Effect of shear-induced contact area and aperture variations on nonlinear flow behaviors in fractal rock fractures

This study experimentally analyzes the nonlinear flow characteristics and channelization of fluid through rough-walled fractures during the shear process using a shear-flow-visualization apparatus.A series of fluid flow and visualization tests is performed on four transparent fracture specimens with various shear displacements of 1 mm,3 mm,5 mm,7 mm and 10 mm under a normal stress of 0.5 MPa.Four granite fractures with different roughnesses are selected and quantified using variogram fractal dimensions.The obtained results show that the critical Reynolds number tends to increase with increasing shear displacement but decrease with increasing roughness of fracture surface.The flow paths are more tortuous at the beginning of shear because of the wide distribution of small contact spots.As the shear displacement continues to increase,preferential flow paths are more distinctly observed due to the decrease in the number of contact spots caused by shear dilation;yet the area of single contacts in-creases.Based on the experimental results,an empirical mathematical equation is proposed to quantify the critical Reynolds number using the contact area ratio and fractal dimension.

Gaps,challenges and possible solution for prediction of wheel-rail rolling contact fatigue crack initiation

The prediction of wheel/rail rolling contact fatigue(RCF)crack initiation during railway operations is an important task.Since RCF crack evolution is influenced by many factors,its prediction process is complex.This paper reviews the existing approaches to predict RCF crack initiation.The crack initiation region is predicted by the shakedown map.By combining the shakedown map with various initiation criteria and the critical plane method,the crack initiation life is calculated.The classification,methodologies,theories and applications of these approaches are included in this paper.The advantages and limitations of these methods are analyzed to provide recommendation for RCF crack initiation prediction.This review highlights that wheel/rail dynamic characteristic,complex working conditions,surface defects and wear all affect the RCF crack initiation.The optimal selection of criteria is essential in the crack initiation prediction.Based on the research gap regarding the challenging process of crack initiation prediction detailed in this review,a proposed prediction process of RCF crack initiation is proposed to achieve a more accurate result.

Wheel-rail contact model for railway vehicle-structure interaction applications:development and validation

An enhancement in the wheel-rail contact model used in a nonlinear vehicle-structure interaction(VSI)methodology for railway applications is presented,in which the detection of the contact points between wheel and rail in the concave region of the thread-flange transition is implemented in a simplified way.After presenting the enhanced formulation,the model is validated with two numerical applications(namely,the Manchester Benchmarks and a hunting stability problem of a sus-pended wheelset),and one experimental test performed in a test rig from the Railway Technical Research Institute(RTRI)in Japan.Given its finite element(FE)nature,and contrary to most of the vehicle multibody dynamic commercial software that cannot account for the infrastructure flexibility,the proposed VSI model can be easily used in the study of train-bridge systems with any degree of complexity.The validation presented in this work proves the accuracy of the proposed model,making it a suitable tool for dealing with different railway dynamic applications,such as the study of bridge dynamics,train running safety under different scenarios(namely,earthquakes and crosswinds,among others),and passenger riding comfort.

Influence of railway wheel tread damage on wheel-rail impact loads and the durability of wheelsets

Dynamic wheel-rail contact forces induced by a severe form of wheel tread damage have been measured by a wheel impact load detector during full-scale field tests at different vehicle speeds.Based on laser scanning,the measured three-dimensional damage geometry is employed in simulations of dynamic vehicle-track interaction to calibrate and verify a simulation model.The relation between the magnitude of the impact load and various operational parameters,such as vehicle speed,lateral position of wheel-rail contact,track stiffness and position of impact within a sleeper bay,is investigated.The calibrated model is later employed in simulations featuring other forms of tread damage;their effects on impact load and subsequent fatigue impact on bearings,wheel webs and subsurface initiated rolling contact fatigue of the wheel tread are assessed.The results quantify the effects of wheel tread defects and are valuable in a shift towards condition-based maintenance of running gear,and for general assessment of the severity of different types of railway wheel tread damage.

