Numerical Investigation of Laser Surface Hardening of AISI 4340 Using 3D FEM Model for Thermal Analysis of Different Laser Scanning Patterns

Laser surface hardening is becoming one of the most successful heat treatment processes for improving wear and fatigue properties of steel parts. In this process, the heating system parameters and the material properties have important effects on the achieved hardened surface characteristics. The control of these variables using predictive modeling strategies leads to the desired surface properties without following the fastidious trial and error method. However, when the dimensions of the surface to be treated are larger than the cross section of the laser beam, various laser scanning patterns can be used. Due to their effects on the hardened surface properties, the attributes of the selected scanning patterns become significant variables in the process. This paper presents numerical and experimental investigations of four scanning patterns for laser surface hardening of AISI 4340 steel. The investigations are based on exhaustive modelling and simulation efforts carried out using a 3D finite element thermal analysis and structured experimental study according to Taguchi method. The temperature distribution and the hardness profile attributes are used to evaluate the effects of heating parameters and patterns design parameters on the hardened surface characteristics. This is very useful for integrating the scanning patterns’ features in an efficient predictive modeling approach. A structured experimental design combined to improved statistical analysis tools is used to assess the 3D model performance. The experiments are performed on a 3 kW Nd:Yag laser system. The modeling results exhibit a great agreement between the predicted and measured values for the hardened surface characteristics. The model evaluation reveals also its ability to provide not only accurate and robust predictions of the temperature distribution and the hardness profile as well an in-depth analysis of the effects of the process parameters.

Stochastic analysis of excavation-induced wall deflection and box culvert settlement considering spatial variability of soil stiffness

In this study, a three-dimensional (3D) finite element modelling (FEM) analysis is carried out to investigate the effects of soil spatial variability on the response of retaining walls and an adjacent box culvert due to a braced excavation. The spatial variability of soil stiffness is modelled using a variogram and calibrated by high-quality experimental data. Multiple random field samples (RFSs) of soil stiffness are generated using geostatistical analysis and mapped onto a finite element mesh for stochastic analysis of excavation-induced structural responses by Monte Carlo simulation. It is found that the spatial variability of soil stiffness can be described by an exponential variogram, and the associated vertical correlation length is varied from 1.3 m to 1.6 m. It also reveals that the spatial variability of soil stiffness has a significant effect on the variations of retaining wall deflections and box culvert settlements. The ignorance of spatial variability in 3D FEM can result in an underestimation of lateral wall deflections and culvert settlements. Thus, the stochastic structural responses obtained from the 3D analysis could serve as an effective aid for probabilistic design and analysis of excavations.

3D finite element method （FEM） simulation of groundwater flow during backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, a 3D numerical approach of the groundwater flow leading to the erosion mechanism of backward erosion piping is presented and discussed. Comparison of the 2D and 3D numerical results explicitly demonstrates the inherent 3D nature of the piping phenomenon. In addition, the influence of the seepage length is investigated and discussed for both piping initiation and piping progression. The results clearly indicate the superiority of the presented 3D numerical model compared to the established 2D approach. Moreover, the 3D numerical results enable a better understanding of the complex physical mechanism involved in backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures.

Evaluating performance of lignite pillars with 2D approximation techniques and 3D numerical analyses

This paper attempts to investigate the use of approximate 2D numerical simulation techniques for the evaluation of lignite pillar geomechanical response, formed via the room and pillar mining method.Performance and applicability of the developing methodology are assessed through benchmarking with a more direct and accurate 3D numerical model. This analysis utilizes an underground lignite mine which is being developed in soft rock environment. Through the decisions made for the optimum room and pillar layout, the design process highlights the strong points and the weaknesses of 2D finite element analysis, and provides useful recommendations for future reference. The interpretations of results demonstrate that 2D approximation techniques come near quite well to the actual 3D problem.However, external load approximation technique seems to fit even better with the respective outcomes from the 3D analyses.

ANN Based Predictive Modelling of Weld Shape and Dimensions in Laser Welding of Galvanized Steel in Butt Joint Configurations

The quality assessment and prediction becomes one of the most critical requirements for improving reliability, efficiency and safety of laser welding. Accurate and efficient model to perform non-destructive quality estimation is an essential part of this assessment. This paper presents a structured and comprehensive approach developed to design an effective artificial neural network based model for weld bead geometry prediction and control in laser welding of galvanized steel in butt joint configurations. The proposed approach examines laser welding parameters and conditions known to have an influence on geometric characteristics of the welds and builds a weld quality prediction model step by step. The modelling procedure begins by examining, through structured experimental investigations and exhaustive 3D modelling and simulation efforts, the direct and the interaction effects of laser welding parameters such as laser power, welding speed, fibre diameter and gap, on the weld bead geometry (i.e. depth of penetration and bead width). Using these results and various statistical tools, various neural network based prediction models are developed and evaluated. The results demonstrate that the proposed approach can effectively lead to a consistent model able to accurately and reliably provide an appropriate prediction of weld bead geometry under variable welding conditions.

