Crack propagation directions in unfilled resins

Posterior composite restorative materials undergo accelerated wear in the occlusal contact area, primarily through a fatigue mechanism. To facilitate the timely development of new and improved materials, a predictive wear model is desirable. The objective of this study was to develop a finite element model enabling investigators to predict crack propagation directions in resins used as the matrix material in composites, and to verify these predictions by observing cracks formed during the pin-on-disc wear of a 60:40 BISGMA:TEGDMA resin and an EBPADMA resin. Laser confocal scanning microscopy was used to measure crack locations. Finite element studies were done by means of ABAQUS software, modeling a cylinder sliding on a material with pre-existing surface-breaking cracks. Variables included modulus, cylinder/material friction coefficient, crack face friction, and yield behavior. Experimental results were surprising, since most crack directions were opposite previously published observations. The majority of surface cracks, though initially orthogonal to the surface, changed direction to run 20 to 30 degrees from the horizontal in the direction of indenter movement. Finite element modeling established the importance of subsurface shear stresses, since calculations provided evidence that cracks propagate in the direction of maximum K(II)(theta), in the same direction as the motion of the indenter, and at an angle of approximately 20 degrees. These findings provide the foundation for a predictive model of sliding wear in unfilled glassy resins.     

Interplay between Field Measurements and Soil Behavior for Capturing Supported Excavation Response

Instruments are installed during the construction of urban excavations to monitor ground response at discrete locations to various construction activities, to verify design assumptions and to effectively apply the observational approach. Inverse analysis approaches are often used to develop improved soil models suitable for representing soil response during excavation from these measurements. We propose that through the integration of inverse analysis and instrument measurements, it is possible to provide information on excavation performance at locations where no instrumentation is available. Therefore, this study examines the relationship between various instruments typically used on an excavation project and the quality of information that can be extracted for excavation modeling. A synthetically generated set of instrument measurements that include inclinometers, surface settlement points, extensometers, heave gauges, piezometers, and strain gauges, using an idealized soil profile are initially used. The analyses show that in addition to the measurements of lateral wall deflections and surface settlement, inclinometers placed some distance behind the wall and measured forces in the struts significantly improve the quality of the extracted soil behavior. These findings are further demonstrated with a well instrumented deep excavation case study in Taipei. The inclinometers at the wall and at farther distance from the wall are used to extract the soil behavior. The extracted soil model used in a numerical analysis provides a good prediction of excavation behavior elsewhere around the excavation including surface settlements.     

Wear analysis of a side dam in twin-roll strip casting

Reducing wear on a side dam can prolong the casting operation life of a twin-roll strip casting process,thus reducing production cost and improving casting stability. To lengthen the service life of the side dam,it is necessary to understand the wear performance of the side dam material. To investigate the wear behavior mechanism of the side dam,in this study,the UMT-2 friction and wear tester was used to determine the relationship between the wear rate of the side-dam material and various parameters. Based on the roughness of the contact area between the side dam and the end of the casting rolls as well as on the amount of deformation of the side dam,which was derived using a thermal-deformation simulation model,the reasons for the uneven wear of the side dam were obtained.     

Novel Approach to Integration of Numerical Modeling and Field Observations for Deep Excavations

Precedent and observation of performance are an essential part of the design and construction process in geotechnical engineering. For deep urban excavations designers rely on empirical data to estimate potential deformations and impact on surrounding structures. Numerical simulations are also employed to estimate induced ground deformations. Significant resources are dedicated to monitor construction activities and control induced ground deformations. While engineers are able to learn from observations, numerical simulations have been unable to fully benefit from information gained at a given site or prior excavation case histories in the same area. A novel analysis method, self-learning in engineering simulations (SelfSim), is introduced to integrate precedent into numerical simulations. SelfSim is an inverse analysis technique that combines finite element method, biologically inspired material models, and field measurements. SelfSim extracts relevant constitutive soil information from field measurements of excavation response such as lateral wall deformations and surface settlement. The resulting soil model, used in a numerical analysis, provides correct ground deformations and can be used in estimating deformations of similar excavations. The soil model can continuously evolve using additional field information. SelfSim is demonstrated using two excavation case histories in Boston and Chicago.     

