Stress analysis of heated concrete using finite elements

Described is a finite element analysis of concrete, which is subjected to rapid heating. Using thermal mass transport calculation, the moisture content, temperature and pore pressure distribution over space and time is obtained first. From these effects, stress at various points of the concrete are computed using the finite element method. Contribution to the stress formulation comes from three components, namely the thermal expansion, pore pressure, and the shrinkage of concrete due to moisture loss (from dehydration). The material properties of concrete are assumed to be homogenous, elastic, and cracking is not taken into consideration.     

Evaluation of aseismic integrity in the HTTR core-bottom structure V. On the static and dynamic behavior of graphitic HTTR key-keyway structures

The graphite components in high temperature gas-cooled reactors are connected to each other through a key-keyway structure that has gaps between the key and the keyway to accomodate thermal expansion. Because a dynamic load concentrates on the key-keyway structure during earthquakes, it is considered to be a crucial element for assessing the integrity of the graphite components. A combination of experiments and analyses was employed to investigate the dynamic behavior of the key-keyway structure, i.e. the equivalent stiffness associated with vibrational characteristics of the graphite components and the stress distribution under dynamic loading. The experiments were performed using a graphite scale model and a dynamic photo-elastic method. The analysis was carried out using the finite element method (FEM) code Abaqus, taking account of the contact between the key and the keyway. The following conclusions were derived. (1) The equivalent stiffness of the key-keyway structure shows nonlinearity, owing to the contact deformation. (2) The equivalent stiffness evaluated by the FEM analysis, taking account of the non-inear contact deformation, is applicable for predicting the vibrational characteristics of ky-keyway structure. (3) The stress concentration under dynamic loading is lower than or nearly equal to that under static loading. The maximum stress concentration of the seismic load can be sufficiently evaluated under static loading conditions.     

Advanced applications of boundary-integral equation methods

Numerical analysis has become the basic tool for both design and research problems in solid mechanics. The need for accuracy and detail, plus the availability of the high speed computer has led to the development of many new modeling methods ranging from general purpose structural analysis finite element programs to special purpose research programs. The boundary-integral equation (BIE) method is based on classical mathematical techniques but is finding new life as a basic stress analysis tool for engineering applications. This paper summarizes some advanced elastic applications of fracture mechanics and three-dimensional stress analysis, while referencing some of the much broader developmental effort. Future emphasis is needed to exploit the BIE method in conjunction with other techniques such as the finite element method through the creation of hybrid stress analysis methods.     

Failure behavior of carbon steel pipe with local wall thinning near orifice

Monotonic four-point bending tests were conducted using pipe specimens having an orifice undergoing local wall thinning. The effects of local wall thinning on the fracture behaviors of pipe were investigated. Local wall thinning was machined on the inside of pipes in order to simulate erosion corrosion metal loss. The configurations of the eroded area were l = 100 mm in axial length, d/t = 0.5 and 0.8 in thickness ratio, and 2θ = 180° in angle. The area undergoing local wall thinning was subjected to either tensile or compressive stress. Failure type could be classified into ovalization, local buckling, and crack initiation, depending on thickness ratio, and stress at the eroded area. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which is able to accurately simulate fracture behaviors. Failure analysis map was constructed for pipes with or without orifice based on the results of finite element analyses in order to investigate the effect of orifice on the failure behaviors.     

Numerical simulation of creep damage for low alloy steel welded joint considering as-welding residual stress

A model to calculate the welding temperature and residual stress was built using finite element code ABAQUS, and a subroutine of creep damage was also developed. Based on the coupling of welding residual stress and creep damage, the welding residual stress and creep damage of a tube made of Cr5Mo steel were simulated. This method can obtain the distributions of complex residual stress, creep damage and stress relaxation, which provide a reference for discussing the effect of residual stress on creep damage. The results show that the welding residual stress is very large at initial stage, then it is relaxed in a short time at high temperature. The distribution of creep and damage is mainly decided by the as-welding residual stress. Welding residual stress has a great effect on the creep and damage, which provides a reference for the design and life prediction of high temperature component.     

