喷丸三维残余应力场的有限元模拟

运用大型有限元计算软件ABAQUS建立了模拟喷丸残余应力场的三维有限元模型,预测了在相同喷丸强度下玻璃丸和钢丸两种类型弹丸喷射所产生的残余应力场。模拟过程中,分析了线性减缩积分单元的沙漏参数、材料的应变硬化率、喷丸覆盖率以及初始残余拉应力等因素对304不锈钢靶材残余应力分布的影响。从计算结果可以看出,钢丸喷丸产生的残余压应力层较深,但在高覆盖率时,玻璃喷丸产生的残余压应力的平均值比钢丸喷丸处理后产生的大。在有初始残余拉应力(250 Mpa)存在的情况下,两种类型的喷丸处理均能使304不锈钢靶材表面形成残余压应力层,这说明喷丸工艺可以提高奥氏体不锈钢焊接构件的抗应力腐蚀开裂能力。本研究成果为进一步探讨喷丸强化不锈钢焊接头抗应力腐蚀性能的机理奠定了基础。     

Shot peening simulation based on SPH method

Shot peening is a complex surface-treating process which is usually employed to improve the fatigue strength of metallic part or members. In dealing with shot peening simulation, existing literatures apply finite element method (FEM) to establish only a single shot or several shots models, thus the effect of a mass of shots impacting repeatedly and the interaction among adjacent shots are ignored. To overcome these defects of FEM models, smoothed particle hydrodynamics (SPH) coupled FEM modeling is presented, in which the shots are modeled by SPH particles and the target material is modeled by finite elements. Contact algorithm is used to simulate the interaction between shots and target. Utilizing this model, material model for shots is established, the relationships between compressive residual stress and peening frequencies, coverage, and velocities are analyzed. Steady compressive residual stress can be gotten by multiple peening; higher coverage can improve the compressive residual stress; faster velocities can induce greater and deeper maximum residual stress in target subsurface. The simulation results agree well with the existing experimental data. The study will not only provide a new powerful tool for the simulation of shot peening process, but also be benefit to optimize the operating parameters.     

Simulation on Residual Stress of Shot Peening Based on a Symmetrical Cell Model

The symmetrical cell model is widely used to study the residual stress induced by shot peening. However, the correlation between the predicted residual stresses and the shot peening coverage, which is a big challenge for the researchers of the symmetrical cell model, is still not established. Based on the dynamic stresses and the residual stresses outputted from the symmetrical cell model, the residual stresses corresponding to full coverage are evaluated by normal distribution analysis. The predicted nodal dynamic stresses with respect to four corner points indicate that the equi-biaxial stress state exists only for the first shot impact. Along with the increase of shot number, the interactions of multiple shot impacts make the fluctuation of the nodal dynamic stresses about an almost identical value more and more obvious. The mean values and standard deviations of the residual stresses gradually tend to be stable with the increase of the number of shot peening series. The mean values at each corner point are almost the same after the third peening series, which means that an equi-biaxial stress state corresponding to the full coverage of shot peening is achieved. Therefore, the mean values of the nodal residual stresses with respect to a specific transverse cross-section below the peened surface can be used to correlate the measured data by X-ray. The predicted residual stress profile agrees with the experimental results very well under 200% peening coverage. An effective correlation method is proposed for the nodal residual stresses predicted by the symmetrical cell model and the shot peening coverage.     

Numerical simulation of Almen strip response due to random impacts with strain-rate effects

Almen intensity is used in the shot peening industry as a standard measure of the residual stress during shot peening and it is employed to ensure the quality of the peening of identical parts. However, relating the intensity to the residual stresses in the strip needs to be established with confidence by realistic simulations. An improved FEM-based approach that simulates the actual process of Almen strip peening is presented in this paper. This approach has some unique features such as employing randomly located shots impacting on a slice of Almen strip and the inclusion of strain-rate dependent target material properties and the lateral deflection of the strip for improved accuracy. The intensity, the corresponding residual stresses and roughness match well with available experimental results. Further, it is shown in this study that the Almen intensity can be correlated well with the residual compressive stresses of the target material using a parameter that depends on the relative dynamic yield strength of the target material.     