Numerical Exploration of Asymmetrical Impact Dynamics: Unveiling Nonlinearities in Collision Problems and Resilience of Reinforced Concrete Structures

This study provides a comprehensive analysis of collision and impact problems’ numerical solutions, focusing ongeometric, contact, and material nonlinearities, all essential in solving large deformation problems during a collision.The initial discussion revolves around the stress and strain of large deformation during a collision, followedby explanations of the fundamental finite element solution method for addressing such issues. The hourglassmode’s control methods, such as single-point reduced integration and contact-collision algorithms are detailedand implemented within the finite element framework. The paper further investigates the dynamic responseand failure modes of Reinforced Concrete (RC) members under asymmetrical impact using a 3D discrete modelin ABAQUS that treats steel bars and concrete connections as bond slips. The model’s validity was confirmedthrough comparisons with the node-sharing algorithm and system energy relations. Experimental parameterswere varied, including the rigid hammer’s mass and initial velocity, concrete strength, and longitudinal and stirrupreinforcement ratios. Findings indicated that increased hammer mass and velocity escalated RC member damage,while increased reinforcement ratios improved impact resistance. Contrarily, increased concrete strength did notsignificantly reduce lateral displacement when considering strain rate effects. The study also explores materialnonlinearity, examining different materials’ responses to collision-induced forces and stresses, demonstratedthrough an elastic rod impact case study. The paper proposes a damage criterion based on the residual axialload-bearing capacity for assessing damage under the asymmetrical impact, showing a correlation betweendamage degree hammer mass and initial velocity. The results, validated through comparison with theoreticaland analytical solutions, verify the ABAQUS program’s accuracy and reliability in analyzing impact problems,offering valuable insights into collision and impact problems’ nonlinearities and practical strategies for enhancingRC structures’ resilience under dynamic stress.

Research on the influence of flexible wheelset rotation effect on wheel rail contact force

Purpose-Under the high-speed operating conditions,the effects of wheelset elastic deformation on the wheel rail dynamic forces will become more notable compared to the low-speed condition.In order to meet different analysis requirements and selecting appropriate models to analyzing the wheel rail interaction,it is crucial to understand the influence of wheelset flexibility on the wheel-rail dynamics under different speeds and track excitations condition.Design/methodology/approach-The wheel rail contact points solving method and vehicle dynamics equations considering wheelset flexibility in the trajectory body coordinate system were investigated in this paper.As for the wheel-rail contact forces,which is a particular force element in vehicle multibody system,a method for calculating the Jacobian matrix of the wheel-rail contact force is proposed to better couple the wheel-rail contact force calculation with the vehicle dynamics response calculation.Based on the flexible wheelset modeling approach in this paper,two vehicle dynamic models considering the wheelset as both elastic and rigid bodies are established,two kinds of track excitations,namely normal measured track irregularities and short-wave irregularities are used,wheel-rail geometric contact characteristic and wheel-rail contact forces in both time and frequency domains are compared with the two models in order to study the influence of flexible wheelset rotation effect on wheel rail contact force.Findings-Under normal track irregularity excitations,the amplitudes of vertical,longitudinal and lateral forces computed by the flexible wheelset model are smaller than those of the rigid wheelset model,and the virtual penetration and equivalent contact patch are also slightly smaller.For the flexible wheelset model,the wheel rail longitudinal and lateral creepages will also decrease.The higher the vehicle speed,the larger the differences in wheel-rail forces computed by the flexible and rigid wheelset model.Under track short-wave irregularity excitations,the vertical force amplitude computed by the flexible wheelset is also smaller than that of the rigid wheelset.However,unlike the excitation case of measured track irregularity,under short-wave excitations,for the speed within the range of 200 to 350 km/h,the difference in the amplitude of the vertical force between the flexible and rigid wheelset models gradually decreases as the speed increase.This is partly due to the contribution of wheelset's elastic vibration under short-wave excitations.For low-frequency wheel-rail force analysis problems at speeds of 350 km/h and above,as well as high-frequency wheel-rail interaction analysis problems under various speed conditions,the flexible wheelset model will give results agrees better with the reality.Originality/value-This study provides reference for the modeling method of the flexible wheelset and the coupling method of wheel-rail contact force to the vehicle multibody dynamics system.Furthermore,by comparative research,the influence of wheelset flexibility and rotation on wheel-rail dynamic behavior are obtained,which is useful to the application scope of rigid and flexible wheelset models.