Response of an Earth Dam to Spatially Varying Earthquake Ground Motion

The stochastic response of the Santa Felicia earth dam, in southern California, to spatially varying earthquake ground motion (SVEGM) is analyzed. An SVEGM model that accounts for both incoherence and propagation of seismic waves is used, the results are compared with those for various simplified excitations, and the sensitivity of the responses to coherency models proposed by different researchers is investigated. A 3D inhomogeneous finite-element model is used to represent the dam. Finite element-based random vibration analysis is performed and the statistical moments of Cartesian displacement, stress, and strain responses are computed. Statistical moments of the maximum shear stress are computed using Monte Carlo simulation that utilizes results from the random vibration analysis. The results indicate that the stress response of stiff material near the base of the dam can be significantly increased due to SVEGM, and that the increase is sensitive to the low-frequency variation of ground motion coherency.     

Position Accommodation and Compliance Control for Robotic Excavation

A robotic excavator must be able to operate in all three states of manipulator motion: free space, contact, and exertion of a force against an environment. A position accommodation control scheme is ideally suited for this task. The position accommodation method described in this study uses a position controller to track a predefined trajectory in the absence of a force at the excavator bucket. If a force is detected, the measured force is used, through a compliance controller, to modify the commanded trajectory. This allows for better control of the complete excavation cycle. This paper describes a rheological method for modeling the soil–bucket interaction force and how that method must be modified to suit the unique case of excavation. Results for both a fixed and variable spring set-point are shown, illustrating the difference between a purely position controlled robot and one using a position accommodation/compliance controller, when used to dig a level trench.     

基坑开挖对临近地表沉降影响的分析

采用FLAC-3D软件对某基坑开挖引起的地表沉降进行了数值模拟,计算中采用摩尔库仑及修正剑桥弹塑性模型模拟土体特征,采用结构单元模拟基坑支护结构。通过模拟开挖过程,获得了地表沉降的一些规律,可供工程设计和施工参考。分析表明：土方开挖对基坑周围土体的影响范围约为2.7倍的基坑深度;修正剑桥模型模拟昆明地区典型地基土能取得较好效果。     

离散元法在球磨机仿真中的研究进展及应用

球磨机作为矿石磨碎阶段的重要设备,对选矿厂提效降耗起着决定性作用.球磨机功耗与能耗分布、技术参数与衬板结构参数设计优化是影响球磨机磨矿效果的重要内容和技术难点,离散元法可以准确模拟颗粒运动的形变过程并记录颗粒的功率及能量分布,在球磨机仿真领域具有良好的应用前景.介绍了离散元法在球磨机仿真过程中应用比较广泛的颗粒模型、接...     

Creep Modeling in Excavation Analysis of a High Rock Slope

Based on the distinct element method, a numerical procedure is presented for simulation of creep behavior of jointed rock slopes due to excavation unloading. The Kelvin model is used to simulate viscous deformation of joints. A numerical scheme is introduced to create incremental contact forces, which are equivalent to producing creep deformation of a rock-joint system. The corresponding displacement of discrete blocks due to creep deformation of contact joints can be calculated by equilibrium iteration. Comparisons of results between the numerical model and theoretical solutions of a benchmark example show that the presented model has excellent accuracy for analysis of creep deformation of rock-joint structures. As an application of the model, residual deformations of the high rock slopes of the Three Gorges shiplock due to excavation unloading and creep behavior are investigated. By simulating the actual excavation process, the deformation history of a shiplock slope is studied. Good agreement has been achieved between numerical prediction and field measurements. It demonstrates the effectiveness of the presented model in analysis of the creep deformation due to excavation unloading of high rock slopes.     

