensile fracture from the point view of microstructure. However, since the wide colloid region is still under compressive stress state, further propagation of boundary cracks through this region obviously needs more external load, thus causing the uniaxial compressive strength of rock much higher than its tensile strength obtained via Brazilian (splitting)

An improved multidirectional velocity model for micro-seismic monitoring in rock engineering

An improved multidirectional velocity model was proposed for more accurately locating micro-seismic events in rock engineering. It was assumed that the stress wave propagation velocities from a micro-seismic source to three nearest monitoring sensors in a sensor's array arrangement were the same. Since the defined objective function does not require pre-measurement of the stress wave propagation velocity in the field, errors from the velocity measurement can be avoided in comparison to three traditional velocity models. By analyzing 24 different cases, the proposed multidirectional velocity model iterated by the Simplex method is found to be the best option no matter the source is within the region of the sensor's array or not. The proposed model and the adopted iterative algorithm are verified by field data and it is concluded that it can significantly reduce the error of the estimated source location.

Improved social force model based on exit selection for microscopic pedestrian simulation in subway station

An improved social force model based on exit selection is proposed to simulate pedestrians' microscopic behaviors in subway station. The modification lies in considering three factors of spatial distance, occupant density and exit width. In addition, the problem of pedestrians selecting exit frequently is solved as follows: not changing to other exits in the affected area of one exit, using the probability of remaining preceding exit and invoking function of exit selection after several simulation steps. Pedestrians in subway station have some special characteristics, such as explicit destinations, different familiarities with subway station. Finally, Beijing Zoo Subway Station is taken as an example and the feasibility of the model results is verified through the comparison of the actual data and simulation data. The simulation results show that the improved model can depict the microscopic behaviors of pedestrians in subway station.

Numerical modeling of time-dependent deformation and induced stresses in concrete pipes constructed in Queenston shale using micro-tunneling technique

Effects of time-dependent deformation（TDD） on a tunnel constructed using the micro-tunneling technique in Queenston shale（QS） are investigated employing the finite element method. The TDD and strength parameters of the QS were measured from tests conducted on QS specimens soaked in water and lubricant fluids（LFs） used in micro-tunneling such as bentonite and polymer solutions. The numerical model was verified using the results of TDD tests performed on QS samples, field measurements of some documented projects, and the closed-form solutions to circular tunnels in swelling rock. The verified model was then employed to conduct a parametric study considering important micro-tunneling design parameters, such as depth and diameter of the tunnel, in situ stress ratio（K;）, and the time lapse prior to replacing LFs with permanent cement grout around the tunnel. It was revealed that the time lapse plays a vital role in controlling deformations and associated stresses developed in the tunnel lining.The critical case of a pipe or tunnel in which the maximum tensile stress develops at its springline occurs when it is constructed at shallow depths in the QS layer. The results of the parametric study were used to suggest recommendations for the construction of tunnels in QS employing micro-tunneling.

Macro and micro mechanics behavior of granite after heat treatment by cluster model in particle flow code

In this paper, a cluster model in particle flow code was used to simulate granite specimens after heat treatment under uniaxial compression. The results demonstrated that micro-cracks are randomly distributed in the specimen when the temperature is below 300?C, and have partial coalescence when the temperature is up to 450?C, then form macro-cracks when the temperature is above 600?C. There is more inter-granular cracking than intra-granular cracking, and their ratio increases with increasing temperature.The micro-cracks are almost constant when the temperature decreases from 900?C to room temperature, except for quartz α–β phase transition temperature(573?C). The fracture evolution process is obviously affected by these cracks, especially at 600–900?C. Elevated temperature leads to easily developed displacement between the grains, and the capacity to store strain energy becomes weaker, corresponding to the plasticity of granite after heat treatment.

A Micro-Mechanism Model for Porous Media

Based on the tortuous-expanding path/channel model,a micro-mechanism model for porous media is developed.The proposed model is expressed as a function of tortuosity,porosity,resistance coefficient,and fluid properties.Every parameter in the proposed model has clear physical meaning.The results show that the model predictions are ingood agreement with those from the existing experimental data.