喷丸强化残余应力对疲劳性能和变形控制影响研究进展

作为机械表面强化技术之一,喷丸强化使工件表层发生形变硬化,引入较高的残余压应力,减少了疲劳应力作用下微裂纹的萌生并抑制其扩展,从而显著提高零件的抗疲劳断裂和抗应力腐蚀开裂的能力。基于喷丸残余应力解析计算模型,从余弦函数模型、接触应力模型和球腔膨胀模型三个方面介绍喷丸强化残余应力的产生,进而对喷丸残余应力的仿真预测及影响规律进行论述。为了提高试件疲劳强度而引入的残余压应力会带来影响形位精度的变形,基于此阐述了喷丸残余应力对疲劳性能的影响及其在疲劳过程中的演化,同时论述了喷丸残余应力变形预测及控制的研究现状,最后对喷丸残余应力未来的研究内容与发展方向进行展望。     

Kinematic modeling and deformation mechanics in shot peening of functional ceramics

The applications of functional ceramics are significantly limited by the brittleness and low reliability. Recent studies have shown that compressive residual stress can be created in ceramics by shot peening, which improves the contact strength and fatigue of ceramic components. However, the formation mechanism of residuals stress in shot peening is yet to understand. In this study, a pressure-dependent plasticity model has been incorporated into a finite element simulation model of shot peening to understand the process mechanism underpinning the residual stress formation. Since shot velocity is the key process parameter to dominate the impact energy which determines the deformation state of the peened surface and the resultant residual stress, a new kinematic model of shots has also been developed by incorporating air drag and travel distance inside and outside the peening nozzle. The results have shown that the shot velocity model can be used to predict shot velocity. The experiment-based model may help understand the process mechanism underpinning the residual stress formation.     

高压水射流喷丸降低焊接残余应力有限元分析

焊接不可避免产生残余应力,对结构完整性造成很大影响。提出利用高压水射流喷丸技术降低焊接残余应力,并利用有限元法进行计算模拟。分别开发了模拟焊接的移动双椭球热源子程序及模拟高压水射流喷丸的移动压力载荷子程序,得到了经高压水射流喷丸处理前后焊接残余应力分布的变化规律。计算结果表明,经高压水射流喷丸处理后,焊缝和热影响区存在的焊态残余应力得到降低,在焊缝区已经产生压缩应力。证明高压水射流喷丸具有降低焊接残余应力的效果。     

喷丸残余应力场有限元模拟研究进展

喷丸强化处理工艺可以显著提高金属材料的抗疲劳和抗应力腐蚀等性能,这与喷丸后在金属表面层形成的残余应力场紧密相关,因此对喷丸残余应力的大小及分布进行预测具有重要意义。对近年国内外喷丸残余应力场的有限元模拟进行评述,总结出6种典型的残余应力分析模型,分别是二维轴对称模型、四对称面模型、三对称面模型、双对称面模型、单对称面模型和无对称边界条件的有限元模型,比较了不同模型的特点及应用现状。介绍了当前几种新的残余应力分析有限元模型:随机三维模型和周期性边界条件模型,根据其原理和特点认为其本质是无对称边界条件模型和对称边界条件有限元模型的延伸。针对空化水喷丸、激光喷丸和超声波喷丸等新工艺,新喷丸过程的数值模拟一般是以机械喷丸工艺的数值模拟方法为基础进行改进,主要采用弹丸撞击法和等效载荷法。对未来喷丸残余应力数值模拟研究进行了展望,认为从组织强化角度深入研究喷丸强化机理、建立更符合实际喷丸工艺的有限元模型、开发新算法以及将有限元法和离散元法进行结合是值得关注的研究方向。     

Compressive Residual Stress into Titanium Alloy Using Cavitation Shotless Peening Method

Cavitation shotless peening (CSP) method, where impacts are generated by a submerged cavitating jet (without shots), was used to introduce compressive residual stress in titanium alloy, Ti-6Al-4V for the purpose of enhancing the conventional fatigue and fretting fatigue life and strength. This method provided higher compressive stress at surface as well as up to a depth of 40 m from the surface than that with the shot peening method. Further, the surface treated by CSP was considerably less rough compared to that by the shot peening method, which is a highly desirable feature to improve the fretting fatigue performance.     

DD3镍基单晶高温合金喷丸层残余应力的X射线衍射分析

对DD3镍基单晶高温合金表面进行喷丸处理,利用X射线衍射方法研究了喷丸层中残余应力沿层深的分布。结果表明:喷丸层中存在的残余压应力在层深为10μm处达到最大值,且随着层深的增加而降低;喷丸表层单晶组分中的残余压应力与多晶组分中的基本接近,次表层中两者存在较大差别,随着材料层深的增加二者的差别更加明显,单晶组分中的残余压应力水平更高,分布深度更大。     

A numerically low-cost and high-accuracy periodic FE modeling of shot-peened 34CrNiMo6 and experimental validation