An integrated approach for the optimization of wheel-rail contact force measurement systems

A comprehension of railway dynamic behavior implies the measure of wheel-rail contact forces which are affected by disturbances and errors that are often difficult to be quantified. In this study, a benchmark test case is proposed, and a bogie with a layout used on some European locomotives such as SIEMENS El90 is studied. In this layout, an additional shaft on which brake disks are installed is used to transmit the braking torque to the wheelset through a single-stage gearbox. Using a mixed approach based on finite element techniques and statistical considerations, it is possible to evaluate an optimal layout for strain gauge positioning and to optimize the measurement system to diminish the effects of noise and disturbance. We also conducted preliminary evaluations on the precision and frequency response of the proposed system.

Nonlinear Contact Between Inner Walls of Deep Sea Pipelines in Buckling Process

In order to study buckling propagation mechanism in deep sea pipelines, the contact between pipeline's inner walls in buckling process was studied. A two-dimensional ring model was used to represent the pipeline and a nonlinear spring model was adopted to simulate the contact between inner walls. Based on the elastoplastic constitutive relationship and the principle of virtual work theory, the coupling effect of pipeline's nonlinear large deformation and wall contact was included in the theoretical analysis with the aid of MATLAB, and the application scope of the theoretical model was also discussed. The calculated results show that during the loading process, the change in external pressure is closely related to the distribution of section stress, and once the walls are contacting each other, the external pressure increases and then remains stable after it reaches a specific value. Without fracture, the pipeline section will stop showing deformation. The results of theoretical calculations agree well with those of numerical simulations. Finally, in order to ensure reliability and accuracy of the theoretical results, the collapse pressure and propagation pressure were both verified by numerical simulations and experiments. Therefore, the theoretical model can be used to analyze pipeline's buckling deformation and contact between pipeline's inner walls, which forms the basis for further research on three-dimensional buckling propagation.

Nonlinear Contact Between Pipeline's Outer Wall and Slip-on Buckle Arrestor's Inner Wall During Buckling Process

In order to theoretically study the buckle propagation of subsea pipelines with slip-on buckle arrestors, a two-dimensional ring model was set up to represent the pipeline and a nonlinear spring model was adopted to simulate the contact between pipeline's inner walls and between pipeline's outer wall and slip-on buckle arrestor's inner wall during buckle propagation. In addition, some reverse springs are added to prevent the wall of left and right sides separating from the inner wall of slip-on buckle arrestors. Considering large deformation kinematics relations and the elastic-plastic constitutive relation of material, balance equations were established with the principle of virtual work. The variation of external pressure with respect to the cross-sectional area of pipelines was analyzed, and the lower bound of the crossover pressure of slip-on buckle arrestors was calculated based on Maxwell's energy balance method. By comparing the theoretical results with experiment and finite element numerical simulation, the theoretical method is proved to be correct and reliable.

Simulation on derailment base on a new wheel-rail contact method

The simulation package for special research on derailment of high speed vehicle is established.The process of derailment is different from other behaviors of vehicle dynamics because of large lateral displacement of wheelsets.To get correct results,a new fast algorithm to computing contact force is adopted and the exact geometry analysis is necessary to judge derailment happened.Variation of contact condition and coefficient of friction with speeds are also considered into vehicle-track coupled model.The structure of the package is presented in detail.The results are particular emphasis on investigation influence of maximum track defect,critical vehicle speed and various contact condition on derailment.The simulation can also be used to define the most risk factor leading to derailment.

Two-Layer Contact Nonlinear Mechanical Analysis of Flexible Drilling Tool in the Wellbore

The lack of research on flexible drilling tool leads to limited application of ultra-short radius horizontal wells.The flexible drilling tool is different from the conventional drilling tool.The flexible drilling pipe involves a mutual transition between the structure and the mechanism during the deformation process.At the same time,the flexible drilling pipe and the eccentric guide tube,the guide tube and the wellbore generate random contact.In this paper,3-D beam elements,universal joint elements,rigid beam elements and the beam-beam contact elements are combined to establish a two-layer contact nonlinear finite element model of the flexible drilling tool in the wellbore.The dynamic relaxation method is introduced for numerical solution.The feasibility of the model and the algorithm is verified by an example.The mechanical analysis of flexible drilling tool under the four hole inclinations in the oblique section is carried out.It is found that the flexible drilling pipe has a“folded line”deformation.The contact force between the flexible drilling pipe and the guide tube is randomly distributed.The contact force between the guide tube and the wellbore in the oblique section is greater than that in the vertical section.As the hole inclinations increase,the torque and axial force transmitted to the drill bit gradually decrease.

