Three-dimensional finite element analysis of cold expansion of adjacent holes

Three-dimensional elasto-plastic finite element analysis was conducted to evaluate the development and growth of the plastic zone and unloading residual stresses resulting from the cold expansion of two adjacent holes. The contact between the mandrel and the hole was modelled using special contact elements. The work examined the effect of the geometry of two adjacent cold-expanded holes upon the resulting three-dimensional residual stress field. Both simultaneous and sequential expansion of the two holes was considered. It was shown that the compressive residual stress varies across the thickness of the specimen and that two-dimensional finite element models are incapable of accurately predicting the residual stresses resulting from the cold expansion process. It was further shown that sequential expansion reduces dramatically the compressive residual stresses. The validity of the developed finite element model was verified with existing experimental results.     

TC4板孔冷挤压强化残余应力分布与疲劳寿命

开展了不同挤压量下TC4钛合金板孔冷挤压强化有限元仿真研究,得到了挤压强化后最小截面的切向残余应力分布规律,分析了挤压量对受载试样孔边应力分布的影响,探讨了挤压量、残余应力和疲劳增益三者之间的内在关系。采用开缝衬套冷挤压强化工艺对TC4带孔板件进行冷挤压和疲劳验证试验。研究结果表明,挤压强化后的孔边切向压缩残余应力可以有效降低孔周应力集中程度,优化受拉试样最小截面应力分布,改变裂纹源的位置并延长疲劳裂纹的萌生和扩展寿命,有效提高试样疲劳寿命。综合仿真和疲劳试验得到TC4板孔最优挤压量为4%。     

压缩载荷对冷胀孔残余应力的影响

用考虑组合强化、增量理论的有限元法计算了２１２４－Ｔ８５１ 板件开缝衬套冷胀孔的残余应力场及其在压缩载荷作用下的变化情况。结果表明冷胀之后的压载循环将引起冷胀残余应力的衰减。一个足够大的压缩载荷能够使冷胀工艺给疲劳寿命带来的增益完全丧失。文中说明压缩载荷对冷胀的作用可以和超载后半周压载对裂纹的作用相比拟,建议在确定合理胀孔量时应当把它对所承受的载荷谱中压缩载荷的敏感性考虑在内,应当加强有关的疲劳对比试验研究及残余应力场测量研究。     

Prediction of fatigue life in cold expanded Al-alloy 2024-T3 plates used in double shear lap joints

To predict fatigue crack initiation and fatigue crack growth lives in cold expanded double shear lap joints a numerical method has been employed. The total estimated fatigue lives were compared with available experimental fatigue test results for plain hole and cold expanded hole specimens of Al 2024-T3 in double shear lap joints. Three-dimensional finite element simulations have been performed in order to obtain the created residual stresses field due to cold expansion and subsequent far field longitudinal loading in the double shear lap joint. The obtained stress and strain distributions from the finite element analyses were employed to predict stress concentration factors to calculate fatigue crack initiation and fatigue crack growth lives using AFGROW computer code. The predicted fatigue lives demonstrate that there is a good agreement between the proposed method and experimental fatigue test results.     

冷胀提高6005铝合金疲劳寿命的机理

针对冷胀对6005铝合金疲劳裂纹产生和扩展的影响,用单边预切口试样研究了含缺陷结构冷胀后的疲劳寿命,分析了不同情况下新裂纹产生的循环数;并用ANSYS软件进行了数值模拟,确定冷胀产生的残余应力场和塑性区及在不同参数下冷胀度对其的影响.结果表明:孔径和冷胀度对最大残余压应力值影响不大,都在材料屈服强度1～1.1倍的限定范围内,而冷胀度对塑性变形区和残余压应力区大小的影响则非常显著.     

