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.     

基于应变硬化模型的复合自增强圆筒的应力分析

自增强是通过提高压力容器的残余应力来增强他们的承载能力和疲劳寿命。自增强复合圆筒可以承受比一个具有相同尺寸的单层圆筒更高的压力。引入了应变硬化模型,对自增强复合圆筒的残余应力进行了分析,并对其分布进行了预测。研究表明:复合圆筒内壁的切向残余应力和应力幅随着超应变度的增大而增大。缩套是提高压缩残余应力的一个非常有效的方法。     

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.     

Autofrettage process analysis of a compound cylinder based on the elastic-perfectly plastic and strain hardening stress-strain curve

The autofrettage process enhances the carrying capacity and fatigue lifetime of pressure vessels by increasing their residual stress. A compound cylinder was introduced in order to increase residual stress. An autofrettaged compound cylinder can resist a higher pressure than a single cylinder having the same dimension. This residual stress can be measured through experimental or calculation processes. In this study, residual stress analysis of an autofrettaged compound cylinder was conducted. The elastic-perfectly plastic and strain hardening models were investigated. The residual stress distribution of the autofrettaged compound cylinder with shrink fit tolerance was predicted. Shrink fit is a very efficient way to extend compressive residual stress. The compressive residual stress of the strain-hardening model is smaller than that of the elastic-perfectly plastic model because of the Bauschinger effect. The compressive residual stress of the strain hardening model decreased by up to 80% overstrain level.     

圆筒形压力容器自增强若干问题研究

按第三强度理论,采用图像法与解析法相结合的方法分析论证圆筒形自增强压力容器弹塑性界面处总应力的当量应力、塑性区深度与反向屈服、承载能力等因素之间的理论联系,提供理论上确定与塑性区深度及承载能力有关的方法与公式、算图及表格。研究表明,以k2lnkj2-k2-kj2+2=0控制区深度最佳,此时可保证:σej≤1,σei'≥-1;以kj=exp(p/σs)控制区深度可使弹塑性界面处总应力的当量应力σej最小。其中k为容器径比、kj为塑性区深度、p/σs为内压/屈服点、σei'为内壁面处残余应力的当量应力。分析论证过程中得到的一些值得注意的规律、关系式及数据、图表等可作为压力容器工程设计时参考的理论基础和依据,也使自增强理论各参数间的关系和变化规律更清晰、透彻和实用。     

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.     

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。     

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。     

Plastic depth and load-bearing capacity of autofrettaged cylinders

Autofrettage technology is usually adopted to even out and reduce stresses as well as improve the load-bearing capacity of a variety of cylindrical ultra-high mechanical apparatuses. The autofrettage of cylinders is theoretically investigated based on maximum shear stress theory or the Tresca criterion to establish the general law for autofrettage theory. The equation for the optimum plastic depth for a certain load and radius ratio is derived to ensure that the equivalent stress of the total stress does not exceed the yield limit and the absolute value of the equivalent stress of the residual stress at the internal surface likewise does not exceed the yield limit. Through this equation, a set of concise equations for total stress and residual stresses are obtained. The safe and optimum load-bearing conditions for cylinders are presented. Results show that, provided the pressure contained in a cylinder is equal to the autofrettage pressure, irrespective of k _j, the equivalent total stress, σ_e, equals the yield limit everywhere in the entire plastic zone, that is, σ_e is a constant. In the elastic zone, σ_e is always lower than the yield limit, but if k _j is outside the quasi-infinite area enclosed by the curves of the sense and possible plastic depth, then either compressive yield occurs or k _j is meaningless. The results based on the Mises criterion and Tresca criteria are compared.     

含缺陷自增强厚壁圆筒塑性极限载荷分析

基于有限元理论,建立内壁含椭球形凹坑的厚壁圆筒有限元模型,模拟厚壁圆筒自增强过程的应力应变。采用三种不同的方法计算含凹坑缺陷的自增强厚壁圆筒的结构极限载荷,给出不同尺寸缺陷对极限载荷的影响规律。通过对比自增强与非增强条件下的极限载荷,表明自增强技术不能有效提高厚壁圆筒的极限承载能力,但在结构极限载荷下,含凹坑缺陷的自增强厚壁圆筒存在一个缺陷尺寸相对不敏感区,对提高结构的安全性是有利的。     

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.     

