Residual stress analysis of an external grooved thick-walled pressure vessel

Residual stress analysis of an autofrettaged thick-walled pressure vessel containing an external groove was described in order to calculate the stress concentration at the external groove. The autofrettage residual stress distributions of the external grooved thick-walled pressure vessel were simulated using an equivalent thermal loading from the analogy of thermal and autofrettage residual stress fields. Thermal stresses due to the simulated thermal loadings for various degrees of autofrettage overstrain level were computed using finite element methods. Very high stress concentration factors due to autofrettage loadings were obtained at the external groove root that contained a sharp root radius. Experimental measurement of residual stresses for a fully autofrettaged smooth thick-walled pressure vessel using an equivalent saw cut method resulted in very close agreement with the theoretical autofrettage residual stress distributions. The stress analysis results implied that the autofrettage residual stress concentration might cause a cracking problem at the external groove root of the thick-walled pressure vessel, indicating that lower autofrettage overstrain and a groove geometry change were desirable for enhanced durability.     

Elastic-plastic stress analysis and fatigue lifetime prediction of cross-bores in autofrettaged pressure vessels

Elastic-plastic stress analysis has been performed to evaluate the fatigue life of an autofrettaged pressure vessel containing cross-bores subjected to pulsating internal pressure of 200 MPa. Finite element analyses were used to calculate the residual and operating stress distributions of the pressure vessel due to the autofrettage process and pulsating internal pressure, respectively. Theoretical stress concentration factors of 3.06, 2.58, and 2.64 were obtained at the cross-bore of the pressure vessel due to internal pressure, 50%, and 100% autofrettage loadings, respectively. Local stresses and local strains determined from the elastic-plastic finite element analysis were employed to calculate the failure location and fatigue life of the pressure vessel with radial cross-bores, incorporating the low-cycle fatigue properties of the pressure vessel steel and fatigue damage parameters. Increase in the amount of overstrain by autofrettage process moved the crack initiation location from the inner radius toward a mid-wall, and extended the crack initiation life, Predicted fatigue life of the fully autofrettaged pressure vessel with cross-bores increased about 50%, compared to the unautofrettaged pressure vessel. At the autofrettage level higher than 50%, the failure location and fatigue life of the pressure vessel were not significantly influenced by the autofrettage level.     

超高压压缩机填料盘润滑油孔自增强的研究

超高压压缩机填料盘结构复杂并承受很高的压力脉动,填料盘润滑油孔附近会产生明显的应力集中,需要自增强处理来得到良好的预应力状态。本文通过建立填料盘三维有限元模型,利用ANSYS双线性随动强化模型模拟自增强处理过程,分析自增强前后润滑油孔对填料盘等效应力分布、等效对称循环载荷、周向应力分布的影响,定量分析对润滑油孔施加自增强的必要性,得到自增强压力推荐范围。     

热-机械载荷下厚壁圆筒自增强压力优化分析

理论上推导了厚壁圆筒在内压及热载荷共同作用下的最佳自增强压力,并基于ANSYS的优化分析结果对理论解进行了验证。结果表明最佳自增强压力的理论解与数值解一致,最大误差不超过1%;另外,不考虑热载荷进行自增强后,会增大工作状态下厚壁圆筒内外壁应力差,降低结构的疲劳强度;工程上可根据本文解析解进行自增强处理,以提高厚壁圆筒的承载能力。     

热预应力自增强厚壁圆筒研究

厚壁圆筒自增强处理技术的关键在于预应力。传统的自增强处理技术采用的是机械预应力方法,即在圆筒投入使用前,对其施加超过操作压力的自增强压力,使之获得残余预应力。考虑到厚壁圆筒内、外壁存在温差时,筒壁中有热应力产生,因此针对厚壁圆筒自增强问题,提出了以热应力作为预应力的自增强技术。具体研究了圆筒壁厚、温差等对热应力与总应力(热应力与操作应力的叠加)的影响、热应力与总应力的变化趋势、各种参数间的约束条件;在分析热应力与总应力特性的基础上,得出最佳设计条件,提出了基于第四强度理论的热预应力自增强厚壁圆筒的设计方法。结果表明,热预应力能有效地降低和均化厚壁圆筒的操作应力;按照所提出的设计方法,在确保圆筒安全的前提下,可使圆筒获得最大的承载能力和最小的壁厚。     