包覆燃料颗粒应力的有限元分析

高温气冷堆的燃料元件由包覆燃料颗粒弥散在石墨基体中组成。在反应堆运行过程中,辐照及各复杂的物理化学反应产生的应力会使包覆燃料颗粒发生破损,对包覆燃料颗粒进行应力分析是评价燃料元件和反应堆运行安全性能的主要内容之一。本文基于压力壳模式,主要考虑内压作用下的球形壳层应力及包覆燃料颗粒的非球形因素,用有限元法对应力进行了分析。     

Finite element analysis of mixed-mode thermoelastic fracture problems

This paper presents a rigorous hybrid finite element procedure for the analysis of the thermoelastic problems with mixed-mode cracks. The singular character of the temperature gradient which significantly affects the distribution of thermal stresses near the crack-tip is precisely described. The calculations of temperature and thermal stress fields are carried out by the finite element assemblage in which hybrid singular elements are used around the crack-tip and high-order isoparametric regular elements are taken elsewhere.To determine the mixed-mode stress intensity factors, both the direct extrapolation method based on the finite element solutions and the indirect method using the modified integrals accounting for thermal effects have been proposed. To avoid the underestimation of stress intensity factors obtained by quarter-point singular elements, the important role of the hybrid singular elements developed is also demonstrated.For verification purposes, several pure mode and mixed-mode problems are solved. Excellent correlations between the computer results and available referenced solutions are drawn.     

Experimental Investigation of Strain Concentration Evaluation Based on the Stress Redistribution Locus Method

Evaluating the local strain in structural discontinuities is an important technology in high temperature design of fast reactor components because the failure mode in high-temperature fatigue or creep fatigue damage usually results from the crack initiation and growth from such locally high strained areas. A rationalized method of evaluating strain concentration that was experimentally studied is discussed. The stress redistribution locus (SRL) method had been proposed to improve the accuracy with which local stress and strain can be evaluated in the structural discontinuities. This method is based on the concept that the locus of stress redistribution from an elastic to an inelastic state, or that during relaxation, strongly depends on the structure, and the locus almost coincides with the locus obtained through elastic-creep analysis. High-temperature fatigue tests of circumferentially notched specimens were conducted accompanied by the measurement of local strain carried out with a capacitance-type strain gauge. The measured strain was compared with the predictions made with SRL, and the method's accuracy was evaluated. SRL improved the accuracy of inelastic strain estimation while remaining reasonably conservative in comparison with Neuber's rule, which is used in high temperature design codes.     

Residual stresses associated with the hydraulic expansion of steam generator tubing into tubesheets

Various methods are being used to expand heat transfer tubes into the thick tubesheets of nuclear steam generators. The residual stresses in the as-expanded tubes and methods for reducing these stresses are important because of the role which residual stresses play in stress corrosion cracking and stress assisted corrosion of the tubing. Of the various expansion processes, the hydraulic expansion process is most amenable to analytical study. This paper presents results on the residual stresses and strains in hydraulically expanded tubes and the tubesheet as computed by two different finite element codes with three different finite element models and by a theoretical incremental analysis method. The calculations include a sensitivity analysis to assess the effects of the expansion variables and the effect of stress relief heat treatments.     

反应堆主泵抗震强度的三维实体模型计算

用有限元方法做泵的强度验算,一般是使用壳单元．泵的厚度为一个固定值本文通过CATIA软件建立了与真实主泵完全一致的三维模型,使用四面体单元对建立的模型划分有限元网格,克服了采用壳体单元的近似,使模型的计算结果更加可靠计算了反应堆主泵的抗震性能．在地震载荷、温度场的作用下,反应堆主泵的最大Mises等效应力为29.9MPa,根据ASME—III ND3400所确定的该材料许用应力极限值132．825MPa,其完全满足相关规范抗震强度的要求.     