This paper proposed a numerically low-cost 3D FE modeling method for multi-shot shot peening. The low computation cost and high prediction accuracy of shot peening are realized at the same time by the incorporation of random multi-shot with defined spacing between the adjacent simultaneously impinging shots, periodicity, and coverage rate of 100%. With this modeling method, one-step and dual-step multi-shot peening of 34CrNiMo6 steel target is modeled and the produced residual stress is predicted. In order to make the predicted residual stress depth profile more comparable with the measured one by XRD method, the redistribution of residual stress due to the layer removal by electrochemical polishing is simulated using Model Change technique. And the comparison between the prediction and experiment indicates that this improved 3D periodic FE modeling of multi-shot impingement provides very accurate simulation models for one-step and dual-step shot peening. It can substitute for the costly and time-consuming optimization experiments of the shot peening process, especially the multi-step shot peening process. Finally, the evolution of residual stress depth profile in dual-step shot peening process is investigated by using the simulation model and a variation of residual stress towards a more uniform distribution on the finished surface taking place in the second step is discovered by RMS analysis.     

Stress concentration analyses of bi-material bonded joints without in-plane stress singularities

The problem of stress concentration in bi-material bonded joint is investigated under the condition of without stress singularities. Disappearance conditions of stress singularity near interface corners and edges are determined based on analyses of eigenvalue equations. Straight-side and curved interface of materials are designed for the bi-material models to avoid singular stress fields around the interface corner and edge. Assuming that one stress component or combined stresses are responsible for failure at or near the interface, the stress concentration becomes critical for the design of bi-material joints with higher interfacial strength. Numerical results show that the stress state near the interface depends strongly on both the interface geometry and the combination of materials, and stress concentration may always occurs at or near the interface. Emphasis is placed on the necessity for geometric optimization of an interface in order to design singularity-free junction with higher interfacial strength.     

Residual stress field of ballised holes

Ballising, involving pushing a slightly over-sized ball made of hard material through a hole, is a kind of cold working process. Applying ballising process to fastener holes produces compressive residual stress on the edge of the holes, and therefore increases the fatigue life of the components or structures. Quantification of the residual stress field is critical to define and precede the ballising process. In this article, the ballised holes are modeled as cold-expanded holes. Elastic-perfectly plastic theory is employed to analyze the holes with cold expansion process. For theoretical simplification, an axially symmetrical thin plate with a cold expanded hole is assumed. The elasticplastic boundaries and residual stress distribution surrounding the cold expanded hole are derived. With the analysis, the residual stress field can be obtained together with actual cold expansion process in which only the diameters of hole before and after cold expansion need to be measured. As it is a non-destructive method, it provides a convenient way to estimate the elastic-plastic boundaries and residual stresses of cold worked holes. The approach is later extended to the case involving two cold-worked holes. A ballised hole is looked upon as a cold expanded hole and therefore is investigated by the approach. Specimens ballised with different interference levels are investigated. The effects of interference levels and specimen size on residual stresses are studied. The overall residual stresses of plates with two ballised holes are obtained by superposing the residual stresses induced on a single ballised hole. The effects of distance between the centers of the two holes with different interference levels on the residual stress field are revealed.     

刀刃半径对不锈钢切削表面残余应力影响的模拟

建立了平面应变有限元模型，采用更新的Lagrange方法模拟了奥氏体不锈钢AISI316L的正交切削过程；研究了刀刃圆弧半径对已加工表面残余应力的影响，发现随着半径的增大，残余拉应力和压应力的数值都增大，压应力层厚度也增大，但是拉应力层厚度不变。将模拟结果与实验结果进行对比，发现二者是吻合的，从而验证了有限元模拟的可用性。     

Plastic Deformation Mechanism in Double-Roller Clamping Spinning of Flanged Thin-Walled Cylinder

Double?roller clamping spinning(DRCS) is a new process for forming a thin?walled cylinder with a complex surface flange. The process requires a small spinning force,and can visibly improve forming quality and production e ciency. However,the deformation mechanism of the process has not been completely understood. Therefore,both a finite element numerical simulation and experimental research on the DRCS process are carried out. The results show that both radial force and axial force dominate the forming process of DRCS. The deformation area elongates along the radial direction and bends along the axial direction under the action of the two forces. Both the outer edge and round corner of the flange show the tangential tensile stress and radial compressive stress. The middle region shows tensile tangential stress and radial stress,while the inner edge shows compressive tangential stress and radial stress. Tan?gential tensile strain causes a wall thickness reduction in the outer edge and middle regions of the flange. The large compressive thickness strain causes material accumulation and thus,an increase in the wall thickness of the round corner. Because of bending deformation,the round corner shows a large radial tensile strain in addition. The inner edge of the flange shows small radial compressive strain and tensile strain in thickness. Thus,the wall thickness on the inner edge of the flange continues to increase,although the increment is small. Furthermore,microstructure analysis and tensile test results show that the flanged thin?walled cylinder formed by DRCS has good mechanical properties. The results provide instructions for the application of the DRCS process.     

