Characterization of stable crack growth through AISI 4340 steel using cohesive zone modeling and CTOD/CTOA criterion

The criteria based on cohesive zone model (CZM) and CTOD/CTOA have been employed for analysis of stable crack growth (SCG) through AISI 4340 steel. Investigations have been reported first concerning characterization of mode I SCG using the CZM and a particular crack size in a CT specimen 8 mm thick. The characteristic data is then verified considering other mode I crack sizes and the same specimen geometry. Similar verifications have also been done considering cracks under mixed mode (I and II) loading. The same mode I cases have been studied later considering some variations of crack tip opening displacement/angle (CTOD/CTOA) with crack extension. Load-load-line displacement (LLD) predictions based on these two approaches have been compared with experimental results reported in the literature. The predictions based on the CZM are found to be closer to the experimental results. The results include the traction-separation law suitable for characterization of SCG through AISI 4340 steel under mode I and mixed mode (I and II). Similarly a CTOD/CTOA variation with crack extension for the similar purpose under only mode I loadings has been reported. Other observations on modeling of crack tip constraint effects by mixed (plane strain core plus plane stress outer domain) discretisation scheme, crack tip element size and J integral, are presented.     

带穿孔三明治结构在爆炸载荷下的动态响应研究

考虑结构被破片穿孔预损伤，采用数值模拟方法进行了带穿孔三明治结构在爆炸冲击载荷下的结构动态响应研究。建立了不同结构参数下的带穿孔三明治平板三维数值模型；得到了带穿孔三明治结构动态响应的3个阶段并分析了各阶段的特征。在此基础上，开展了不同穿孔数对三明治平板结构的动态结构响应、最大变形量和各部分内能的影响研究，结果表明:对于三明治结构，穿孔造成的孔径方向横向动能显著减少了结构垂直方向上的变形量；穿孔数增加使结构的承载方式由二维“面承力”过渡为一维“梁承力”，使垂直方向变形增大和承载能力明显下降。研究成果为爆炸载荷下的带穿孔三明治结构动态结构响应和易损性建立了基础，同时也为三明治结构防护设计和研究提供了参考依据。      

Liquid metal embrittlement of T91 martensitic steel evidenced by small punch test

In this work, the sensitivity of liquid metal embrittlement of the T91 martensitic steel is investigated with the small punch test (SPT). The material was studied in three tempering conditions (as quenched, tempered at 500 and 750 °C), at 300 °C in air and in the liquid lead bismuth eutectic (LBE). The load–displacement curves (four stages, low maximum force and large displacement to fracture) obtained for one test condition of the 750 °C tempered material is in general very different from those of the two other materials. An effect of LBE has been observed for the as quenched and 500 °C tempered steels. For these materials, the curves tend to be linear with a reduced displacement to fracture suggesting a brittle behavior. This ductile to brittle transition induced by liquid metal has been confirmed from the fracture surface analysis where cleavage was observed. In comparison with conventional tensile tests, small punch tests appear to be more sensitive to evidence liquid metal embrittlement.     

Effect of the cascade energy on defect production in uranium dioxide

The primary damage induced by a displacement cascade in a pure uranium dioxide matrix was investigated using classical molecular dynamics simulations. Cascades were initiated by accelerating a uranium primary knock-on atom (PKA) to a kinetic energy ranging from 1 keV to 80 keV inside a perfect UO₂ lattice at low temperature (300 K and 700 K). There is little effect of temperature in the temperature range studied. Following the cascade event, the damage level, defined as the total number of defects irrespective of whether they form clusters or not, is proportional to the initial kinetic energy of the PKA, in agreement with the literature relating to other materials. The linear dependence of damage upon initial PKA energy results from the formation of subcascades at high energy and constitutes a simple law which can be applied to any material and used in order to extrapolate molecular dynamics results to high energy PKAs. The nature of irradiation induced defects has also been studied as a function of the cascade energy.     

A transient creep solution for uniaxial tension rectangular thin plates with central circular holes

A general method for the solution of non-linear creep problems is presented. This method reduces the creep problem to a sequence of elastic problems with initial strains. The solution of the elastic problem with initial strains is determined using four displacement functions together with the point-matching method. The method is based on the extension of Goodier's concept of reducing a thermoelastic problem to one at constant temperature with no body force. Using this method, creep behaviour in the vicinity of a circular hole in a uniaxial tension thin rectangular plate has been investigated for a time-hardening material law. The results show how stress concentration factors vary with time for various combinations of geometrical shape and stress index.     