A study on integrated design for improving fatigue life of common rail pipe considering stress concentration at complex shape

Common rail pipe in a clean diesel vehicle plays a key role in supplying fuel from a rail to an injector of each cylinder connecting engine under a repeated internal pressure. For satisfying EU emission standards, fuel injection pressure is increased to be over 200 MPa, and it causes stress concentration at the outer surface of neck part with discontinuity and complexity of shape of the pipe. For preventing folding defects and for improving a fatigue life, integrated design of a heading process and an autofrettage process are required because the methods for reducing the stress concentration by using changing design, material and adding heat treatments cause considerable effects on performance and price of the pipe. In this study, the heading process for checking folding defects of pipe head is performed by FEA, and an autofrettage process is designed for improving a fatigue life by considering the stress concentration at the complex and discontinuous shape. The optimal autofrettage pressure is required not only to relieve the stress concentration but also to be not beyond the allowable internal pressure for the process safety. The allowable internal pressure for the autofrettage process is determined by using theoretical analysis and FEA, and the stress distributions through the autofrettage process are obtained by using the commercial software, ANSYS-Workbench. On the basis of the FEA results, the autofrettage pressure is optimized, and SEM (scanning electron microscope) and fatigue tests were performed for prototypes to validate analysis of the integrated process design (Heading process and the autofrettage process).     

机械自紧工艺控制参量的实验研究

机械自紧技术在超高压容器和身管制造过程中得到广泛采用,但国内尚无完整的设计方法和工艺规范。采用实验方法来研究机械自紧问题,设计了自紧冲头,搭建由液压泵站提供动力的机械自紧实验装置,建立应变、冲头行程及推力测量系统。对厚壁圆筒试件进行机械自紧实验后,获得了冲头挤进过程中推力的变化数据,结合实验结果和经验公式归纳了机械自紧冲头推力计算公式,得到的推力与实验测量值的误差在±10%以内。测量了机械自紧过程中的重要工艺控制参量外表面应变、残余应变和冲头位移-外表面应变曲线。机械自紧后进行稳定化、机械加工对再屈服压力影响的实验。     

自增强疲劳寿命的理论分析

自增强处理是化工、石油行业提高高压设备承载能力和疲劳寿命的重要举措。文中利用断裂力学理论和疲劳损伤理论,探讨研究了自增强处理后残余应力对高压设备寿命的影响,得出引入残余压应力能提高高压设备寿命的结论,对今后在自增强处理方面的实验与理论研究具有指导意义。     

Autofrettage of thick-walled pipe bends

The bending of a thick-walled cylinder to a given radius involves an elastic–plastic deformation that results in a residual, axial stress distribution. The latter alternates from maximum tension to maximum compression between top and bottom halves of the cross-section. The residual stress levels depend upon the depth of plastic penetration and may be determined as a closed solution when they arise from a bending moment applied to either a non-hardening or linearly-hardening material. When the bent pipe receives an autofrettage treatment without an intermediate heat treatment, this produces a further residual, triaxial stress state. The interaction between the residual states from bending and autofrettage has an important effect upon the net axial stress and the equivalent stress. It is shown that large plastic penetrations arising from bending and autofrettage can residually stress the section beyond its yield point: in tension and in compression across both its halves. With the unloading from each process, a Bauschinger effect reduces the yield point to assist with the onset of reversed plasticity. The latter is far less beneficial than when unloading is elastic. It is shown how a nonlinear kinematic hardening model can be employed to avoid unloading plasticity at the inner and outer diameters. The consequence of interacting residual stresses is that axial stress can play as important a role as hoop stress when designing for safe service loadings. In general, an enhanced residual stress state is beneficial when compressive but detrimental when tensile. Pre-compression is often employed in practice to reduce tensile stress arising from internal pressure, axial force and self-weight. Here, the compressive residuals arising from an autofrettage treatment have long been exploited to enhance the fatigue life of process piping and weaponry.     

