超高强度钢喷丸表面残余应力在疲劳过程中的松弛规律

对超高强度钢23C014Ni12Cr3MOE进行了喷丸强化，采用X射线衍射应力分析方法研究了旋转弯曲疲劳过程中喷丸表面残余应力的松弛变化规律。结果表明，在疲劳循环过程中，残余应力的松弛主要发生在疲劳的初始循环100周次内，疲劳循环100周次后残余应力基本稳定在某一个应力水平上，而且其中的大幅度松弛发生在疲劳的初始循环10周次内。对比不同应力水平下的松弛行为，在疲劳极限以上的应力水平下，残余应力松弛的幅度和速率都较大。对喷丸后的试样在疲劳试验前先进行200℃／2h的保温提前应力松弛处理，然后再进行疲劳试验，200℃／2h的保温处理可降低残余应力松弛的幅度和速率甚至在低于疲劳极限的应力水平时只发生小的松弛或不发生松弛。但由于喷丸强化试样疲劳裂纹往往从次表层萌生，表面残余应力的松弛只能作为评价材料疲劳性能的一个参考数值，在工程应用时不能只依据表面残余应力来判定材料的疲劳性能。     

A 3D FE model with plastic shot for evaluation of equi-biaxial peening residual stress due to multi-impacts

A 3D multi-impact finite element (FE) model for evaluation of peening residual stress is presented. Combined peening factors by Kim et al. are applied to the 3D symmetry-cell originally contrived by Meguid et al. To describe the feature of multi-impacts, concepts such as FE peening coverage, impact sequence and cycle-repetition are introduced. We successfully extracted the equi-biaxial stress from the simulations of diverse single-cycle and multi-cycle impacts. At four impact locations of FE symmetry-cell, surface and maximum residual stresses converge to equi-biaxial stress, and convergence improves with the number of repetitions of cycle. Impact velocity needed for comparing the FE solution with the XRD result is determined from the Almen arc height and coverage. It is further found that the simulation set with plastic shot produces residual stress consistent with the experimental XRD result.     

Prediction model of surface residual stress within a machined surface by combining two orthogonal plane models

The variation of surface residual stress within a machined surface layer caused by face turning was studied. The size of the tool’s corner radius and the feed rate affect residual stress. A process model using the finite element method is proposed and the mechanical effects of the corner radius and feed rate on a machined surface were discussed. When a tool with a small corner radius is used, surface residual stress perpendicular to the cutting direction becomes compression stress. As well, surface residual stress changes from tension to compression as the feed rate decreases. The process model consists of an orthogonal cutting simulation and an indentation-like simulation of a corner radius into a work piece surface. The simulated results show quantitative agreement with the residual stress measured experimentally. The integrity of the machined surface will be controlled more efficiently if the cutting conditions during finishing are determined with the proper consideration of the surface generating process.     

Iterative inversion method for eddy current profiling of near-surface residual stress in surface-treated metals

Because of their frequency-dependent penetration depth, eddy current measurements are capable of mapping the near-surface depth profile of the electric conductivity. This technique can be used to nondestructively characterize the subsurface residual stress distribution in certain types of shot-peened metals, e.g., in nickel-base superalloys. In this paper, a highly convergent iterative inversion procedure is presented to predict the frequency-independent intrinsic electric conductivity depth profile from the frequency-dependent apparent eddy current conductivity (AECC) spectrum. The proposed technique exploits three specific features of the subsurface electric conductivity variation caused by near-surface residual stresses in shot-peened metals. First, compressive residual stresses are limited to a shallow surface region of depth much less than typical probe coil diameters. Second, the change in electric conductivity due to residual stresses is always very small, typically less than 1%. Third, the electric conductivity depth profile is continuous and fairly smooth. The accuracy of the proposed iterative inversion procedure is one order of magnitude better than that of the previously developed simpler method (J Appl Phys 2004;96:1257).     

