RESULTS RESULTING FROM AUTOFRETTAGE OF CYLINDER

Autofrettage is used to introduce advantageous residual stresses into wall of a cylinder and to even distributions of total stresses. Basic theory on autofrettage has been functioning for several decades. It is necessary to reveal profound relations between parameters in the theory. Therefore, based on the 3rd strength theory, σej/σγ, σej/σγ, σej/σγ, σej/σγ and their relations, as well as p/σγ, are studied under ideal conditions, where σej/σγ is equivalent stress of total stresses at elastoplastic juncture/yield strength, σej/σγ is equivalent stress of total stresses at inside surface/yield strength, σej′/σγ is equivalent stress of residual stresses at elastoplastic juncture/yield strength, σej′/σγ is equivalent stress of residual stresses at inside surface/yield strength, p/σγ is load-bearing capacity of an autofiettaged cylinder/yield strength. Theoretical study on the parameters results in noticeable results and laws. The main idea is： to satisfy ｜σej′｜=σγ the relation between kj and k is k^2ln kj^2 -k^2 -kj^2 ＋2=0, where k is outside/inside radius ratio of a cylinder, kj is ratio of elastoplastic juncture radius to inside radius of a cylinder; when the plastic region covers the whole wall of a cylinder, for compressive yield not to occur after removing autofiettage pressure, the ultimate k is k=-2.218 46, with k=-2.218 46, a cylinder＇s ultimate load-bearing capacity equals its entire yield pressure, or p/σγ =（k2 -1）/k2=lnk; when kj≤√e =1.648 72, no matter how great k is, compressive yield never occurs after removing Pas; the maximum and optimum load-bearing capacity of an autofrettaged cylinder is just two times the loading which an unautofrettaged cylinder can bear elastically, or p /σγ = （k2 - 1） / k2, thus the limit of the load-bearing capacity of an autofrettaged cylinder is also just 2 times that of an unautofrettaged cylinder.     

ULTIMATE LOAD-BEARING CAPACITY OF CYLINDER DERIVED FROM AUTOFRETTAGE UNDER IDEAL CONDITION

According to the basic theory on autofrettage and according to the 4th strength theory, several parameters and their relations are studied under ideal condition, including σej/σy, the equivalent stress of total stresses at elastoplastic juncture; σei/σy, the equivalent stress of total stresses at inside surface; σej＇/σy, the equivalent stress of residual stresses at elastoplastic juncture; σei＇/σy, the equivalent stress of residual stresses at inside surface; and p/σy, load-bearing capacity of an autofrettaged cylinder. By theoretical study on relations between the parameters, noticeable results and laws are achieved： to satisfy ｜σei＇｜=σy. the relation between kj and k is, k^2lnkj^2-k^2-kj^2＋2=0, when k→∞, kj = √e = 1.648 72, as based on the 3rd strength theory, where k is the outside/inside radius ratio of a cylinder, kj is the ratio of elastoplastic juncture radius to inside radius of a cylinder; If the plastic region covers the whole wall of a cylinder, for compressive yield not to occur after removing autofrettage pressure, the ultimate k is k=-2.218 46 as based on the 3rd strength theory; With k=2.218 46, a cylinder＇s ultimate load-bearing capacity equals its entire yield pressure, or p/σy=21nk/√3; The maximum and optimum load-bearing capacity of an autofrettaged cylinder is just 2 times the loading which an unautofrettaged cylinder can bear elastically, or p/σy=2（k^2-1）/√3 k^2, and the limit of the load-bearing capacity of an autofrettaged cylinder is also just 2 times that of an unautofrettaged cylinder. The conclusions are the same as based on the 3rd strength theory, but some equations are different from each other.     