Modeling of Energy Processes in Wheel-Rail Contacts Operating under Influence of Periodic Discontinuous Forces

In this paper we present new numerical simulation approaches for determining the energy processes under periodic conditions caused by time-discontinuous forces in the wheel-rail contacts. The main advantage of the presented method is the total elimination of frequency analysis, which in effect introduces important simplifications in the identification of the effects in the contact. The second important feature is the fact that the method is based on the analysis of appropriate loops on the energy phase plane leading to an easy estimation of the rail strength through the evaluation of the loop’s area. That model based simulation in the applied dynamics relies on advanced methods for model setup, robust and efficient numerical solution techniques and powerful simulation tools for practical applications. Fundamental properties of contact displacements of the rail surface have been considered on the basis of the newly established method. The contact zone between railway wheels and the rail surfaces made of bulk materials is perceived as strong enough to resist the normal (vertical) forces introduced by heavy loads and the dynamic response induced by track and wheel irregularities. The analysis is carried out for a wheel running on an elastic rail rested on sleepers arranged on completely rigid foundation. The equations of displacement motion are established through the application of the Lagrange equations approach. The established model of the wheel-rail contact dynamics has been applied to that same roll plane but with taking into account a nonlinear characteristic of the sleeper with respect to the ground. Attention then is focused completely on the modeling of the energy absorbed by the rail. The applied method employs the energy state variables as time functions leading to determine the susceptibility of a given contact on the strength induced by the rail roll.

扣件刚度非线性对波磨区轮轨瞬态滚动接触行为的影响

WJ-8型扣件橡胶垫板刚度在长期服役过程中表现出非线性特征,静刚度随荷载增加而降低,为提高三维瞬态滚动接触有限元模型计算精确性,本文以LMA踏面车轮及CHN60型钢轨为基础,基于显式积分算法,将以往研究中线弹性扣件转化为非线性扣件,建立考虑扣件刚度非线性特征的三维轮轨瞬态滚动接触有限元模型,研究刚度非线性对车轮与钢轨波磨间高频动态响应及瞬态接触行为的影响,并重点分析了波磨工况下时频域内轮轨接触力、轴箱加速度的变化信息.结果表明:扣件非线性对轮轨接触力变化影响明显,主要表现为车轮行驶至扣件前端时强振动导致橡胶垫板表现为柔软特性使轮轨接触力减小,车轮行驶至扣件上方时在轴重作用下振动减弱表现为刚硬特性使轮轨接触力增大,轮轨力变化差异最大达到13.1%.

盘式制动器温度场的分析研究

针对制动器温度场的研究分析,建立额定起重量32 T的钳盘式制动器三维模型,同时在Workbench中添加制动器材料的各个参数,设置摩擦副间的接触关系,施加制动盘和制动衬片载荷、约束,对制动器制动过程产生的温度场进行仿真。使用LM算法建立包括时间、摩擦因数、制动压力、初始转速的多元非线性数学模型,通过Matlab进行回归分析。最终拟合结果和有限元仿真结果吻合,验证了起重机制动器温度模型的正确性。

基于UEL的水电机组导轴承支架非线性连接模态特性分析

模态分析是水电机组故障诊断的重要方法,但其传统方法缺少对接触部分非线性接触的模拟手段,直接影响模态分析的准确性,也无法对链接部位的失效过程及趋势进行分析。该文基于用户自定义单元子程序接口(user-defined element subroutine interface,UEL),建立了结合部三维非线性专用分析单元模型。推导所提模型有限元格式的基本方程,获得表征材料非线性特性的本构关系;给出螺栓连接的固定结合部的静态分析,以及立式水轮发电机导轴承支架的模态分析算例;给出导轴承支架的支臂末端螺栓连接变刚度计算的工程应用实例。实验与仿真对比结果表明了建立的UEL三维非线性接触分析专用单元模型的正确性;连接失效分析表明了随着导轴承支架接触刚度的下降,会在整机模态频率计算中引入新的频率段这一结果的重要性。

一类具有非线性出生率和饱和恢复率的随机SEIR传染病模型的动力学行为

研究了一类具有非线性出生率和非线性接触率的随机SEIR传染病模型的绝灭性、持久性与平稳分布问题。同时利用计算机数值模拟验证了所得结论。本文所得结果改进了相关文献的结论。