局部锤击法对水轮机转轮焊接残余应力场的影响

针对混流式水轮机转轮在服役过程中出现裂纹的情况,在利用数值模拟技术研究局部锤击法对转轮焊接残余应力的影响的基础上,对某水电站转轮的焊接残余应力进行了研究.结果表明,普通工艺下,转轮叶片出水边附近的焊接残余拉应力的最大值出现在焊缝的熔合线附近;局部锤击法可以起到降低锤击部位的焊接残余拉应力的作用.当锤击部位的温度接近或超过材料本身的塑性温度时,叶片危险区域焊接残余应力峰值的降低效果非常明显;当锤击温度比较低时,残余应力的效果不明显.     

混流式转轮局部处理技术与局部压应力区的优化

焊接残余压应力的存在能够有效提高焊接结构的抗疲劳性能．在优化混流式转轮焊接工艺、实现转轮叶片出水边焊接接头熔合区附近易开裂区域压应力化的基础上,对局部加热及锤击处理对转轮易开裂区域的应力场分布的影响进行了数值模拟和真机试验测试分析．结果表明,采用局部加热和局部锤击的方法均可以对转轮易开裂区域的焊接残余应力场进行调控,优化转轮的局部压应力区,进一步提高转轮易开裂区域的残余压应力值,形成高压应力区,扩大压应力区范围,促进转轮易开裂区域压应力化的实现,有效的提高转轮的抗疲劳性能．     

激光喷丸诱导的残余应力的有限元分析

激光喷丸成形是一项新技术,是利用强激光辐照材料表面产生的高压冲击波对材料内部诱导的残余应力来成形金属板料的,控制残余应力场的分布是精确成形板料的关键。有限元模拟技术是预测激光诱导的残余应力场的有效方法之一。该文建立激光诱导残余应力模拟的模型,讨论了模拟过程中关键问题的技术处理,如加载历史、本构关系、网格划分、求解时间等。数值模拟的过程主要包括动态分析和静态分析两个步骤,模拟结果表明,残余压应力的大小和残余压应力层的深度随激光的功率密度的增加而增加,最后趋于饱和。模拟结果和试验结果较为一致。     

微粒子喷丸对靶材残余应力场的影响

为了研究微粒子喷丸多颗粒冲击靶材时的残余应力场,建立了1个中心粒子周边均布6个粒子的7粒子冲击模型。应用有限元软件,对微粒子搭接率、冲击速度、冲击角度、直径等喷丸工艺参数对中心粒子冲击坑中心点处残余应力场的影响进行了仿真研究。结果表明:当搭接率ζ≤0.5时,ζ的变化对残余压应力场深度没有影响,而ζ=0.75是表面残余压应力S_S变化的分界线,只有在ζ＞0.75时,残余压应力场的4个特征值才均随着ζ的增大而增大;增大微粒子直径、减小冲击角度,有利于增大残余压应力场的深度,但同时也会减小残余压应力场的大小。增大微粒子的冲击速度,残余压应力场的深度增大,而残余压应力场的大小呈现一定的波动性,但是两者均在冲击速度为200 m/s时取得最大。     

16Mn钢U形焊接接头残余应力和变形

利用有限元分析软件ANSYS/APDL语言编写程序,选用U形焊接接头,采用生死单元方法的热源加载模式,进行多步循环,实现了焊接全过程残余应力场的三维动态模拟,获得了焊接接头残余应力和残余变形的分布规律;采用CO2气体保护焊对16Mn钢U形焊接件进行焊接,得到了焊接接头冷却后最终的残余变形情况,仿真结果与实验所得的规律相吻合,为进一步研究和消除焊接残余应力和应变提供了依据。     

Metastable pitting behaviour of austenite stainless steel under compressive residual stress

Metastable pitting behaviour of austenite stainless steel after compressive deformation in an acidic NaCl solution was investigated. The results show that the pit initiation rate increases while both metastable pit lifetime and growth rate decrease after compressive deformation. Meanwhile, the ratio of the surface oxide fraction of Cr to Fe increases whereas the average size of stable pits is smaller and shallow disk-shaped under compressive residual stress. The results reveal that with the aid of enrichment of Cr oxides and compressive residual stress, which helps break the remnants of the undermined passive film, pit repassivation can be accelerated.     