A Phenomenological Model for Tool Wear in Friction Stir Welding of Metal Matrix Composites

Friction stir welding (FSW) of metal matrix composites (MMCs) is advantageous because the solid-state nature of the process precludes formation of deleterious intermetallic phases which accompany melting. FSW of MMCs is complicated by rapid and severe wear of the welding tool, a consequence of contact between the tool and the much harder abrasive reinforcement which gives the workpiece material its enhanced strength. The current article demonstrates that Nunes’s rotating plug model of material flow in FSW, which has been successfully applied in many other contexts, can also help us understand wear in FSW of MMCs. An equation for predicting the amount of wear in this application is developed and compared with experimental data. This phenomenological model explains the relationship between wear and FSW process parameters documented in previous studies.     

细化渗碳体对高碳纳米贝氏体轴承钢的影响

研究了GCr15Si1Mo轴承钢中渗碳体细化后,对材料微观组织、常规力学性能、耐磨性及滚动接触疲劳性能的影响。通过工艺的调控,使材料组织中的渗碳体尺寸从0.49细化到了0.20 μm,贝氏体铁素体板条尺寸从66细化至41 nm,并且渗碳体的分布密度也随之提高。随后通过SEM、TEM、XRD、硬度、冲击、摩擦磨损及滚动接触疲劳等试验,得到了材料的宏观性能与微观组织。研究结果表明,细化渗碳体后,材料的耐磨性能和滚动接触疲劳性能比常规工艺的优异,但是韧性相对于常规工艺有显著降低。研究证明了较细小渗碳体可以对轴承钢的组织结构、常规力学性能、耐磨性能及疲劳性能产生一定的影响,为后期轴承钢中渗碳体的调控研究提供支持。     

Influence of Mesh Geometry on Three-Dimensional Finite-Element Analysis of Tunnel Excavation

Finite element (FE) analysis has become an important tool for predicting building response to tunnel-induced ground movement. Because tunnel construction is a three-dimensional (3D) process, the trend is to apply 3D FE analysis to tunnel-soil-building interaction problems instead of applying the plane-strain models that are commonly used in engineering practice. Since 3D FE analyses require large amounts of computational resources, the geometric dimensions of the 3D models are often kept to a minimum to reduce calculation time. There is, however, a lack of published information concerning appropriate mesh dimensions. This paper investigates the influence of the geometry and the dimension of a 3D FE model on tunnel-induced surface settlement predictions. The paper shows how the vertical boundaries can influence the results. It demonstrates that reasonable results can be obtained by increasing the length of incremental tunnel excavation and by scaling back the settlement values to give a required tunnel volume loss. This study therefore not only highlights the limitations of 3D modeling but also shows its potential for engineering practice.     

Simulation of Shield Tunneling Behavior along a Curved Alignment in a Multilayered Ground

The kinematic shield model has been proposed to simulate shield behavior during excavation based on equilibrium condition, taking into account ground displacement around the shield. The model has been validated by the simulation of an earth pressure balanced shield behavior in a straight alignment for a single ground layer. To verify the model performance for shield tunnel excavation at curved alignment in multilayered ground, the slurry shield behavior is simulated in this study using the in situ data, and the immediate ground movements around the shield are computed by three-dimensional FEM employing the enforced displacement, which is obtained from the shield behavior simulation. As a result, it is found that the simulated shield behavior and the computed immediate ground movements around the shield during shield tunneling are in good agreement with the observations. Furthermore, the results reveal that the excavated area including the area generated by copy cutter is a predominant factor affecting the shield behavior, and the ground displacement, at the excavated surface plays an important role in the surrounding ground movements during shield tunneling.     

Moment Equation Analysis of Base-Isolated Buildings Subjected to Support Motion

The stationary response of base-isolated buildings subjected to support motion is studied. The isolation device consists of a friction device, while the ground motion is assumed to be a stationary Gaussian white noise random process. The moment equation approach is preferred in this study to characterize the structural response statistically. A non-Gaussian closure is adopted to reveal the degree of non-Gaussianness of the response. The proposed approach is compared with the stochastic equivalent linearization and the numerical simulation, resulting in more accuracy when predicting the moments of the response which is markedly non-Gaussian. The models of cascaded and coupled system for the base isolator and the building are also compared, finding the former to be quite in error.     