残余应力松弛和自增强处理压力对在役高压容器的安全影响

为了确定残余应力松弛和自增强处理压力对在役高压容器安全性能的影响,通过分析测试结果获得了残余应力的松弛规律,计算了在工作压力、残余应力作用下的当量等效应力沿壁厚分布情况,模拟计算出了不同的工作压力、自增强处理压力下的安全系数,推导出了最佳自增强处理压力。结果表明所研究的高压聚乙烯反应管在使用10年后,环向应力在近内壁区衰减最快,从-600MPa衰减到-333MPa,衰减率达45%;在弹性区衰减较小,残余应力峰值位置外移,但其峰值大小变化不大。对于自增强处理后的压力容器,在工作压力作用下,随着残余应力的松弛,内壁面当量等效应力增大,当量等效应力在弹塑性交界处最大,应该按此处的当量等效应力计算安全系数。依据示例聚乙烯反应管尺寸,模拟计算出在工作压力分别为180、280、380MPa时,经过自增强处理压力分别为606、677、743MPa的最佳自增强处理后,其安全系数比残余应力全部衰减为0时分别高16%、26%、37%。压力容器工作压力越大,经最佳自增强处理后安全系数增大得越多,但残余应力衰减对其安全影响越大。     

An integrated CAD/CAM system for CNG pressure vessel manufactured by deep drawing and ironing operation

The Tiber reinforced composite material is widely used in the multi-industrial field because of their high specific modulus and specific strength. It has two main merits which are to cut down energy by reducing weight and to prevent explosive damage proceeding to the sudden bursting which is generated by the pressure leakage condition. Therefore, Pressure vessels using this composite material can be applied in the field such as defence industry and aerospace industry. In this paper, for nonlinear finite element analysis of E-glass/epoxy filament winding of composite vessel subjected to internal pressure, the standard interpretation model is developed by using the ANSYS with AutoLISP and ANSYS APDL languages, general commercial software, which is verified as useful characteristic of the solution. Among the modules of the system, both the process planning module for carrying out the process planning of filament wound composite pressure vessel and the autofrettage process module for obtaining higher residual stress will minimize trial and error and reduce the period for developing new products. The system can serve as a valuable system for experts and as a dependable training aid for beginners.     

基于LabVIEW的超高压自增强测试系统设计

为了提高超高压缸体的承载能力并延长疲劳寿命,对制造缸体的原材料采取自增强处理,同时将LabVIEW技术应用到自增强处理过程中,开发了一个专门的超高压自增强测试系统.在LabVIEW环境下完成对自增强系统重要参数的采集、显示、存储以及相关性分析和曲线拟合等数据处理工作,取得了较好的效果.     

Effect of optimum plastic depth on stresses and load-bearing capacity of autofrettaged cylinder

Autofrettage is an effective measure to even distribution of stresses and raise load-bearing capacity for (ultra-)high pressure apparatus. Currently, the research on autofrettage has focused mostly on specific engineering problems, while general theoretical study is rarely done. To discover the general law contained in autofrettage theory, by the aid of the authors’ previous work and according to the third strength theory, theoretical problems about autofrettage are studied including residual stresses and their equivalent stress, total stresses and their equivalent stress, etc. Because of the equation of optimum depth of plastic zone which is presented in the authors’ previous work, the equations for the residual stresses and their equivalent stress as well as the total stress and their equivalent stress are simplified greatly. Thus the law of distribution of the residual stresses and their equivalent stress as well as the total stress and their equivalent stress and the varying tendency of these stresses are discovered. The relation among various parameters are revealed. The safe and optimum load-bearing conditions for cylinders are obtained. According to the results obtained by theoretical analysis, it is shown that if the two parameters, namely ratio of outside to inside radius, k, and depth of plastic zone, kj, meet the equation of optimum depth of plastic zone, when the pressure contained in an autofrettaged cylinder is lower than two times the initial yield pressure of the unautofrettaged cylinder, the equivalent residual stress and the equivalent total stress at the inside surface as well as the elastic-plastic juncture of a cylinder are lower than yield strength. When an autofrettaged cylinder is subjected to just two times the initial yield pressure of the unautofrettaged cylinder, the equivalent total stress within the whole plastic zone is just identically equal to the yield strength, or it is a constant. The proposed research theoretically depicts the stress state of ultra-)high pressure autofrettaged cylinder more accurately and more reasonably and provides the reference for design of (ultra-)high pressure apparatus.     