带裂纹压力容器內压爆破的试验研究

一、压力容器传统强度计算方法及其局限性传统的压力容器设计方法常用的有: 对薄壁容器(外内径比k=D外/D内≤1.2) 多采用中径公式: P/[δ]=t/R_m=2(k-1)/(k 1) (1) 式中P:内压; [σ]:许用设计应力,系对材料的屈服强度σ_y或强度极限σ_u取一定的安全系数n_y     

用云纹法测定压力容器中疲劳裂纹尖端的应变场

云纹法是近年来新发展起来的一种实验力学测试方法,它可以比较直观地在较大面积上测量应力应变集中部位的位移场和应变场,并可应用于任何可展曲面上,适宜于大应变情况。本文介绍了用云纹法测量压力容器中疲劳裂纹尖端附近的位移场和应变场的方法。并在三种应力状态下(σ_2/σ_1=0,σ_2/σ_1=0.5,σ_2/σ_1=1)计算了裂纹尖端附近塑性区的精确解,得到了裂纹尖端附近应变分布的规律。     

异种耐热钢焊接接头低塑性蠕变失效力学因素的研究

用有限元法计算了异种耐热钢接头邻近焊缝界面低强度材料区的蠕变应变分布,建立了蠕变拘束区（CRZ）应力三轴度（σ(av)/σ）与蠕变应变速率的比值（εw/εb）之间的数学关系模型,并通过蠕变断裂试验证明了数学模型的正确性。理论分析和试验结果表明,焊缝界面低强度材料蠕变拘束区的应力三轴度是接头发生低塑性蠕变失效的主要力学因素。     

统一强度理论在厚壁圆筒自增强中的应用

采用统一强度理论分析了厚壁圆筒自增强中的一些关键问题,得出了非自增强厚壁圆筒弹性极限载荷和塑性极限载荷的统一解的形式,以及弹塑性界面上当量应力最小时的弹塑性界面半径,并导出了当材料拉、压强度不同,及考虑中间主应力的情况下,自增强处理不发生反向屈服时的圆筒径比。另外,利用统一强度理论公式比较了现有的几种强度理论在自增强分析中所得的结果。     

简单结构平面应力等比例加载塑性失稳研究

对平面应力状态下几种简单结构等比例加载条件下塑性失稳研究表明,考虑材料应变强化性能和结构非线性变形,结构塑性失稳由应变控制。对于应力—应变关系符合Swift类型的材料,可以推导几种简单结构的失稳应变,并以应变作为失效判据。同时,根据应力—应变关系,求出相应应力,进一步确定结构的最大承载能力。进行圆筒压力容器有限元计算和爆破实验,证明以应变作为结构失效判据准确合理,可应用于压力容器弹塑性应力分析。     

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。     

折线裂纹的当量应力强度因子

对于折线裂纹端点的应力强度因子,目前尚无理论计算公式。华东化工学院提出了用圆弧裂纹当量方法以近似计算扩展后斜裂纹的应力强度因子,分别在单向受载及x、y双向受载(σ_2/σ_1=0.5)情况下作了计算,并与试验     

织构强化计算及在钛制压力容器设计中的应用

本文首先介绍了织构强化的概念,然后根据Hill的屈服条件和关系式σ_(1s)(1 1/R_1)1/2=σ_(2s)(1 1/R_2)~(1/2)导出了正交各向异性金属板织构强化的一个新算式,并分析了织构强化的效果与织构强化因子及应力状态的关系。最后,指出了此算法在压力容器设计中的应用,并举出一个数字实例,以示此算法与常规算法的区别。     

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.     

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.     

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.     

Safety and collapse of elastic–plastic beams against dynamic loads

The dynamic load-bearing capacity of elastic–plastic beam structures is analysed by the apparatus of shakedown theory. The reduced kinematic formulation for bending beams, which is equivalently deduced from Koiter’s kinematic theorem, combined with the plastic collapse’s method of hinge mechanisms appears effective in solving practical problems. The safety limits on the quasiperiodic dynamic loads as well as respective collapse mechanisms for a number of practical beams are determined.     