基于DEM的无钟高炉布料料流轨迹研究

摘要：离散单元仿真技术被广泛用于研究高炉布料过程中炉料运动的规律,其计算散料运动的方法得到了国内外专家的认可。利用SOLIDWORKS和EDEM的Geometry建立无钟炉顶高炉几何模型,基于DEM结合炉料物理参数进行数学模拟,研究了溜槽倾角、料线深度和溜槽结构对料流轨迹的影响。结果表明：焦炭落点半径随溜槽倾角的增大而增大,倾角为13.3°时,炉料在下落过程中不与溜槽接触;随着料线深度的增大,焦炭的落点半径随之增大,但增量减小;在相同倾角下,矩形溜槽布焦炭时的落点半径比半圆形溜槽的小0.3m,且料流更集中,布料效果略优;有衬板溜槽比光面溜槽布焦炭时的落点半径小0.13m,料流宽度也较小,且能通过料与料之间的摩擦降低炉料对溜槽表面的磨损。     

Review of Resistive Force Models for Earthmoving Processes

The force required to insert a tool into a medium is of major interest for the design and automation of earthmoving machinery. The present work reviews previous investigations on forces encountered during earthmoving processes by cyclic (but nonrotary) excavation machines. The objective is to integrate the formulation for cutting and penetrating forces to those for excavation. Common practices for characterizing an unfrozen medium and the associated tool actions are discussed, followed by a general overview of various models describing earthmoving tasks of penetration, cutting, and excavation. Observation and analysis of cutting and excavation models reveals that there is not a common ground for their validation, but it also identifies a core of key parameters, reduced in number and essential to any further excavation model. The paper suggests a normalized experimental verification and comparison of the models before they can be further used.     

Friction and wear behavior of sintered antifriction materials at high sliding speeds

Conclusions ZhNBM material exhibits very high wear resistance with good surface quality in unlubricated rubbing operation in air at sliding speeds of up to 50 m/sec. It can therefore be recommended as a suitable material for end-face seals intended for operation in liquid and gaseous environments. In operation without lubrication MG30ZhN1K material possesses good wear resistance only under light loads (0.1–0.2 MPa). The formation of spots which are rubbed off during operation leads to the conclusion that a plasticized film may be present in the contact zone of an end-face seal, and this substantially reduces the thermal load which the seal must withstand and prolongs its useful life.Translated from Poroshkovaya Metallurgiya, No. 11(239), pp. 58–61, November, 1982.     

Fiber Element for Cyclic Bending and Shear of RC Structures.?II: Verification

The fiber beam element with shear modeling developed in the companion paper is calibrated and verified by comparison with test data. The verification is carried out for the material constitutive behavior and for single beam and column elements using available test results from literature. A structural analysis of a shear sensitive viaduct pier subjected to ground input motion is presented. Details of the algebraic expressions used for the concrete and steel constitutive behaviors are provided.     

Three-Dimensional Effects for Supported Excavations in Clay

This paper presents the results of 150 finite-element simulations conducted to define the effects of excavation geometry, i.e., length, width, and depth of excavation, wall system stiffness, and factor of safety against basal heave on the three-dimensional ground movements caused by excavation through clays. The results of the analyses are represented by the plane strain ratio (PSR), defined as the maximum movement in the center of an excavation wall computed by three-dimensional analyses normalized by that computed by a plane strain simulation. A simple equation for PSR is presented based on excavation geometry, wall system stiffness, and factor of safety against basal heave. This PSR equation reasonably represents trends in results of the 150 simulations as well as those simulations reported in literature. When the excavated length normalized by the excavated depth of an excavation wall is greater than 6, results of plane strain simulations yield the same displacements in the center of that wall as those computed by a three-dimensional simulation.