Elastoplastic constitutive model for particle-dispersed composites accounting for interfacial damage

For establishing the constitutive law, the property of a composite material is generally described by focusing on the relation between the average stress and the average strain in multiple phases. While the interface between matrix and inclusions undergoes damage, this relation should be modified accordingly. The effects of damaged interface on the strain field in composite are considered in two ways. First, the degradation of matrix-inclusion interface makes the strain field inside inclusions no longer uniform as that of inclusions with perfectly bonded interface. Secondly, it contributes to the average strain in composite by an additional strain, which is yielded from an integration of relative displacement between matrix and inclusion over their interface. In present paper, the first part is considered by using a modified Eshelby's S-tensor. After deriving the local relative displacement distributions between matrix and inclusion at the interface, the second effect of damaged interface on the average strain can be expressed in terms of the corresponding eigenstrain, by introducing a damage-relevant tensor D, which is a fourth order tensor, and tends to zero when the interface is perfect. Both the tangential and normal discontinuities at the interface are independently modeled. The numerical results are also shown. It is found that the interface conditions of debonding and/or sliding give detrimental effects on the overall properties of composites. Thus, the establishment of the most appropriate model describing properly the meso-local phenomena.     

Thermoplastic analysis of a sphere-pac fuel pin

This analysis is concerned with the thermomechanical response of a sphere-pac fuel pin. The fuel is modeled as an elastic-plastic continuum governed by the Mohr-Coulomb yield criterion and associated flow rule under plane strain conditions. Yielding is found to initiate at the outer edge of the fuel, and a plastic zone progresses inward. When the fuel has completely yielded, there exist three distinct plastic zones corresponding to stress states on different facets of the yield surface. Closed form expressions for the displacement and stresses in each of the plastic zones are presented. Numerical results illustrating the variation of displacement and stress with radial position are given.     

Effect of parameter setting and spectral normalization approach on study of matrix effect by laser induced breakdown spectroscopy of Ag–Zn binary composites

The complex nature of laser-material interaction causes non-stoichiometric ablation of alloy samples.This is attributed to matrix effect, which reduces analyzing capability. To address this issue, the analytical performance of three different normalization methods, namely normalization with background, internal normalization and three point smoothing techniques at different parameter settings is studied for quantification of Ag and Zn by Laser induced breakdown spectroscopy(LIBS).The LIBS spectra of five known concentration of silver zinc binary composites have been investigated at various laser irradiances(LIs). Calibration curves for both Ag(I) line(4d~(10)5s~2S_(1/2)→4d~(10)5p~2P_(1/2) at 338.28 nm) and Zn(I) line(4s5s~3S_1→4s4p~3P_2 at 481.053 nm) have been determined at LI of 5.86?×?10~(10)W cm~(-2). Slopes of these calibration curves provide the valuation of matrix effect in the Ag–Zn composites. With careful sample preparation and normalization after smoothing at optimum parameter setting(OPS), the minimization of sample matrix effect has been successfully achieved. A good linearity has been obtained in Ag and Zn calibration curve at OPS when normalized the whole area of spectrum after smoothing and the obtained coefficients of determination values were R~2?=?0.995 and 0.998 closer to 1. The results of matrix effect have been further verified by analysis of plasma parameters. Both plasma parameters showed no change with varying concentration at OPS. However, at high concentration of Ag, the observed significant changes in both plasma parameters at common parameter setting PS-1 and PS-2 were the gesture of matrix effect. In our case, the better analytical results were obtained at smoothing function with optimized parameter setting that indicates it is more efficient than normalization with background and internal normalization method.     

Development and benchmarking of a mechanical model for GFR plate-type fuel

The reference fuel design currently being considered within the Generation-IV Gas-cooled Fast Reactor (GFR) project is a ceramic plate matrix with a honeycomb inner structure containing small fuel cylinders. The fuel is mixed plutonium–uranium carbide, while the matrix material is silicon carbide. The present paper describes the mechanical part of a thermal–mechanical model being developed for studying the transient behavior of this highly heterogeneous fuel type. Benchmarking has been carried out against detailed finite-elements modeling (FEM).The resultant thermal–mechanical model can provide reliable fuel and cladding (matrix) stress/strain conditions to evaluate temperatures and neutronic feedbacks. As such, it has been integrated into PSI’s coupled code system “FAST”, which aims at the comprehensive safety analysis of advanced fast reactor systems.The detailed FEM analysis of the GFR fuel has been useful not only for benchmarking the new model, but also for obtaining an in-depth understanding of fuel stress/strain characteristics, which cannot be reproduced with simplified models. Thereby, the range of applicability of the new model has clearly been defined. In particular, the 3D FEM analysis has revealed a concentration of stresses at the pellet corners during pellet/matrix contact, which could lead to fuel element failure. This effect is found to be mitigated considerably, if the fuel pellets are shaped in a manner which enhances the contact area.     

