厚壁圆筒最佳自紧压力的有限元分析

考虑厚壁圆筒在发生塑性变形后材料的强化及鲍辛格效应,建立与实际材料一致的数学模型来求解自紧压力与弹塑性分界半径的关系,以确定自紧压力的取值范围.并提出了最佳自紧压力的评定方法及求解过程,然后使用Chabeche循环塑性本构模型,模拟材料循环硬化及鲍辛格效应对残余应力的影响,并通过Marc有限元软件对厚壁圆筒的自紧及工作过程进行仿真,分析残余应力及工作过程中的最大应力与自紧压力的关系,通过比较,找出厚壁圆筒最大等效应力(包括平时及工作状态)与自紧压力的关系曲线,最后求出最佳自紧压力.     

基于ABAQUS的身管外自紧仿真分析

自紧身管是高膛压火炮普遍采用的身管，目前均采用内自紧方式进行加工。为检验外自紧方式加工自紧身管的可行性，采用ABAQUS软件对身管内外自紧加工的应力分布进行了仿真分析。研究表明：内外自紧都会导致身管内壁先于外壁塑性变形并达到一定的自紧度；内自紧可减小射击应力，外自紧会使射击应力增大；对于理想弹塑性材料，内自紧身管射击应力小于材料屈服极限，外自紧身管射击应力的塑变区域不超过制造时塑变区域，内外自紧方式均可以提高身管强度；达到同样的自紧度时，内外自紧身管具有同样的强度，但是外自紧需要的压力要小于内自紧压力。     

基于自紧弹工艺的自紧身管强度数值仿真分析

自紧身管是高膛压火炮普遍采用的身管,目前多采用液压和机械自紧方法。自紧弹自紧是一种较新的自紧方法,具有很高的效率,但是只能实现局部自紧。文中采用数值仿真方法,分析了某型火炮自紧弹自紧身管的相关问题,包括身管使用中要承受的最大压强、单筒身管和全长全塑身管的强度、自紧弹自紧身管的强度以及永久胀大比等。研究表明:单筒身管强度不能满足要求,自紧弹自紧后,身管膛底部分强度得到提高,身管各截面安全系数均大于1.2,自紧区域自紧度不超过0.45,膛线起始部永久胀大比不超过0.01%,自紧弹自紧身管能够满足强度要求。     

枪炮身管外自紧研究

目前枪炮自紧身管都是通过内自紧来实现的。文中提出用外自紧来完成身管自紧的设想,推导了外自紧相关理论公式,仿真分析了内、外自紧身管应力分布和强度的区别。研究标明:外自紧也能达到身管自紧的目的,外自紧身管强度低于内自紧身管。研究为自紧身管制造指出了新的思路。     

基于数值仿真的四种类型身管强度比较

强度是枪炮身管的核心指标。为更好地理解不同类型身管强度机理,借助数值仿真方法,基于第二强度理论对单筒、筒紧、衬管和自紧四种类型身管的应力分布和强度机理进行了比较。研究表明,结构和材料参数相同情况下,衬管、单筒、筒紧、自紧身管强度依次提高;当内外管之间相对紧缩量从负值到零到正值变化时,身管类型从衬管变为单筒再变为筒紧;自紧身管类似于多层筒紧身管。研究为枪炮身管设计提供了参考。     

基于Matlab和ABAQUS的身管应力分析比较

身管强度与身管应力密切相关。身管应力既可以基于相关解析公式用Matlab软件计算，也可以采用有限元软件ABAQUS分析，对特定单筒身管和自紧身管实例，采用前述两种方法进行应力分析并比较，结果表明：对弹性变形的单筒身管，两者分析结果基本一致，特征参数误差不超过2%；对塑性变形的自紧身管，两者分析结果差异也很小，两者计算出的自紧压力差异随着自紧度增大而增大，但不超过2%;ABAQUS计算的自紧压力和内壁径向应力绝对值差异也随着自紧度增大而增大，但不超过0.5%。     

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。     

金属合金中的Bauschinger效应

本文对金属合金中(尤其是两相合金中)的Bauschinger效应的研究进展进行了评述.文中介绍了描述Bauschinger效应的参量和测试方法,讨论了阐明了Bauschinger 效应的微观力学模型.最后讨论了Bauschinger 效应在管线制造,尺寸稳定性,氢脆,校直工艺的制定等方面的实际意义.     