西门子F级联合循环机组D型锚固盒设计研究

锚固盒为燃气轮机与基座的主要连接构件之一,其应力状态复杂,目前国内研究较少。本文采用有限元分析方法,研究西门子F级联合循环机组D型锚固盒应力状态,并进行足尺试验,为锚固盒设计提供参考方案。

AN L^(∞) SECOND ORDER CARTESIAN METHOD FOR 3D ANISOTROPIC INTERFACE PROBLEMS

A second order accurate method in the infinity norm is proposed for general three dimensional anisotropic elliptic interface problems in which the solution and its derivatives,the coefficients,and source terms all can have finite jumps across one or several arbitrary smooth interfaces.The method is based on the 2D finite element-finite difference(FEFD)method but with substantial differences in method derivation,implementation,and convergence analysis.One of challenges is to derive 3D interface relations since there is no invariance anymore under coordinate system transforms for the partial differential equations and the jump conditions.A finite element discretization whose coefficient matrix is a symmetric semi-positive definite is used away from the interface;and the maximum preserving finite difference discretization whose coefficient matrix part is an M-matrix is constructed at irregular elements where the interface cuts through.We aim to get a sharp interface method that can have second order accuracy in the point-wise norm.We show the convergence analysis by splitting errors into several parts.Nontrivial numerical examples are presented to confirm the convergence analysis.

柴油机活塞的温度场、热变形与应力三维有限元分析

本文应用 MARC有限元分析软件 ,采用三维有限元法计算大功率柴油机活塞在热载荷作用下的温度场和换热情况 ,并在此基础上分析活塞的热变形及热应力场 ,确定活塞失效的主要位置 ,从而为改进设计提供参考。

在温度和机械载荷作用下活塞应力与变形的三维有限元分析

根据有限元法解得的温度场，利用微机对ω型燃烧室的柴油机活塞进行温度载荷和机械载荷作用下应力和变形的三维有限元分析。单元网格划分中采用了３１８６个节点、２２１０个８节点六面体等参元，将计算输出的各单元形心的六个应力分量换算为三个主应力分量，再选择合适的插值求得各节点的主应力分布。绘制各截面在温度和机械载荷作用下的变形图和三个主应力分量的等应力曲线图。为了评估计算的可靠性，还进行了沿活塞轴线方向的静力校核。

基于子结构技术的复杂齿轮系统有限元三维接触分析

基于子结构分析技术,建立了复杂齿轮系统的三维整体有摩擦弹性接触计算模型,用有限元参数二次规划法求解非线性部分,对其进行了比较全面、精细的有限元分析。以韶山7电力机车牵引齿轮系统为例,分析了齿轮啮合处接触状态（齿面相对滑动状态、齿面接触应力、齿面接触区域、齿向载荷分布）的变化规律。分析结果表明,齿轮系统的支承方式对齿轮齿面接触状态有较大影响。同时也说明,对于齿轮系统这种重复结构多的工程结构,子结构分析技术在解题规模和计算效率上都有优越性。

大跨度悬索桥的三维应力分析方法

根据悬索桥的受力特点 ,将空间杆系与三维应力仿真分析模型相结合 。

三维地形直流电阻率有限元法模拟

基于稳定电流场的基本方程、三维区域满足的边值问题以及相应的变分问题,研究了三维起伏地形条件下电阻率的有限单元数值模拟算法.离散积分区域时,以三棱柱为最小研究单元,推导了含有地形特征信息的三线性插值型函数以及单元刚度矩阵.采用变带宽、一维数组方式只存储稀疏刚度矩阵中非零元素,能够节约内存.利用Cholesky分解法只分解一次大型稀疏矩阵,通过回代可以求出方程组的全部解,当求解有多个供电点的测深问题时可以缩短计算时间.模型计算表明,在水平层状介质模型上,三维计算结果与解析解或二维数值解十分吻合,计算精度满足误差要求.在二维山脊上的二极剖面或三维山谷上的中间梯度剖面上,其三维计算结果与相应模型的土槽实验结果或边界元法计算结果也非常接近.