孔挤压强化和工艺参数研究

孔挤压强化可以显著提高机械连接的疲劳强度。采用有限元方法建立了孔挤压强化轴对称模型,得到了孔壁残余应力分布状态,建立了孔壁残余应力和孔径、板件厚度、挤压强化过盈量等工艺参数之间的关系曲线。结果表明,周向残余应力和孔径、挤压强化过盈量之间为对数关系,和板件厚度之间为线性关系;径向残余应力和孔径、挤压强化过盈量之间为对数关系,和板厚(以2.5mm为分界线)之间为线性关系,为孔挤压强化效果的定量化认识奠定了基础。     

交变载荷下孔边残余应力变化规律的无损研究

用高灵敏度的云纹干涉法，无损研究了冷挤孔边残余应力在交变载荷下的变化规律。使用１２００线／ｍｍ的闪耀正交光栅，实时测量了冷挤孔边正交方向上因残余应力衰减而引起的应变量，估算出了这些方向上的残余应力的衰减量，并研究了不同挤压量下的残余应力衰减规律。试验研究表明，ＬＹ１２ＣＺ试件的孔边残余应力在交变载荷作用下发生了衰减，孔边残余应力的衰减量受挤压量影响，挤压量越大，孔边的残余应力相对衰减量就越小。     

A fundamental investigation on rotating tool cold expansion: numerical and experimental perspectives

In this study, we present the development of a novel technique for cold expansion using a rotating tapered mandrel that friction processes the cylindrical wall of the fastener hole and simultaneously cold expands it. The developed technique, named as rotating tool cold expansion (RTCE), is experimentally and numerically investigated. A 3D thermomechanical finite element model for predicting the compressive residual stress, responsible for delaying crack propagation from the edges of the holes, is introduced. The efficacy is that RTCE is assessed for varying degrees of cold expansion under different lubricating conditions at the tool–workpiece interface, such as dry, metal working fluid, and nanopowder. The plastic deformation combined with friction stirring at the tool–workpiece interface helps the RTCE in controlling the surface damage at entry and exit of the hole that is most often observed with the conventional cold expansion technique. Enhanced friction due to the nanopowder at the tool–workpiece interface helps in sustaining efficacy of the RTCE even at a higher degree of cold expansion which otherwise leads to surface damage with other mediums.     

Numerical and experimental studies of the build-up and redistribution of residual stresses obtained from stress-coining

The build-up and redistribution of intentionally introduced residual strains and stresses after the coining of holes in AA6082-T6 aluminium plates has been studied numerically in a nonlinear finite element analysis. Verifying experiments have been performed. Strain gauges have been used to monitor the strains at the bore of the hole during the coining process and a part of the subsequent simulated service load. The positive effect of compressive coining residual stresses on the propagation of a through-the-thickness crack emanating from the hole is quantified by integrating Paris' law for fatigue crack growth where the entering stress intensity factors are evaluated by use of the weight function technique. The finite element method is shown to be useful for the simulation of the coining process, for the stress redistribution during subsequent service loading and for the search of combinations of coining parameters that will provide a beneficial coining process.     

电弧气刨过程应力场特点分析

建立一个既考虑电弧热作用又考虑材料去除的电弧气刨三维有限元模型,详细分析电弧气刨过程应力场演变及最终残余应力的分布特征,结果表明：应力场的演变过程及最终残余应力的分布和平板焊接过程的较为相似。在电弧气刨过程中,由于电弧强烈的热作用,导致金属材料的热胀冷缩,在坡口附近的一圈薄层中出现很高的拉应力,纵向应力远大于横向应力,且存在三向拉应力的集中,这对电弧气刨的工程应用有相当不利的影响。在此基础上,进一步建立考虑初始应力场的电弧气刨应力场分析模型,揭示出电弧气刨对板材初始应力的影响机理。     

高强度钢材料小孔孔边R冷挤压强化工艺

小孔孔边R挤压强化工艺是基于开缝衬套冷挤压理念,作为在疲劳敏感孔处延缓裂纹扩展的一种方法。该工艺利用在孔边施加一定的压力,引起材料径向塑性流动,并形成沿孔边缘向外扩展的环形残余压应力区,提高工件的抗疲劳强度。     

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.     

Influence of inclined holes in measurement of residual stress by the hole drilling method

The hole drilling method is widely used in measuring residual stress in surfaces. In this method, the inclination of holes is one of the sources of error. This paper presents a finite element analysis of the influence of inclined holes on the uniaxial residual stress field. The error in stress has been found to increase proportionally to the correct inclined angle of the hole. The correction equations by which one may easily obtain the residual stress, taking account of the inclined angle and direction, have been derived. The error of stress due to the inclined hole has been reduced to around 1% using the correction equations.     