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。     

双相钢中的Bauschinger效应和背应力

本文研究了Mn-V热处理双相钢中的Bauschinger效应和背应力。结果表明：Mn-V热处理双相钢中存在有Bauschinger效应和永久软化,在预应变引入基体中的位错是可动的条件下,背应力帮助位错运动,导致反向流变时屈服应力下降和反向流变曲线圆化,产生Bauschinger效应。对双相钢中的背应力与显微组织之间的关系测定与计算表明：双相钢的背应力硬化机制与弥散硬化合金类同,时效可以消除双相钢中的Bauschinger效应,但不改变背应力大小,而反向流变可使Bauschinger效应和背应力同时下降或消除。     

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。     

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。     

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.     

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.     

Analysis on autofrettage of cylinders

Autofrettage is an effective technique to improve load-bearing capacity and safety for pressure vessels.For autofrettaged cylinder,the depth of plastic zone,or overstrain is a key factor which affects load-bearing capacity and safety.The previous research on overstrain was not done in terms of the point of view of raising load-bearing capacity as far as possible and simultaneously avoiding compressive yield for cylinders experiencing autofrettage handling,and there were no analytic solutions of autofrettage in the above view point presented,the 3rd and 4th strength theories were not applied synthetically in the research to compare the results from these two theories.In this paper,with the aid of the analytic method,based on summing up the authors’ previous research,results from autofrettage of a cylinder based on the 3rd and 4th strength theories are studied and compared,and the laws contained in the results are looked into.Then,the essential cause and reason for the obtained laws are analyzed and the inherent and meaning relations between various parameters in autofrettage theory are revealed.It is shown that the maximum radius ratio for equivalent residual stress at inside surface never exceeds the yield strength even for a cylinder experiencing wholly yielded autofrettage,or the critical radius ratio is kc=2.218 457 489 916 7…,irrespective of the 3rd or 4th strength theories.The equation relating the depth of plastic zone with the thickness of a cylinder is identical for the 3rd and 4th strength theories.In form,the optimum load-bearing capacity of an autofrettaged cylinder is two times the initial yield pressure of the unautofrettaged cylinder irrespective of the 3rd or 4th strength theory.The revealed inherent relations between various parameters and varying laws of the parameters as well as the forms of the relations under the 3rd and 4th strength theories not only have theoretical meanings but also have prospects in engineering application.     

厚壁管自增强残余应力计算模型

基于对材料应变硬化行为的不同简化得到不同的自增强理论模型 ,如理想弹塑性模型、线性应变硬化模型、幂硬化模型以及这些简化的应变硬化模型的组合模型等。不同的模型适用于不同的应变硬化行为的材料。本文提出了一种无需对材料的应力应变行为进行简化 ,考虑材料应变硬化行为和包辛格效应的厚壁管自增强理论一般模型。由于包辛格效应系数是预应变的函数 ,本模型用参数来考虑这种影响。该模型模型中用到的计算参数通过对材料拉压曲线的拟合得到 ,理想弹塑性模型 ,线性强化模型以及幂硬化模型等都是本模型的特例。     

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.     

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.     

The overstrain of Thick-Walled cylinders considering the bauschinger effect factor (BEF)

An independent kinematic hardening material model in which the reverse yielding point is defined by the Bauschinger effect factor (BEF), has been defined for stainless steel SUS 304. The material model and the BEF are obtained experimentally and represented mathematically as continuous functions of effective plastic strain. The material model has been incorporated in a non-linear stress analysis for the prediction of reverse yielding in thick-walled cylinders during the autofrettage process of these vessels. Residual stress distributions of the independent kinematic hardening material model at the onset of reverse yielding are compared with residual stresses of an isotropic hardening model showing the significant effect of the BEF on reverse yielding predictions. Critical pressures of direct and reverse yielding are obtained for the most commonly used cylinders and a range of permissible internal pressures for an efficient autofrettaged process is recommended.