Development of the evaluation method for crack propagation due to thermal striping

The evaluation code “THERST” was developed to estimate the fatigue crack propagation behavior under thermal stresses due to high-frequency temperature fluctuations, called “thermal striping”. This paper presents fundamental formulations of the evaluation method and verifications of the evaluation method by FEM analyses. Experimental data were obtained in high cycle thermal fatigue tests and the effect of a multiple crack which is characteristic for a crack under thermal stress is discussed in addition to the results of the FEM analyses. A modification of the evaluation method was performed to take multiple crack effects into account.     

Analytical evaluation method of J-integral in creep-fatigue fracture for type 304 stainless steel

Creep-fatigue crack growth at the operating temperature of LMFBR can be characterized by ΔJ_F and J′ (same as C^*). Type 304 stainless steel, the main structural material of the Japanese LMFBR, shows notable cyclic hardening at elevated temperatures. Evaluation of these J-integrals with the finite-element method is strongly affected by the reference strain range when the cyclic hysteresis' is used as the stress-strain relation.In this paper, an evaluation method for ΔJ_F and J′ with a cyclic stress-strain curve (Δσ – Δ relation) is proposed and verified by experimental results. The evaluation method proposed here does not require cyclic calculations but is monotonic and the effect of the reference strain range is relatively small.     

Comparison of crack initiation life estimation procedures under creep-fatigue conditions

This paper describes the application of “high temperature structural integrity assessment procedures” developed in the UK and Japan to creep-fatigue crack initiation in welded Type 316 features tests. The components were subjected to both fatigue and creep-fatigue loading at 630 °C. The loadings are representative of those on the upper seal gimbal joint in an advanced gas cooled reactor (AGR), except that the tests were isothermal and the imposed dwell times were reduced. It is demonstrated that application of the procedures gives accurate predictions of the observed crack initiation in the weldment, based on two different advanced inelastic constitutive models (BE and CRIEPI models) and best estimate materials data. Application of simplified assessment methods based on elastic analysis is shown to be conservative. Where appropriate, contrasts between the UK and the Japanese assessment procedures and inelastic modelling techniques have been highlighted.     

A comparison between Japanese and French A16 defect assessment procedures for creep-fatigue crack growth

This paper presents the results of a benchmark on creep-fatigue crack growth evaluation for a plate subjected to cyclic bending loads with a 1 h dwell. The simplified creep-fatigue crack growth evaluation methods of JNC in Japan and A16 procedures proposed by CEA in France are presented. The methods, based on the reference stress approach, are compared each other. They are found to differ in the expression used for the reference stress solution used to estimate the creep strain. It is also pointed out that in contrast to the A16 procedures, the JNC method takes heterogeneous creep strain distribution into account for small scale yielding condition. The predictions obtained by the methods are also compared to the experimental data. It is found that the methods exhibit conservatisms which are significantly reduced when integrating the creep curve continuously without initialisation during the experiment [Proceeding of SMiRT 14(G13/2), Creep-Fatigue Crack Growth on CT25 Specimens in 316L(N) stainless steel at 650 °C].     

Elastic creep-fatigue damage evaluation: less restrictive design rules for SNR 2 components

To achieve less conservative design rules for LMFBR Class 1 components in elevated temperatures service, two modifications of the elastic creep-fatigue evaluation in use are proposed: (1) In the creep-damage evaluation, the relaxation of residual stresses during hold-times shall be taken into account; (2) In the fatigue-damage evaluation, the creep effects on the cyclic stress strain hysteresis loop shall be considered by adding the net relaxation strain during holdtimes to the elastic-plastic strain range, and design fatigue curves without hold-time shall apply.     

基于PepS的高温核一级管道蠕变疲劳分析方法研究

钍基熔盐堆(Thorium Molten Salt Reactor-Liquid Fuel,TMSR-LF1)回路管道最高运行温度达650℃,高温服役下的管道蠕变-疲劳损伤分析及评定至关重要。目前仅ASME-BPVC-III-5-HBB规范中有适用于高温核一级管道的蠕变-疲劳损伤暂行评定方法,但该方法对于复杂管道系统使用起来过于繁琐。本文旨在使用管道分析软件PepS软件实现高温核一级复杂管系的分析与结构完整性评估。首先结合管道结构在多种载荷组合作用下的截面应力状态解析解,进行管道截面应力分析及应力线性化,并将结果与有限元数值解进行对比分析,两者的误差结果基本一致。随后,利用PepS软件对TMSR-LF1回路管道进行了力学分析和结构完整性评估,结其蠕变疲劳损伤结果位于包络线以内,满足蠕变疲劳极限的要求。该研究将管道分析软件与ASME评定规范进行了有效衔接,明确了评定方法,实现了高温核一级复杂管系的蠕变疲劳评估。     

A simplified analysis of creep crack growth using local approach

The use of Fracture Mechanics parameters (K, J, C^*) is subjected to serious limitations when viscoplastic strain appears in a cracked structure.It is therefore one of the cases where a local approach of fracture might be the only mean to help understand the complex mechanical phenomena which influence crack growth.Using a very simple local fracture law, called sudden damage model, various closed form solutions of creep damage zone growth are proposed. With the help of a Double Cantilever Beam (DCB) model to simplify the geometry of the cracked structure, crack initiation and growth are analytically studied.Points such as creep zone size, stress redistribution or influence of local fracture criterion are discussed using closed form solutions.     

