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。     

动载荷作用下区分厚壁圆筒和薄壁圆筒的一个准则

从厚壁圆筒弹性动应力解出发,给出了厚壁圆筒内外径处的环向应力的简化计算公式,并提出了在动载荷作用下区分厚壁圆筒和薄壁圆筒的一个准则,认为当K≤1.14时,可以将圆筒视作薄壁圆筒。     

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。     

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.     

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.     

陶瓷构件形状对其抗热震性的影响

在陶瓷材料临界应力断裂理论的基础上,通过分别求解无限大平板、无限长圆柱体和球体三种形状构件在第三类边界条件下的瞬态温度场及瞬态热应力场,研究了不同形状陶瓷构件的热冲击行为,并据此分别建立了引起三种形状陶瓷构件表面临界热应力的临界温差表达式作为它们的抗热震参数。计算结果表明,在相同Biot模数的条件下,无限大平板表面达到临界热应力的量纲-时间最长,无限长圆柱体次之,圆球体表面达到临界热应力的量纲-时间最短;圆球体的临界温差最大即抗热震性最好,无限长圆柱体次之,无限大平板的抗热震性最差。     

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。     

Experimental and analytical investigation of the heat transfer and thermal stresses in a cylinder head of a diesel engine

An experimental and theoretical study has been carried out to investigate the mechanism of crack development in cylinder heads of two-stroke diesel engines. The cracks initiate in the valve bridge region, between a pair of exhaust valves, where high temperatures exist during operation of the engine. The experiments included steady state and transient modes (cold start, loading and shut off), and temperature distributions were measured in the metal and the cooling water. The hot surface temperature as well as the maximum stresses are estimated using an analytical model. It is concluded that the dominant process of cracking is low cycle thermal fatigue and residual tensile stresses which appear after engine shut off.     

The effect of CNT volume fraction on the magneto-thermo-electro-mechanical behavior of smart nanocomposite cylinder

In this article, using analytical approach, the stress analysis of a long piezoelectric polymeric hollow cylinder reinforced with carbon nanotube (CNT) under combined magneto-thermo-electro-mechanical loading is investigated. Considering three combined loading conditions such as pressure-electric, pressure-electric magnetic and pressure-electric thermal, the governing equation of the problem is obtained. The rule of mixture and modified multiscale bridging model are used to predict effective properties of nanocomposite. The magneto-thermo-electro-mechanical stresses in hollow cylinder are discussed in detail. It can be concluded that increasing CNT volume fraction enhances strength of the nanocomposite cylinder. The results of this work could be useful in view of optimum design of the smart nanocomposite cylinder under magneto-thermo-electro-mechanical loadings and could also be as a reference for future related works.     

Thermal ratchetting of a tube with non-uniform thickness

Two-dimensional (plane stress) finite-element analyses have been carried out to investigate the thermal ratchetting behaviour of a thin tube with an eccentric bore (eccentricity/mean wall thickness = 0.2). The tubes were subjected to steady internal pressure and cyclic, linear through-thickness temperature gradients. If the mechanical loading is characterized by the nominal mean stress (σ_m) at the thinnest section, the ratchetting boundary is accurately predicted by a simple, analytical, beam model. For high thermal loads ( ) the maximum mid-thickness ratchet strains are conservatively predicted. However, for lower thermal stresses the mid-thickness ratchet strains are underpredicted. The value of the creep stress exponent has no effect on the ratchet strains obtained under complete redistribution conditions. The first cycle ratchet strains under no-creep conditions are practically identical to the complete redistribution values.     

Nonlinear transient heat transfer and thermoelastic analysis of thick-walled FGM cylinder with temperature-dependent material properties using Hermitian transfinite element

Nonlinear transient heat transfer and thermoelastic stress analyses of a thick-walled FGM cylinder with temperature-dependent materials are performed by using the Hermitian transfinite element method. Temperature-dependency of the material properties has not been taken into account in transient thermoelastic analysis, so far. Due to the mentioned dependency, the resulting governing FEM equations of transient heat transfer are highly nonlinear. Furthermore, in all finite element analysis performed so far in the field, Lagrangian elements have been used. To avoid an artificial local heat source at the mutual boundaries of the elements, Hermitian elements are used instead in the present research. Another novelty of the present paper is simultaneous use of the transfinite element method and updating technique. Time variations of the temperature, displacements, and stresses are obtained through a numerical Laplace inversion. Finally, results obtained considering the temperature-dependency of the material properties are compared with those derived based on temperature independency assumption. Furthermore, the temperature distribution and the radial and circumferential stresses are investigated versus time, geometrical parameters and index of power law. Results reveal that the temperature-dependency effect is significant.     