胀接工艺对镍基合金换热管残余应力的影响

胀接残余拉伸应力是造成核岛换热设备换热管应力腐蚀开裂的主要原因。选取SB -163 UNS N06690镍基合金换热管开展胀接试验,检测分析换热管胀接残余应力。研究结果表明：换热管胀接过渡区残余拉应力最大,胀接区域次之,未胀接区域最小;机械胀接工艺使管子变形量明显大于液压胀接,过渡区域残余拉应力相对较大,且随壁厚减薄率的增加而变大。     

Effects of the indentation process on fatigue life of drilled specimens: optimization of the distance between adjacent holes

The generation of permanent compressive stresses around the holes is recognized as a valuable mean to delay the onset and propagation of the defects and to extend the fatigue life of the mechanical components. In the work, a bilateral indentation process, performed on each side of the component, is widely used in order to create a residual circumferential stress field around the area to be drilled and that persists after the drilling operation. In order to evaluate the process parameters and to identify optimal geometric solutions, several static and fatigue tests are conducted on AW 6082-T6 aluminum alloy specimen where two holes are created. In particular, experimental tests on only drilled specimens (OD) and specimens subjected to Indentation process (IP) varying the indentation depth and the center-tocenter distance of the holes are performed. Several numerical analyses, conducted in ANSYS APDL environment with explicit solver, allow to determine the influence of the distance between two adjacent holes and the effect of the indentation depth on the residual stress distribution.     

7075铝合金板淬火残余应力模拟及实验研究

铝合金板在淬火过程中产生较大的残余应力,导致构件出现变形、翘曲。采用准耦合法对7075铝合金板在淬火过程中的温度场和应力场进行数值模拟,利用盲孔法对淬火后的残余应力分布进行实验测量。结果表明,7075铝合金板淬火后残余应力呈现出外压内拉的分布状态,淬火过程中形成的较大温度梯度造成的不均匀塑性变形是应力产生的主要因素,铝板表层和心部冷却速度的变化导致了应力分布状态的转变。实验结果验证了准耦合模拟方法的可行性,研究结论为残余应力控制提供重要依据。     

曲面冲击头随焊冲击旋转挤压法控制40Cr钢冷裂纹

采用随焊冲击旋转挤压法控制高强钢冷裂纹。为了增强加载区域的塑性延展,进行了曲面冲击头的设计,对强拘束度条件下40Cr钢工件,利用冲击旋转挤压曲面头进行了间距分别为80mm、100mm和110mm的随焊加载。采用小孔法测量了接头处的残余应力。实验结果表明,随焊冲击旋转挤压法能够显著减小残余应力,从减小接头残余应力的角度控制了高强钢焊接冷裂纹。     

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。     

Prediction of error due to eccentricity of hole in hole-drilling method using neural network

The measurement of residual stresses by the hole-drilling method has been used to evaluate residual stresses in structural members. In this method, eccentricity can usually occur between the hole center and rosette gage center. In this study, we obtained the magnitude of the error due to eccentricity of a hole through the finite element analysis. To predict the magnitude of the error due to eccentricity of a hole in the biaxial residual stress field, it could be learned through the backpropagation neural network. The prediction results of the error using the trained neural network showed good agreement with FE analyzed results.     

Récents développements de la mesure des contraintes résiduelles par méthode ultrasonore. Les principales sources d'erreurs

Recent developments in measurements of the residual stresses by ultrasonic method. The main sources of errors. . Welding causes deformation of welded joints leading to the occurrence of residual stresses. Now, it is possible to evaluate the field of residual stresses, whether experimentally, or by calculation. The knowledge of the distribution and level of the residual stresses in welded components is an important factor, particularly for predicting the damaging and ruin phenomena of these components. This paper presents recent advances obtained in the field of an ultrasonic method for stress measurements. This method is potentially advantageous because it is nondestructive, easy and fast to use in situ. In the paper are presented the results obtained with ultrasonics on steel welded plate, and comparison is made with stress measurements obtained by incremental step-by-step hole drilling method. These results are also discussed after offset and “microstructure” corrections. The paper discusses also the influence of the principal sources of errors and the precision obtained.