高温屈服极限对高强钢焊接变形及残余应力影响的仿真研究

由于缺乏材料的高温数据,进行焊接仿真时所取的高温屈服极限通常有差异。采用双椭球热源模型,对某高强度结构钢的高温屈服极限设计了五种方案进行焊接仿真计算,研究了高温屈服极限对焊接变形和残余应力的影响规律。结果表明:高温屈服极限差异对焊接变形与焊接残余应力有不同的影响;高温屈服极限差异对变形影响较大,越靠近焊缝影响越明显;对残余应力影响较小,仅在近缝区的局部区域产生影响。焊接变形与残余应力均随高温屈服取值的增大而增大。     

HT250冷焊表面残余应力试验

利用亚激光瞬间熔冷焊机对柴油发动机缸体所用材料HT250表面缺陷进行修复,并用x射线法测定焊缝两侧表面残余应力,以此研究冷焊残余应力分布规律以及与不同工艺参数间的关系．结果表明,靠近焊缝处为拉应力,远离焊缝处为压应力：冷焊残余应力范围很小,峰值主要分布在离焊缝中心5mm的范围内,离焊缝中心20mm处,残余应力接近为零;时间参数对残余应力峰值的影响较小,而能量参数对残余应力峰值的影响较大;通过改变时间与能量参数组合,可以得到较小的残余应力峰值,满足发动机再制造零部件表面损伤的修复要求．     

焊接残余应力在热处理过程中的演变

利用有限元法,对Q345R钢焊后热处理进行模拟分析.结果表明,焊态残余应力在焊缝和热影响区具有较高的应力水平,远离焊缝热影响区,应力逐渐降低.热处理后,残余应力降低了近60%,最大值位于热影响区浅表区域.热处理不仅使应力降低,同时使得应力发生再分布,母材热处理后应力增加.热处理过程中,随着温度升高,屈服强度降低,材料发...     

超声高速研磨陶瓷表面残余应力特性研究

工程陶瓷材料的表面残余应力对其使用性能影响很大.本文采用固着磨料超声外圆研磨装置在精密车床上,对ZrO2、Al2O3和纳米ZrO2增韧Al2O3(ZTA)三种陶瓷材料,在不同的磨粒尺寸、研磨压力、研磨速度下的表面残余应力进行了实验研究.研究结果表明:精密平面磨削残余应力产生机理主要为热塑效应和相变效应的综合作用,而固着磨料和游离磨料研磨残余应力产生机理主要是机械冷挤压作用;前者在两个方向上均表现为拉应力,后者均为较小的残余压应力;其值随着磨料尺寸、研磨压力与研磨速度的增大而增大;但超过一定值之后残余应力的增加变得缓慢.在相同条件下,Al2O3陶瓷研磨表面在两个方向上均表现为残余拉应力,而ZrO2和ZTA陶瓷研磨表面均为残余压应力.     

锻造工艺参数对TC4钛合金锻件残余应力的影响

针对非等温锻造时TC4钛合金锻件极易产生较大残余应力的问题,依据物理实验,建立并调控仿真模型,通过数值模拟的方法,对不同锻造工艺参数下的TC4钛合金锻件的残余应力进行分析。研究了变形温度、变形程度以及变形速度3个关键参量对TC4钛合金锻件残余应力的影响,揭示了残余应力的变化以及分布规律。结果表明:TC4钛合金锻件的径向残余应力关于中心轴线呈对称分布,但分布不均匀,从外表层至心部,残余应力由拉应力转化为压应力,在中心处应力较为集中;而TC4钛合金锻件的轴向残余应力从上表面到下表面分布较为均匀,且明显小于径向残余应力;关键参量中的变形温度和变形程度对TC4钛合金锻件残余应力的影响较为显著,而变形速度的影响较小,适当提高变形温度和变形速度、减小变形程度,可降低TC4钛合金锻件的残余应力。     