Development of new Z-factors for the evaluation of the circumferential surface crack in nuclear pipes

The purpose of this study is to develop new Z-factors to evaluate the behavior of a circumferential surface crack in nuclear pipe. Z-factor is a load multiplier used in the Z-factor method, which is one of the ASME Code Sec. XI's recommendations for the estimation of a surface crack in nuclear pipe. It has been reported that the load carrying capacities predicted from the current ASME Code Z-factors, are not well in agreement with the experimental results for nuclear pipes with a surface crack. In this study, new Z-factors for ferritic base metal, ferritic submerged arc welding (SAW) weld metal, austenitic base metal, and austenitic SAW weld metal are obtained by use of the surface crack for thin pipe (SC.TNP) method based on GE/EPRI method. The desirability of both the SC.TNP method and the new Z-factors is examined using the results from 48 pipe fracture experiments for nuclear pipes with a circumferential surface crack. The results show that the SC.TNP method is good for describing the circumferential surface crack behavior and the new Z-factors are well in agreement with the measured Z-factors for both ferritic and austenitic pipes.     

放射性碘化IBZM的快速制备及鉴定

采用自行设计的低浓度过氧化氢乙酸法碘化标记(s)-BZM,Sep-Pak C18小柱分离纯化反应产物,无菌无热原滤器除菌.结果表明低浓度过氧化氢乙酸法是最佳的碘化标记(s)-BZM的方法,其碘化标记率≥90%,无副产物,易于进行无菌无热原处理.纯化产物的放射化学纯度为96.8%±1.4%(n=5),全程产率为81.9%±2.8%(n=5).由此可见采用低浓度过氧化氢乙酸法,配合Sep-Pak C18小柱分离纯化和无菌无热原滤器除菌,能简便、快速、高产率地制备无菌无热原的放射性碘化IBZM,是极具实用的临床方法.     

Fatigue crack propagation properties from small sized rod specimens

Mechanical properties characterization is needed in many industrial applications yet sufficient amount of material for fabricating standard-sized testing specimens is often not available. Techniques for testing miniaturized specimen must be adopted. Much effort has been made to develop techniques for impact, fracture toughness and tensile properties of sub-sized specimens. Work on the testing of fatigue properties is more limited. In this study, fatigue crack propagation behavior is evaluated from the growth of surface crack in a cylindrical rod under tension. Rods of various lengths and diameters were tested. As the size of the rod specimen is reduced, the fatigue crack growth rate tends to increase when correlated using the stress intensity factor range. This increase is explained largely by the decrease in the degree of premature crack closure in the small specimens. Valid fatigue crack growth data can be obtained among the specimens examined except on the crack growth on the surface of the smallest specimen, which has a length of 26 mm and diameter of 8 mm. Even so, valid data can still be elucidated on the latter specimen if the interior growth is considered. The dimensions of the latter specimen allow fatigue properties to be evaluated using broken remnants from impact or other test specimens.     

Creep-fatigue crack propagation behavior in a surface cracked plate

A series of experiments were performed in order to clarify the surface crack growth behavior under creep-fatigue condition. Type 304 stainless steel was tested at 550°C and 650°C. Specimens were plates with a surface notch. Loading patterns were axial fatigue, bending fatigue, axial creep-fatigue and bending creep-fatigue. As results were obtained: (1) the beach mark method was available to measure the changes of the crack front shape after the test; (2) the electrical potential method was available to measure the changes of the crack front shape in real time; (3) the crack front shape was affected by the loading mode; and (4) ΔJ and ΔJ_c calculated from the proposed simplified method could characterize the surface crack growth rate.     

Fatigue crack propagation testing using subsized rotating bending specimens

In many industrial applications, mechanical properties characterization is needed yet sufficient amount of material for standardsized specimens is not available. Miniaturized specimen testing technique has to be employed. Currently there are a lot of effort in developing subsized specimen technique for impact, fracture toughness and tensile properties. Work on fatigue properties testing is limited and largely confined to stress/strain life tests. In the current work, evaluation of fatigue crack propagation behaviour using surface crack growth in a rotating bending rod has been attempted. Nine different rod geometries have been tested. The resulting fatigue crack propagation data is more sensitive to rod diameter than to rod length. Difference in crack growth behaviour can largely be understood when crack closure is taken into account. All in all, the crack propagation data obtained from these small sized rods agree well with those obtained from standard testing employing compact tension specimens. A more precise picture about the crack growth behaviour can be obtained if crack closure is considered. If crack closure is not monitored, rods with longer length and smaller diameter are more likely to give the conservative upper bound fatigue crack propagation behaviour.     