Fretting Wear Behavior of Laser Peened Ti-6Al-4V

This work deals with the influence of laser peening on the fretting wear behavior of Ti-6Al-4V. Laser peening was carried out on Ti-6Al-4V. The laser-peened surface was characterized by transmission electron microscopy. Surface roughness, nanoindentation hardness, residual stress, and tensile properties of the material in both laser-peened and unpeened conditions were determined. Fretting wear tests were conducted at different normal loads using a ball-on-flat contact geometry. Laser peening resulted in the formation of nanocrystallites on the surface and near-surface regions, increased hardness, and compressive residual stress. Laser peening did not affect the tensile properties and surface roughness significantly. There was no considerable difference between the values of the tangential force coefficient of laser-peened and unpeened samples. The fretting scar size, wear volume, and wear rate of laser-peened specimens were lower than those of unpeened samples. This may be attributed to an increase in surface hardness due to strain hardening and grain refinement at the surface and near-surface regions, higher compressive residual stress, and higher resistance to plastic deformation of laser-peened samples.     

Finite element analysis of shot-peening effect on fretting fatigue parameters

Shot peening is widely used to improve the fretting fatigue strength of critical surfaces. Fretting fatigue occurs in contacting parts that are subjected to fluctuating loads and sliding movements at the same time. This paper presents a sequential finite element simulation to investigate the shot peening effects on normal stress, shear stress, bulk stress and slip amplitude, which are considered to be the controlling parameters of fretting damage. The results demonstrated that among the modifications related to shot peening, compressive residual stress has a dominant effect on the fretting parameters.     

TiB_2/Al复合材料喷丸后微区残余应力的有限元模拟

采用ANSYS/LS-DYNA有限元分析软件建立了颗粒增强TiB2/Al复合材料的喷丸模型,并对喷丸后残余应力分布进行了预测;然后对复合材料进行了喷丸试验,对残余应力进行了检测;将试验结果与模拟结果进行了对比。结果表明:该复合材料喷丸后残余应力分布的试验结果与模拟结果基本相符;喷丸后最表层部分增强体呈拉应力状态,在材料残余压应力场内,由于增强体和基体材料力学性能的差异,增强体的残余应力值普遍大于基体中的。     

The use of ultrasonic cavitation peening to improve micro-burr-free surfaces

Advances in machining technology, particularly in the field of micro-machining, have led to the design and creation of miniature components suitable for use in the precision engineering industry. However, the need to contain ubiquitous burrs still exists and has to be addressed. Previous studies on deburring have mostly focused on the parametric investigations of orientation, temperature, type of liquid media and abrasives, frequency, deburring time and power. It is hypothesized that by inducing compressive residual stresses on a pre-machined workpiece surface, the resulting burrs caused by machining can be minimized or even eliminated. The paper presents the findings of an investigative study into the possibility of inducing compressive residual stresses on machined surfaces by the use of ultrasonic cavitation, with the aim of reducing or eliminating burr formation. The paper also briefly reviews the development of ultrasonic cavitation and covers published work on deburring by ultrasonic cavitation. Experimental results are presented on the performance of ultrasonic cavitation peening on the residual stress in Stavax stainless steels and on micro-burr formation.     

Multiphysics simulation on electromagnetic peening of predrilled holes

The areas around predrilled holes are highly stressed regions where cracks can easily form, which decreases the resistance of metal components to fatigue and causes serious problems in industry, such as aerospace, automobile, and ship building. In this paper, a novel method utilizing the effects of a strong pulsed electromagnetic field is introduced to effectively produce residual compressive stress into the surface of a predrilled hole. Basic principles of electromagnetic peening (EMP) are investigated using a multiphysics simulation approach. Simulations of EMP process on the surface of a work piece with predrilled blind hole are carried out by COMSOL Multiphysics 3.4. The distributions of stress and strain are presented and discussed, which demonstrate the feasibility of EMP on inducing compressive residual stress into the interior surface around a hole. In addition, the distribution of residual stress varies with various hole depth and work piece thickness is investigated. Finally, the influence of process parameters is also studied. The results of this work will help design process of strengthening 3D surfaces in metal components in manufacturing industry.