氮化硅陶瓷磨削表面残余应力的测试与计算

利用X射线衍射应力分析的sin2 ψ法测量、计算出氮化硅陶瓷试样的残余应力。分析表明 ,经平面磨磨削后的气压烧结氮化硅陶瓷试样表面存在的残余应力为拉应力 ,而再经过表面抛光处理 ,则可以适当地降低表面残余应力。     

不同点固焊形式对焊接变形和残余应力的影响

借助有限元分析手段对不同点固焊形式的对接焊焊接过程进行模拟,经分析得到焊接变形和应力分布情况,采用实验手段验证试验结果。结果表明,点固焊位置相同,长度越长,获得的变形量越小;起、收弧的两端面点固焊变形在起、收弧两侧翘起,焊缝位置点固焊翘起的位置发生在与焊缝位置平行的两侧自由端;点固焊长度越长,仿真和实测结果的残余应力分布波动性越大;在起、收弧两端面点固焊获得的残余应力峰值最小;起、收弧两端面点固焊、点固焊2处残余应力值均降低,点固焊3处残余应力值增大。证明了有限元模型的正确性和有效性。     

An accurate,efficient envelope approach to modeling the geometric deviation of the machined surface for a specific five-axis CNC machine tool

Geometric deviation, defined as the difference between the nominal surface and the simulation model of the machined surface, is the fundamental concern of five-axis tool path planning. Since the machined surface is part of the cutter envelope surface generated by the cutter motion, it is necessary to calculate the envelope surface in order to obtain the geometric deviation. In the stage of tool path planning, current approaches calculate the cutter envelope surface by using the cutter motion along the given tool path. However, the cutter motion of practical machining on a specific five-axis CNC machine tool is different from the given tool path. Moreover, the computation is very challenging when the accurate cutter motion of practical machining is applied to calculate the envelope surface. To overcome these two problems, a geometric envelope approach with two major distinctions is proposed in this paper. First, the envelope surface of the cutter undergoing a general motion is efficiently obtained as a closed-form vector expression. Second, the accurate cutter motion, which is determined by machine kinematic and interpolation scheme in practical machining, can be easily applied to calculate the accurate envelope surface. With the envelope surface, the geometric deviation is calculated to estimate the overcut or undercut in five-axis milling. An example is given to demonstrate the validity of the proposed method.     

残余压应力场对B780CF钢焊接接头疲劳裂纹扩展的影响

为了明确不同强度残余压应力场对裂纹扩展速率及寿命的影响,采用埋弧自动焊通过不同焊接热输入控制不同焊缝宽度,从而在制备标准CT试样时通过机械加工时的残余应力释放得到各组不同强度残余压应力场的各组焊接接头,并根据裂纹闭合原理,提出基于Elber的三参数分段公式,根据分段公式条件从MTS试验机采用柔度法获得的试验数据中提取、归纳数据,利用断裂力学方法和数理统计方法进行分析,得出结论,裂尖存在的焊接残余压应力场在一定程度上可以明显降低裂纹扩展速率,提高裂纹扩展寿命,对焊接接头裂纹扩展有着关键性作用。     

翻转次数对20MnMoNb特厚管板焊接残余应力与变形影响的数值分析

基于有限元ABAQUS软件,对390mm厚20MnMoNb特厚板双U形坡口拼焊制造EO反应器管板过程进行焊接残余应力与变形有限元模拟,考察两面交替焊接时的翻转次数对特厚管板焊后残余应力与变形的影响．结果表明,特厚管板焊后发生一端翘起的角变形,在靠近表层的焊缝及热影响区存在较大的残余拉应力,在焊缝内部为残余压应力;随着翻转次数的增加,焊接残余应力和变形均降低,但当往返翻转次数超过7次后,进一步增加翻转次数对残余应力和变形的降低作用不明显．