Short fatigue crack propagation and effect of notch plastic field

In engineering application, almost all structures and components contain notches or holes. They often experience severe fatigue loading, and have been recognized as a potential site for small fatigue crack initiation and propagation. In this paper, the effects of notch plastic field on small fatigue crack initiation and propagation from notch member, under cyclic tensile loading control, are investigated. Experiment shows that small crack initiates from notch specimen at far higher rate than that of smooth specimen; small crack propagation is still faster than that of smooth specimen within notch plastic field, though this difference is progressively lessening; beyond notch plastic zone, small crack growth rate is approaching long crack growth rate. Analysis via finite element and analytical method reveals that notch plasticity has key influence on small crack initiation, crack tip generated plasticity has critical impact on small crack propagation within notch plastic field, while plasticity induced crack closure has dominant effect on crack propagation out of notch plastic field. A comparison between experimental and analytical results is made to identify the mechanisms of small fatigue crack initiation and propagation within notch plastic field.     

Stress intensity factor evaluation of a circumferential crack in a finite length thin-walled cylinder for arbitrarily distributed stress on crack surface by weight function method

A weight function to evaluate the stress intensity factor (SIF) of a circumferential crack, subjected to arbitrarily distributed stress on the crack surfaces, in a finite length thin-walled cylinder was derived based on the closed form SIF equation previously developed by the authors. It is easy to evaluate the effects of structural parameters and stress distribution on the SIF with this weight function. Numerical examples confirmed the validity of the weight function. These examples showed that the effect of cylinder length on the SIF is quite large.     

A survey of the french creep-fatigue design rules for LMFBR

The paper provides a survey of the creep-fatigue design rules for the LMFBR in France. These rules are the ones currently implemented in French component manufacturing. The background of each item is discussed and the trends for improvements currently investigated are described. The cree-fatigue rules apply to elastic analysis only.     

Development of an evaluation method for the HTTR-IS nuclear hydrogen production system

A thermochemical water splitting hydrogen production system based on the iodine sulphur (IS) process is presently under development in JAEA. The hydrogen production system is to be connected to the HTTR operating test reactor in JAEA. An important development goal for the HTTR-IS system is design and construction of the IS process to the standards of a conventional chemical industrial plant in order to simplify the cost and operation of the overall nuclear hydrogen production.     

Space asymptotic method for the study of neutron propagation

Space asymptotic theory is shown to be a suitable model for the study of pulsed experiments in neutron multiplying systems. After a short revisitation of the basic aspects of space asymptotic theory applied on the Laplace transformed one-group transport equation, the full solution is derived. It is shown how results are exact in representing localized pulse propagation in the first portion of the transient, until the boundary is reached by the neutron signal, since it propagates with a finite velocity. Approximate models are then derived starting from the exact formulation and the B_N method is used to account for anisotropy effects. Numerical results are presented for one-dimensional systems, discussing the physical phenomena and noting the distortions introduced by approximate models, which may then turn out to be inadequate for the simulation of realistic pulsed experiments situations.     

用于CANDU堆换料方案快速评价的线性敏感矩阵方法

为了实现CANDU堆换料方案的快速评价,本文提出了线性敏感矩阵方法(LSM).该方法基于各种扰动引起的堆芯状态变化互不干涉且堆芯对扰动的响应与扰动量成正比的假设,借助于事先针对参考堆芯形成的敏感矩阵,主要的堆芯参数经简单代数运算就可得到,无需耗时的三维扩散方程求解.秦山三期1号机组部分运行历史检验结果表明,该方法不但可大幅提高换料方案评价的速度,而且还具有较高的精度,能满足现场工程应用的要求.     

Finite element analysis of crack propagation in three-dimensional solids under cyclic loading

This paper is concerned with an efficient computational procedure for analyzing crack propagation in solids. The method is general; however, its application to semi-elliptical surface cracks in thick plates is discussed in particular. The strain energy release rate G for a crack in mode I is a function of the crack geometry, the direction of crack propagation and the state of loading. When G is known, the stress intensity factor K_I can easily be obtained. In this paper the strain energy of the plate is computed numerically for a wide range of crack geometries using the finite element method. A 20-node isoparametric solid element is employed in modelling the structure. Certain special techniques for increasing the computational efficiency of the method, such as multilevel subdivision of the structure (substructuring) and condensation of degrees of freedom that are not needed in the crack propagation analysis, are emphasized. In fact, analysis of a large number of crack geometries requires only insignificantly more computational efforts than treating a single crack. Certain other aspects of the finite element modelling are also discussed.Two methods for replacing the computed discrete values of strain energy by continuous functions are presented. These functions are expressed in terms of the two half-axes defining the geometry of the elliptical crack and they are determined using a least square technique. G and K_I are easily deduced from these functions. As an example, a semi-elliptical, part-through, surface crack in a thick nickel steel plate is analyzed. The crack is subjected to a combination of axial and bending loading, applied cyclically. From the finite element calculations of the strain energy and the stress intensity factors which are computed accordingly, crack propagation along the two half-axes of the ellipse is calculated by utilization of a formula suggested by Paris. The results are checked against laboratory fatigue tests. The method has proved to be very efficient and accurate, and due to its generality it can also be applied to complicated geometries and complex states of loading.