复合机械自紧技术研究

由于Bauschinger效应的影响，自紧厚壁管反向屈服现象普遍存在，使得内壁残余应力减少，厚壁管承载能力下降，因此，通过复合机械自紧技术，可消除Bauschinger效应，提高自紧厚壁管的弹性强度。本文在理论上和工艺上对复合自紧技术进行了研究，结果表明可大幅提高自紧 厚壁管的强度，这对自增强压力容器和管道设计具有指导意义。     

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。     

增压器浮环轴承润滑过程数值分析

以流体滑动轴承的润滑理论为基础，分析研究了内燃机增压器浮环轴承的工作机理和结构参数与性能的关系。建立了浮环轴承力平衡、力矩及摩擦功率损失的方程式，探讨了轴承内外膜承载能力与相应转速比、间隙比、偏心率等参数之间的关系。结果表明：浮动套内外半径比增大，偏心率减小，承载能力增大；间隙过大或过小，难以形成润滑油膜，影响承载能力；索氏数（Sommerfeld数）越大，则轴承的承载能力也越大。     

厚壁半球形液压缸的强度计算及其与圆筒形液压缸对比分析

半球形液压缸是缸梁一体式压力机的主要承力构件,它与传统的三梁四柱式液压机的圆筒形液压缸相比,结构上和受力状态都发生很大的改变,从而使承压能力有近一倍的提高。导出半球形液压缸强度计算公式,对于厚壁球壳,最薄弱区域是球的内壁,在该处有最大的经(纬)向拉应力与代数值最小(绝对值最大)的径向压应力。按照Tresca 强度准则或Mises强度准则均可算出数值相同的最大当量应力,强度计算公式表明该应力应小于或等于材料的许用应力。并将该式与圆筒形液压缸强度计算公式进行对比,在相同条件下(相同许用应力、相同内压及相同内半径),计算结果表明,半球形液压缸的壁厚要远小于圆筒形液压缸,减重效果明显。     

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。     

采用有限元法计算CNG2型气瓶自紧压力

利用有限元法可精确计算CNG2型气瓶各部分应力的特点,提出了以内胆材料屈服强度为基础、以屈服率为判据的计算CNG2型气瓶自紧压力的新方法,开辟了快速准确计算并指导气瓶生产的新途径。     

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.     

A comparative study of thermal and hydraulic autofrettage

Thermal autofrettage is a potential process to generate beneficial compressive residual stresses at and around the inner wall of a thickwalled cylinder for increasing its pressure carrying capacity. Due to its simplicity and inexpensive arrangement, it can compete with the conventional hydraulic autofrettage process. In this work, a comparative study of the thermal autofrettage and the hydraulic autofrettage is carried out. As the thermal autofrettage does not require hydraulic power pack, the process is more economical than the hydraulic autofrettage. The thermal autofrettage process is also studied for the thick-walled cylindrical vessels subjected to high thermal gradient with or without pressure and is compared with the hydraulic autofrettage. Comparison shows that for cylinders subjected to high thermal gradient without pressure, the thermal autofrettage is superior to the hydraulic autofrettage.     

帽形截面耐撞薄壁结构的棱边强化与优化设计

为了满足车身轻量化要求,乘用车耐撞车身的关键承载结构越来越多采用超高强度薄钢板制作。但是为减小板厚而提高材料强度可能引发平板屈曲,导致薄壁箱型结构受压侧的极限承载力的降低。为此,根据平板和圆柱壳棱边的屈曲理论,讨论帽形截面的薄壁箱型结构受压侧的有棱边强化平板的后屈曲极限承载力。通过CAE数值模拟,求得单向均匀压缩带棱边强化平板的横截面应力分布和极限承载力。从而明确用棱边强化钢板部件替代热成形钢板部件在乘用车耐撞结构中应用的可行性。最后,梳理了棱边强化设计应用的前景和后续待研究的课题。