基于摩擦和胶接机理的混合锚固技术的研究

纤维增强复合材料加固的一般施工方法是利用树脂类胶结材料将FRP片材粘贴于混凝土表面,从而达到对结构构件补强加固及改善结构受力性能的目的。但是此种方法在利用FRP加固结构时,容易发生FRP-混凝土界面的早期剥离破坏,从而导致其加固效果受到限制,无法充分利用FRP的高强材料特点。为了解决上述破坏问题,本文研究一种新型的FRP片材粘结锚固技术,即基于摩擦和胶接机理的混合锚固技术(简称FAHB)。本文利用ANSYS软件对FAHB进行了非线性有限元分析,并与常规单粘结锚固技术进行对比。结果表明,FAHB粘结锚固技术可以有效避免整体粘结失效,从而显著提高构件的极限承载力,较单粘结锚固技术具有明显优势。     

Thermal stresses due to time exponentially decaying laser pulse: elasto-plastic wave propagations

In the present study, thermal stress developed in the substrate material, which is subjected to a laser heating pulse is formulated. The closed form solutions for the temperature and stress fields due to time exponentially decaying laser pulse are presented. The Laplace transformation method is employed when deriving the governing equations. The elastic and plastic propagation of the stress waves are considered and the depth of the plastic zone is predicted. In order to account for the recoil pressure generated during the evaporation process, stress boundary at the free surface of the workpiece is considered. It is found that the magnitude of stress wave, due to stress boundary at the surface, well exceeds the elastic limit of the substrate material. Once the magnitude of the recoil pressure reduces considerably, elastic wave is generated. This occurs after t^*=0.032. Since the elastic wave propagates faster than the plastic wave, both waves meet at some depth below the surface. This, in turn, defines the depth of the plastic zone. In the present case, the depth of elastic zone extends to about x^*=9.2 below the surface. The magnitude of the stress wave generated due to temperature gradient is less than the yield strength of the substrate material; in which case, its magnitude decreases with increasing depth from the surface.     

From local failure toward global collapse of elastic–plastic structures in fluctuating fields

Application of shakedown theory to study the load-bearing capacity of truss structures subjected to varying loads is presented. Inadaptation may cause local fractures not leading to the global collapse (the loss of load-bearing capacity) of a structure. A full analysis requires a step-by-step application of the reduced kinematic formulae constructed recently by the author to check the occurrence of a local fracture by alternating plasticity and a possible spreading of the fracture zone until the critical state of global incremental (or instantaneous) collapse is reached. This basic phenomenon, in somehow more sophisticated appearance, might be observed in many more general structures and in inhomogeneous materials working in changing fields, as in some fiber bundle models presented. The solution procedure could also help to improve the design of a structure for particular working conditions.     

Effect of axial restraints on the deflection of strain hardening beams

Centrally loaded, rigid-linear strain hardening rectangular beams with variable axial restraints are investigated. Load-deflection solutions are presented for beams with four common types of end conditions. It is found that the effect of strain hardening cannot be neglected, especially for beams of larger depth to length ratios. By considering strain hardening, the influence of such beam depth to length ratio on the load-deflection curve can be quantified and the length of the plastic zone estimated. Compared with the available experimental data for simply supported and fully clamped beams of various depth to length ratios, the results give realistically closer agreement than those given by a rigid-perfectly plastic analysis.     

真空平衡型气体静压止推轴承的数学模型研究

讨论了真空平衡型气体静压止推轴承的工作机理,建立了该轴承的承载能力的测度的数学模型,并通过计算机真及实验的方法对其进行分析验证,提出了气膜厚度为真空度变量的参数,使其与导管参数,轴承参数联系起来,为定量分析真空度对轴承特性的影响提供了设计依据。     

Machined surface plastic strain in orthogonal cutting by subsequent recrystallizations technique

The subsequent recrystallization technique, an experimental strain measurement method by use of recrystallization phenomena, has been successfully applied for machined surface plastic zones with equivalent plastic strain ϵ≥0.5, 0.12 and 0.02 of type 304 stainless steel. The depth of the zone with ϵ≥0.5 is very small, 10–50 μm, while zones with ϵ≥0.12 and 0.02 are 100–200 μm and 200–450 μm, respectively. The depths increase with increasing depth of cut and with decreasing rake angle. The relation between the depth of the zones and the cutting parameters are shown. The deformation state ahead of the quick-stop cut tip was also well visualized by the technique.