参变量变分原理的提出、发展与应用

参变量变分原理及其参数二次规划算法是由钟万勰院士1985年针对弹性接触边界非线性问题首次提出来的,经过将近40年的不断发展,目前参变量变分原理已经成功应用于各个领域,其中包括弹塑性分析、接触问题、润滑力学、岩土力学、变刚度杆系结构、先进材料性能分析、材料的蠕变与损伤、柔性结构力学和LQ最优控制等各个工程领域.本文首先回顾了参变量变分原理的起源,介绍了参变量变分原理的基本概念,然后以弹塑性分析问题为例,阐明建立参变量变分原理的理论模型以及实现数值参数二次规划求解原理,最后详细回顾了参变量变分原理的基本理论与相应数值算法在各个领域的发展及其工程应用,展示了参变量变分原理在求解各类非线性问题的特色与优势.

多叶式径向动压气体箔片轴承预紧力仿真

为获取多叶式径向动压气体箔片轴承大预紧装配过程中各部件状态,基于显式动力学方法建立了该类轴承的非线性接触模型,并讨论了预紧量、静态偏心率对转轴表面应力分布的影响,研究结果表明:多叶式箔片结构在平箔片搭接区域及与转轴接触区域应力较大;当无静态偏心时,转轴表面应力分布呈周期对称状态,有静态偏心时,则呈余弦形式,且应力波峰波谷数量与箔片数量一致;同时,预紧量越大,转轴表面应力越大,而静态偏心率越大,转轴表面主要受压区峰谷间应力差越大。

等效锥度非线性研究及其在踏面设计中的应用

为研究等效锥度非线性特性对车辆系统动力学和磨耗性能的影响,提出可以用于踏面优化设计的非线性特性建议指标。通过对轮轨关系进行细致推导,应用踏面逆向设计快速递推算法,得到具有不同等效锥度非线性特性的踏面,对不同踏面的轮轨关系进行分析。建立车辆系统动力学模型和基于Archard理论的磨耗预测模型,对不同非线性特性踏面的临界速度、横向平稳性、磨耗性能进行对比分析。研究结果表明:非线性因子值过高或过低都会导致临界速度下降,但该指标过高对临界速度影响更大,最大降幅可达20%;等效锥度非线性特性还会影响到踏面的磨耗发展,踏面的初始非线性因子越高,名义等效锥度在磨耗过程中的增长越慢,对延长车轮服役周期是有利的。综合动力学性能及磨耗性能,建议高速列车踏面的非线性因子应取尽可能接近于0的负值。在这一结论的指导下,对S1002CN踏面进行优化设计,得到名义等效锥度为0.1,等效锥度非线性因子为0.013(与S1002CN持平)、-0.002的优化踏面S1002CN_opt1、S1002CN_opt2,优化踏面S1002CN_opt2与S1002CN相比临界速度和横向平稳性有所优化,同时磨耗性能也有所提升。S1002CN_opt2与S1002CN_opt1之间的等效锥度非线性关系在动力学性能及磨耗性能中的表现证明以等效锥度非线性因子限值指导踏面进行优化设计的可行性,研究结果可为踏面设计及轮轨关系研究提供参考。

考虑结构与加锚岩体联合受力的船闸闸首抗震性能研究

为研究地震作用下结构与加锚岩体的非线性接触对闸首抗震性能的影响,以某双线五级船闸的第一级闸首为例,在ADINA有限元软件中建立库水-结构-锚杆-基岩动力相互作用的整体三维有限元模型,基于约束函数的非线性接触单元模拟了闸首结构中的各类接触问题,采用动力时程分析方法模拟了地震作用下闸首结构的动力响应、锚杆受力的分布规律以及不同接触面的接触状态。结果表明:设计地震作用下闸首各边墩均向航槽侧变形,位移沿高度方向递增;锚杆轴向拉应力的最大值小于其抗拉强度;不同介质间的接触面上部有往复开合的现象,接触面最大张开度沿高度方向递增,地震结束时接触面恢复到震前的紧贴状态;设计地震作用下该船闸闸首的结构变形、结构应力、锚杆受力情况及接触面接触状态均满足规范要求,具有良好的抗震性能。