盾构隧道纵向地震响应分析

为了探讨盾构隧道的纵向地震响应特性,采用地层-隧道整体三维有限元模型,对武汉长江越江盾构隧道的地震响应进行了分析,主要研究了合理的盾构隧道力学模型、隧道与地层之间的相互作用以及隧道的振动特性.通过隧道与地层的整体分析,得到了盾构隧道位移和应力的分布及其随时间的变化曲线.计算结果表明:压缩波引起的纵向拉、压应力和剪切波引起的扭曲变形是隧道抗震设计的关键.

复杂地形三维直流电阻率有限元数值模拟

系统地论述了用有限单元法研究复杂地形条件下三维直流电阻率的正演计算技术.首先给出了三维构造中点源电场的边值问题以及相应的变分问题;然后利用有限单元法求解变分问题,采用四面体单元对研究区域进行剖分,在单元中进行三线性函数插值,将变分方程化为线性代数方程组;最后,考虑到节约计算时间,利用对称超松弛预条件共轭梯度迭代算法求解大型线性方程组,得到了各节点的电位值,进而计算出地表的视电阻率.通过理论模型的计算检验了算法的可行性之后,给出了几种常见纯地形异常的数值模拟结果和一个组合模型的计算结果,其研究工作为研究三维直流电阻率反演奠定了基础.

固定点源测深激电异常研究

电阻率法三维有限元数值模拟方法的实现,给研究形状复杂的地质体的电性异常提供了有力工具.本文利用该数值模拟程序,通过先模拟计算模型的视电阻率,然后利用视电阻率和视极化率的转换公式转换,从而实现对激电异常的数值模拟.文章计算并分析了固定点源测深法在各种1-D、2-D、3-D模型下的激电异常规律及定量解释结果,力求使人们从空间不同角度了解该方法的IP异常规律,从而指导野外生产实践,提高解释效率和精度.

内燃机排气消声器性能的三维有限元计算及分析

首先用三维有限元法计算出消声器的四端子参数,然后利用所求得的四端子参数预估消声器的性能,克服了由于高次波影响,一维理论计算公式在高频区计算不准确的缺陷,也克服了二维有限元法不适用于非对称结构消声器的缺陷,取得了计算结果、理论分析与试验值吻合的良好效果。在此基础上,对扩张式消声器最高有效上限频率及扩张室长度对消声器性能的影响进行了讨论,其结论对消声器设计有一定的参考价值。

东坪金矿采空区开挖过程的三维有限元数值模拟

结合东坪金矿南山采区1号空区的工程实际,采用Plaxis 3D Tunnel对其建立三维数值模型,并对其不同开挖顺序的开挖过程进行数值模拟。通过对开采过程中的地压活动规律和围岩稳定性进行分析,论证了地下采空区开挖方案的可行性,揭示了采空区不同开挖阶段应力的集中部位和围岩的潜在破坏部位。计算表明该空区由上向下开挖顺序较为有利,其开挖过程总体可以保持稳定,但也存在不安全隐患。计算结果对该矿山的安全生产具有指导意义。

可升卧式翻板闸门的静力数值分析

基于大型有限元分析软件ANSYS建立了可升卧式翻板闸门的有限元模型,分析了滑块反力特性及闸门在不同工况下的变形和应力变化规律,并校核了闸门强度和刚度,评估了闸门的安全性。结果表明,该文分析方法可供同类闸门结构的设计计算借鉴。

一种表面-内置式永磁转子同步电机三维全域温度场分析

针对表面-内置式永磁转子同步电机(SIPMSM)具有结构紧凑和功率密度高的特点,准确计算SIPMSM各部件的温度分布非常重要。采用电磁场-温度场耦合分析的方法对SIPMSM的三维全域温度场进行计算。建立SIPMSM的电磁场和温度场有限元模型,分析在同步运行速度下负载和永磁体退磁对SIPMSM三维全域温度场的影响,也分析了在额定负载下运行速度对SIPMSM三维全域温度场的影响。通过仿真与实验结果的对比分析,验证了样机模型的合理性和计算结果的正确性。

可膨胀管膨胀过程三维有限元数值模拟

为合理设计膨胀工具及制订膨胀工艺,采用三维弹塑性接触问题有限单元法,建立了实体可膨胀管膨胀过程的三维非线性接触问题有限元分析模型.通过对所建模型的求解,对可膨胀管膨胀过程中膨胀变形力、套管膨胀后的残余应力及轴向位移、膨胀工具模角的关系进行了深入细致的研究,找出了膨胀管与膨胀工具在不同的接触条件、可膨胀管在不同的内径膨胀率及不同的壁厚条件下,各参数之间相互制约的变化规律.通过实验测得的膨胀变形力与用本模型求得的结果符合得很好,相对误差仅为0.65%,完全可以满足实际工程的要求.