厚壁圆筒自增强压力的优化分析

采用考虑材料应变强化效应和包辛格效应的双线性材料模型,建立了厚壁圆筒自增强理论模型。基于工作时的等效应力及周向应力,提出了最佳自增强压力的评定方法并给出了理论求解过程。采用有限元软件对自增强厚壁圆筒涉及的三个加载过程进行模拟分析,模拟结果与理论计算结果相吻合。由模拟结果得到了厚壁圆筒工作时的最大等效应力和最大周向应力与自增强压力的关系曲线,并采用直接加权组合法进行优化,得到了最佳自增强压力。研究结果为厚壁圆筒最佳自增强压力的求解提供了新思路,具有一定的工程意义。     

高压液压能源系统热特性及热控制仿真分析

高压液压技术是未来飞机液压系统的主要发展趋势,由于损耗功率增加,系统温度变化将更加剧烈并影响飞行安全,因此,高压液压系统的热特性与热控制技术是未来飞机液压系统设计需要考虑的一个重要因素。以某高压液压能源系统为例,对液压能源系统的主要液压元件进行生热和散热机理分析。利用AMESim软件开展液压系统温度特性分析,权衡系统是否需要热交换器。结果表明:热交换器有效降低系统油液温度至安全温度内;同时,得到燃油-液压油热交换器位于不同位置、系统在不同飞行阶段下不同环境温度、机翼处管路引入冷气流等工况下温度变化趋势,为飞机高压液压能源系统热设计提供参考。     

自增强技术的研究

介绍目前实现白增强技术的主要方法，如机械式挤压法、直接静液压法、爆炸胀压法和固体自增强法等。根据自增强技术原理，结合材料的应力应变关系，总结自增强技术对材料的要求。     

A residual stress analysis program using a Matlab GUI on an autofrettaged compound cylinder

A program for the residual stress analysis of an autofrettaged compound cylinder is designed using a Matlab graphical user interface (GUI) and program design technique. The high-pressure vessels are autofrettaged in order to increase their operating pressure and fatigue life. An autofrettage process causes plastic expansion of the inner section of the cylinder, adding residual compressive stress to the bore after relaxation. Such a compound cylinder is produced via a shrink-fit procedure that incorporates a monobloc tube that has previously undergone autofrettage. This paper presents a simple and visual tool to calculate the residual stress and describe the distribution of residual stress for both the elastic-perfectly plastic model and the strain-hardening model.     

基于三剪统一强度准则的厚壁圆筒自增强分析

基于三剪统一强度准则,考虑材料应变强化效应、包辛格效应、拉压异性及中间主应力的影响,采用双线性强化材料模型对厚壁圆筒进行自增强分析,得到了厚壁圆筒加载应力、残余应力和工作应力的解析解,提出了最佳自增强压力的计算方法,探讨了拉压比、强度准则变化参数的影响,比较了自增强处理和非自增强处理及双线性强化模型和理想弹塑性模型厚壁圆筒的应力分布差异。研究结果表明：厚壁圆筒的最佳自增强压力随半径比和强度准则参数的增大而增大;工作时的最大等效应力随半径比和强度理论参数的增大而减小,随拉压比的增大而增大;自增强等效应力的最大值在弹塑性分界面处,且应力沿壁厚的分布较均匀;与理想弹塑性模型相比,双线性强化模型所对应的弹塑性分界面半径和残余应力较小,且随着自增强压力的增大,两种模型的差值越来越大;等效应力随半径比的变化规律可为厚壁圆筒选择合理的壁厚提供一定的参考;自增强技术可改善厚壁圆筒工作时的实际应力分布,提高其极限承载能力。     

Analysis of the autofrettage effect in improving the fatigue resistance of automotive CNG storage vessels

Type II compressed natural gas (CNG) storage vessels for automobiles are becoming widely used. They are not only supplied to automakers in Korea (such as Hyundai Motors), but are being exported overseas in increasing numbers. Autofrettage is a process that produces beneficial residual stresses in a vessel by subjecting it to excessive internal pressure. This strengthens the vessel and improves its fatigue resistance. This paper presents research into the autofrettage process and the residual stresses it produces in type II CNG storage vessels. Finite element analysis and a closed-form equation are used. Fatigue resistance is then analyzed via a fatigue evaluation performed according to ASME section VIII.     

《超高压水切割机》标准的制定

介绍了标准中规定的选材，试验方法，投料，超高压系统，制造技术和自动控制的要求，主要易损件和压力下降的范围，切割平台的测量精度和试验方法的要求也作了相应的阐述。