Combination of simulation and experiment in designing repair weld strategies: A feasibility study

Numerical simulations, based on an off-the-self commercial finite element (FE) code and experimental tests using the neutron diffraction (ND) technique, are combined in an attempt to evaluate post-weld heat treatment (PWHT) of a letterbox-type repair weld, in respect of its effect on the residual stress field. 21/4CrMo steel plates with an 18-pass repair weld were heat treated at various temperature levels and for different durations. Due to the prohibitive cost of a complete residual stress mapping, ND tests were performed only at selected specimen locations. In this sense, FE simulation acts as a supplement to ND, since it predicts the complete residual stress field. Uncoupled quasi-static thermoelasticity in conjunction with an element activation/deactivation technique, simulating deposition of new weld material, are combined in a 3D FE analysis. Grouping of the 18 weld beads in lumps, following a sensitivity analysis, reduces computational costs to feasible levels, whereas a creep strain hardening law is used to simulate stress relaxation during PWHT. Computed residual stresses are compared to ND measurements for verification purposes. Comparison of heat treated specimen measurements to heat treated and untreated specimen predictions illustrates that PWHT has a strong effect on the residual stress field, achieving significant stress relaxation.     

Finite element elastic-plastic analysis of LMFBR components

The present effort involves the development of computationally efficient finite element methods for accurately predicting the isothermal elastic-plastic three-dimensional responses of thick and thin shell structures subjected to mechanical and thermal loads. This work will be used as the basis for further development of analytical tools to be used to verify the structural integrity of liquid metal fast breeder reactor (LMFBR) components. The methods presented here have been implemented into the three-dimensional solid element module (HEX) of the Grumman PLANS finite element program. These methods include the use of optimal stress points as well as a variable number of stress points within an element. This allows monitoring the stress history at many points within an element and hence provides an accurate representation of the elastic-plastic boundary using a minimum number of degree of freedom. Also included is an improved thermal stress analysis capability in which the temperature variation and corresponding thermal strain variation are represented by the same functional form as the displacement variation. Various problems are used to demonstrate these improved capabilities.     

Local stress factors of a pipe-nozzle under internal pressure

A comprehensive study of local pressure stresses at the juncture of a pipe-nozzle is presented. Currently, neither experimental nor analytical data exists that is sufficient for pressure vessel designers to analyze the local pressure stresses at the juncture of a pipe-nozzle. In order to provide a comprehensive database to calculate these pressure stresses, this paper, through a finite element technique, provides a series of local pressure stress factor plots as a function of pipe-nozzle geometrical parameter β (nozzle mean radius/pipe mean radius) with a range from 0.1 to 1.0 and γ (pipe mean radius/pipe thickness) with a range from 10 to 300. These local pressure stress components supplement the stress calculations presented by the Welding Research Council Bulletin 107 for other external loadings.     

Elasto-plastic analysis of three-dimensional structures using the isoparametric element

A method for elasto-plastic analysis of three-dimensional structures is presented. The finite element method, implemented by a three-dimensional isoparametric element is used in the study. An “initial” stress approach is employed in the range of plastic behavior.     

Thermal stresses in rectangular plates: Variational and finite element solutions

This paper deals with the development of an approximate method for the analysis of thermal stresses in rectangular plates (plane stress problem) and an evaluation of the relative accuracy of the finite element method. The stress function is expanded in terms of polynomial coordinate functions which identically satisfy the boundary conditions, and a variational approach is used to determine the expansion coefficients. The results are in good agreement with a finite element approach.     

核安全一级高温管道系统结构分析与安全评估方法研究

为解决复杂核安全一级高温管道系统结构分析与评定工程问题，在管道分析软件与核级高温评定规范ASME-NH之间建立了一座桥梁。首先，对管道结构（直管及弯管）在不同载荷作用下的应力状态解析解进行了详细推导分析，并且与有限元数值解进行了误差分析。结果显示，给出的直管及弯管结构应力状态解析解具有很好的准确性。随后，将一维管线力学分析模型与截面三维应力状态解析解相结合，给出了高温管道系统结构分析、评定方法及应用步骤，将ASME-NH-3650规范内容明确化。      

Review of continuum, finite element and hybrid techniques in the analysis of stress concentrations in structures