Effect of particle content, size and temperature on magneto-thermo-mechanical creep behavior of composite cylinders

The effects of particle content, particle size, operating temperature and magnetic field on steady-state creep behavior of thick-walled rotating cylinders made of Al-SiC composites have been investigated. Loading is composed of a uniform magnetic field in axial direction, steady-state heat conduction in radial direction and an inertia body force due to rotation. The composite creep constitutive equation has been described by Norton’s law in which the creep parameters are functions of particle size, temperature and particle content. The composite properties are radial dependent based on volume fraction of SiC reinforcement. It has been found that the minimum effective creep strain rate belongs to a composite identified by 25% SiC at the inner and 5% at the outer surfaces. Therefore this composite has been selected for the design of the cylinder. It has been concluded that increasing particle size and operating temperature significantly increases the effective creep strain rates. It has also been illustrated that magnetic field decreases the stresses and the effective creep strain rates.     

功能梯度压电材料壳体热残余应力

功能梯度压电材料结构成型冷却后会出现热残余现象,影响结构强度.借鉴复合材料层合结构的研究方法,将功能梯度压电材料球壳和圆柱壳沿厚度分为若干层,各层视为均匀材料,根据层间连续条件导出递推关系,得到显式的力—电—热多场耦合热残余解.统一了多层功能梯度压电材料壳体和连续功能梯度压电材料壳体热残余解.对于前者,其解为精确解;对于后者,其解为渐近解,随层数增加而收敛于精确解.其解也适用于功能梯度压电材料涂层.该方法对材料性能的变化方式(函数)没有要求,适应性强.并讨论影响热残余应力和界面强度的因素,球壳因双曲率的影响,热残余应力显著大于柱壳.     

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。     

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。     

Numerical analysis of temperature fields and thermal stresses in the friction discs of a multidisc wet clutch

The subject of the considerations in this paper is a multidisc wet clutch embodying alternately assembled discs of two types: discs made entirely of steel and discs with a steel core lined with layers of friction material. A model of the unsteady heat conduction process in such discs has been developed with account taken of the non-homogeneity of the thermal properties of these discs. Heat conduction equations have been derived and initial and boundary conditions given. To solve this problem, the finite difference method was used. Next, the thermal stresses in the steel discs, regarded as quasi-static, were considered. The stresses were calculated by the finite element method. Calculations of the temperature fields and thermal stresses were performed for a specific typical clutch engagement. Discussion of the results obtained is presented. In the analysis of the temperature distributions and stresses, attention has been focused on the determination of factors permitting a reduction in stresses.     

ANALYTICAL MODELING OF THE EFFECT OF THE TOOL FLANK WEAR WIDTH ON THE RESIDUAL STRESS DISTRIBUTION

Tool flank wear has significant effects on the cutting process, as it affects cutting forces, temperature and residual stresses. In this article, analytical models are developed to predict the cutting temperature and residual stresses in the orthogonal machining of a worn tool. In these models, measured forces, cutting conditions, tool geometry, and material properties are used as inputs. Stresses resulting from thermal stresses, fresh tool stresses and stresses due to tool flank wear are used in this analytical elasto-plastic model, and the residual stresses are determined by a relaxation procedure. The analytical model is verified experimentally with X-ray diffraction measurements. With the analytical model presented here, accurate residual stress profiles in worn tools are shown, while the computational time is significantly reduced from days, typical for finite-element method (FEM) models, to seconds.     

气缸壁温度预测研究

基于包含热交换的气缸运动数学模型,提出了一种计算气缸充放气过程中等效热功率的理论方法,预测气缸在达到热稳态平衡时的气缸壁面温度变化情况,并利用该预测模型结果与实验测量的气缸壁面温度变化情况进行对比。结果表明,等效热功率理论分析得到的气缸壁面温度变化与实验结果基本相符,等效热功率模型可以预测不同工况气缸壁面的温度变化。     

压力容器接管部位的可靠性设计研究

使用ANSYS软件对某压力容器筒径,不同开孔直径壳体,共3个模型的开孔接管部位进行分析设计,同时提出最弱环模型对接管部位应力强度进行可靠性评定。研究结果表明,在接管需要补强时采用补强圈或厚壁接管补强,均可有效地改善孔边应力集中;随着开孔率的增大,厚壁补强接管部位的可靠性大于等面积补强法所得结果;最后提出厚壁补强时合适的可靠度取值。     

Temperature and stress analysis and simulation in fractal scanning-based laser sintering

A model for calculating the evolution of temperature and thermal stresses within a single metallic layer formed on the powder bed using different scanning patterns in selective laser sintering (SLS) was proposed. The model allows for the non-linear behavior of thermal conductivity, specific heat, and elastic modulus due to temperature changes and plastic deformation capabilities with a bilinear isotropic hardening behavior. The effect of laser scanning patterns on temperature, residual thermal stresses and distortion were investigated. It was shown that the distortion and transient stresses of a layer processed by a moving laser beam is decreased with fractal scanning pattern.