PROTOTYPE SURFACE MICRO- PRECISION IN FUSED DEPOSITION MODELING PROCESS

To aim at prototype parts fabricated with fused deposition modeling （FDM） process, the problems how to improve and enhance their surface micro-precision are studied. The producing mechanism of surface roughness is explained with three aspects concretely including the principle error of rapid prototyping （RP） process, the inherent characteristics of FDM process, and some mi- cro-scratches on the surface of the extruded fiber. Based on the micro-characters of section shape of the FDM prototype, a physical model reflecting the outer shape characters is abstracted. With the physical simplified and deduced, the evaluating equations of surface roughness are acquired. According to the FDM sample parts with special design for experimental measurement, the real surface roughness values of different inclined planes are obtained. And the measuring values of surface roughness are compared with the calculation values. Furthermore, the causes of surface roughness deviation between measuring values and calculation values are respectively analyzed and studied. With the references of analytic conclusions, the measuring values of the experimental part surface are revised, and the revised values nearly accord with the calculation values. Based on the influencing principles of FDM process parameters and special post processing of FDM prototype parts, some concrete measures are proposed to reduce the surface roughness of FDM parts, and the applying effects are better.

Micro Hydro Potential Modelling: Integrating GIS into Energy Alternatives for Climate Change Mitigation

This research focused on integrating GIS into energy alternatives for climate change mitigation by creating a GIS-based hydrologic model that can be used to identify sites that have significant potential for micro hydropower development within the River Perkerra catchment area. Hydropower is a clean and renewable energy source that remains largely untapped in the country and its development can be used to mitigate anthropogenic climate change by reducing reliance on fossil or biomass derived fuels. This research established the extent of this resource and whether the available sites with significant micro hydropower potential within the study area were amply copious to warrant further development. Currently, such identification is done physically using means that are menial, costly and significantly time consuming. A 90-metre resolution Digital Terrain Model (DTM) data obtained from the Shuttle Radar Topography Mission and various GIS tools were used to create a hydrologic framework which was used to identify potential sites along River Perkerra that suited any desired head requirement for the purposes of locating micro hydropower plants. The derived model demonstrated that it was possible to identify sites at discrete geographic locations along any stream drainage network using GIS. In addition, the model also provides a decision support system that integrates a powerful graphical user interface, spatial database management system and a generalized river basin network flow model for the purposes of exploiting and developing micro hydropower. With sufficient data on catchment discharge and use of higher resolution DTM, the model can be further enhanced to accurately obtain the total microhydro potential of River Perkerra by aggregating the respective potentials of every steam segment.

Model construction of micro-pores in shale: A case study of Silurian Longmaxi Formation shale in Dianqianbei area, SW China

Based on scanning electron microscopy and nitrogen adsorption experiment at low temperature, the pore types and structures of the Longmaxi Formation shale in the Dianqianbei area, SW China were analyzed, and a molecular model was built. According to mathematical statistics, the validation of the model was solved by converting it into a mathematical formula. It is found by SEM that the pores in clay mineral layers and organic pores occupy most of the pores in shale; the nitrogen adsorption experiment at low temperature reveals that groove pores formed by flaky particles and micro-pores are the main types of pores, and the results of the two are in good agreement. A molecular model was established by illite and graphene molecular structures. Moreover, based on the fractal theory and the Frenkel-Halsey-Hill formula, a modified Frenkel-Halsey-Hill formula was proposed. The reliability of the molecular model was verified to some extent by obtaining parameters such as the fractal dimension, replacement rate and fractal coefficients of correction, and mathematical calculation. This study provides the theoretical basis for quantitative study of shale reservoirs.

Modeling and simulation of a high-frequency micro-pump based on giant magnetostrictive material (GMM)

A new type of high-frequency micro-pump was designed, in which GMA (Giant Magnetostrictive Actuator) was employed to replace the traditional motor drive, owing to its significant characteristics of fast response, high accuracy, easily miniaturized and so on. Both the mathematic and simulation models of the micro-pump were built.A set of raw data was used for simulation studies.The results show that the micro-pump based on GMA has achieved the features of high-frequency response and high accuracy, theoretically displaying the performance merits of a giant magnetostrictive material (GMM) high-frequency micro-pump.

Collinear micro-shear-bands model for grain-size and precipitate-size effects on the yield strength

Numerous experimental evidences show that the grain size may significantly alter the yield strength of metals.Similarly,innickel-based superalloys,the precipitate size also influences their yield strength.Then,how to describe such two kinds of size effects on the yield strength is a very practical challenge.In this study,according to experimental observations,a collinear micro-shear-bands model is proposed to explore these size effects on metal materials’yield strength.An analytical solution for the simple model is derived.It reveals that the yield strength is a function of average grain-size or precipitate-size,which is able to reasonably explain size effects on yield strength.The typical example validation shows that the new relationship is not only able to precisely describe the grain-size effect in some cases,but also able to theoretically address the unexplained Hall-Petch relationship between theprecipitate size and the yield strength of nickel-based superalloys.