When a uniform flow of any nature is interrupted, the readjustment of the flow results in concentrations and rare-factions, so that the peak value of the flow parameter will be higher than that which an elementary computation would suggest. When stress flow in a structure is interrupted, there are stress concentrations. These are generally localized and often large, in relation to the values indicated by simple equilibrium calculations. With the advent of the industrial revolution, dynamic and repeated loading of materials had become commonplace in engine parts and fast moving vehicles of locomotion. This led to serious fatigue failures arising from stress concentrations. Also, many metal forming processes, fabrication techniques and weak-link type safety systems benefit substantially from the intelligent use or avoidance, as appropriate, of stress concentrations. As a result, in the last 80 years, the study and and evaluation of stress concentrations has been a primary objective in the study of solid mechanics.Exact mathematical analysis of stress concentrations in finite bodies presents considerable difficulty for all but a few problems of infinite fields, concentric annuli and the like, treated under the presumption of small deformation, linear elasticity. A whole series of techniques have been developed to deal with different classes of shapes and domains, causes and sources of concentration, material behaviour, phenomenological formulation, etc. These include real and complex functions, conformal mapping, transform techniques, integral equations, finite differences and relaxation, and, more recently, the finite element methods. With the advent of large high speed computers, development of finite element concepts and a good understanding of functional analysis, it is now, in principle, possible to obtain with economy satisfactory solutions to a whole range of concentration problems by intelligently combining theory and computer application. An example is the hybridization of continuum concepts with computer based finite element formulations. This new situation also makes possible a more direct approach to the problem of design which is the primary purpose of most engineering analyses. The trend would appear to be clear: the computer will shape the theory, analysis and design.     

核电厂人员闸门数字样机应用技术研究

为提高核电厂人员闸门的安全、可靠性，基于数字样机技术，构建人员闸门整机和关键零部件的参数化模型，结合故障模式及影响分析（FMEA）方法，对人员闸门参数化模型进行可靠性分析，找到薄弱环节。并借助动力学仿真分析软件（ADAMS）和有限元分析软件（ANSYS），对人员闸门顶升机构和筒体与密封门组成的承压部件薄弱环节进行了动力学仿真分析和应力计算与评定。结果表明，人员闸门自身存在的薄弱环节不会导致其产生功能或结构完整性失效事故，验证了人员闸门数字样机结构设计的合理性、装配关系的正确性。      

先进压水堆一回路地震反应谱分析及参数敏感性研究

压水堆一回路系统包含压力容器、蒸汽发生器、主泵、稳压器、主管道和波动管等重要部件,各部件在地震激励下的动态响应与整个系统的结构形式密切相关。本文从系统的角度,以非能动先进压水堆一回路为研究对象,运用ANSYS建立了其三维有限元模型,在模态分析的基础上,进行了三正交方向输入下的反应谱分析,得到了系统在地震载荷下的响应。并对反应谱输入角度和支撑刚度进行了敏感性研究,给出了这些特性参数对结构设计和分析的指导性意见。此外,通过直接积分法得到系统的地震时程响应,并与谱分析的模拟结果进行了对比,同时也为主泵等单个部件的详细地震分析提供位移、加速度输入。最后通过三维实体模型与集中质量模型抗震计算结果的比较,说明建立三维实体模型的必要性。本文为核电站一回路重要设备的结构分析提供了技术支持。     

Structural and Fracture Mechanics Analysis for the Bracket of ITER Upper ELM Coil

The brackets are the important components of ITER edge localized modes (ELM) coils to connect the coils and rails. In order to assure the structural integrity and security of the bracket, the maximum tresca stress and stress intensity factor are examined from the viewpoints of structural and fracture mechanics. Based on the finite element method, the global upper ELM coils with simplified and detailed bracket are investigated. Since it is difficult to perform in-service inspection due to inaccessibility of in-vessel coils, it is important to estimate the allowable initial defect. Assuming an initial crack in the maximum first principal stress region on the bracket, the fracture mechanics analyses under different loads are performed. Results show that the bracket design is valid and feasible and the calculation method of finite element for stress intensity factor is feasible and reliable. Assuming the initial crack of 7 mm depth, the bracket can meet the crack growth criteria. The stress intensity factor of the bracket is mainly caused by electromagnetic (EM) load and the thermal load can reduce the stress intensity factor under EM load. The thermal load can make the crack grow on the surface of the bracket and the EM load can cause the crack to extend in the inner of the bracket.