Dynamic plastic deformation of axi-symmetric circular cylindrical shells

The endochronic theory of plasticity has been applied to study the axially symmetric motion of a finite circular cylindrical shell subjected to an arbitrary pressure transient applied to its inner surface. The constitutive equations for thin shells have been obtained. The governing equations are then solved by means of the characteristics method. It is found that there exist two characteristic waves: one corresponds to the longitudinal wave while the other represents the shear wave. Several numerical examples are presented for specific boundary conditions and a prescribed loading history along the axial direction. The radial displacement profiles in this analysis show the coupling effect of both longitudinal and shear waves. Furthermore, the resultant displacement history seems in good agreement qualitatively with the pressure input function.     

Deflection-based method for seismic response analysis of concrete walls: Benchmarking of CAMUS experiment

A number of shake table tests had been conducted on the scaled down model of a concrete wall as part of CAMUS experiment. The experiments were conducted between 1996 and 1998 in the CEA facilities in Saclay, France. Benchmarking of CAMUS experiments was undertaken as a part of the coordinated research program on ‘Safety Significance of Near-Field Earthquakes’ organised by International Atomic Energy Agency (IAEA). Technique of deflection-based method was adopted for benchmarking exercise. Non-linear static procedure of deflection-based method has two basic steps: pushover analysis, and determination of target displacement or performance point. Pushover analysis is an analytical procedure to assess the capacity to withstand seismic loading effect that a structural system can offer considering the redundancies and inelastic deformation. Outcome of a pushover analysis is the plot of force–displacement (base shear–top/roof displacement) curve of the structure. This is obtained by step-by-step non-linear static analysis of the structure with increasing value of load. The second step is to determine target displacement, which is also known as performance point. The target displacement is the likely maximum displacement of the structure due to a specified seismic input motion. Established procedures, FEMA-273 and ATC-40, are available to determine this maximum deflection. The responses of CAMUS test specimen are determined by deflection-based method and analytically calculated values compare well with the test results.     

Mechanical aspects of the Charpy impact test

A comparative experimental and FEM study has been carried out, in order to investigate dynamic and constraint aspects of the Charpy test. Standard V-notch Charpy specimens were tested under dynamic and static loading conditions. 2-D plane strain and 3-D models were employed in numerical analysis. In order to incorporate strain-rate effects, an elastic-viscoplastic constitutive equation has been applied, based on actual test data obtained for a low-alloy structural steel. Fully dynamic analysis clearly indicated inertial effects. Modal analysis enabled the confirmation of the origin of the oscillations on the load–displacement curve as beam vibration of the specimen resulting from interaction with the elastic striker.     

核电站主泵机组地震响应谱分析及应力评定

用三维实体建模软件Inventor建立某核电站主泵的三维实体模型。对模型进行简化,灵活运用ANSYS的单元属性和接触功能,建立有限元动力学模型。通过模态分析,得出前13阶固有频率。在此基础上,用SRSS振型组合法分析多地震谱、多角度下核主泵的地震谱响应,得到了相应的应力和位移响应。对主泵进行静力学分析,将地震动应力与静应力相叠加,分析不同工况下主泵机组的应力值。按ASME规范进行校核,结果表明：应力值满足标准要求。     

Modeling deformation and failure of fast reactor cladding during simulated accident transients

An analysis is made of burst experiments performed on neutron irradiated cladding tubes. This is done by employing a generalized Voce equation to describe the mechanical deformation of type 316 stainless steel, combined with an empirical creep crack growth law, each modified to account for the effects of irradiation matrix hardening, and irradiation induced grain boundary embrittlement, respectively.The results of this analysis indicate that for large initial hoop stress, failure occurs at relatively low temperature and is controlled by the onset of plastic instability. The increase in failure temperature of irradiated material, in low temperature region, is due to irradiation strengthening. Failure in the case of relatively small initial hoop stress occurs at high temperature where the Voce equation reduces to a power law creep formula. The ductility of irradiated material, in this high tem-temperature region, is adequately described through the use of an empirical intergranular crack growth law used in conjunction with the creep law. The effect of neutron irradiation is to reduce the activation energy for crack propagation from the value for creep to some lower value correlated to independent Dorn rupture parameter measurements. The result is a predicted reduced ductility which translates into a reduction in failure temperature at a given hoop stress value for irradiated material.     

中子位移损伤监测技术研究

利用硅双极晶体管直流增益倒数与中子注量具有线性关系的特点,将其作为位移损伤监测器以获取不同中子辐射场的损伤特性。采用两种不同的数据分析方法,分别得到了两种位移损伤监测器阵列的相对损伤常数。研究结果为在现有的实验条件和测试手段基础上选择位移损伤监测器和分析监测结果奠定了基础。     

The various creep models for irradiation behavior of nuclear graphite

Graphite is a widely used material in nuclear reactors, especially in high temperature gascooled reactors (HTRs), in which it plays three main roles: moderator, reflector and structure material. Irradiation-induced creep has a significant impact on the behavior of nuclear graphite as graphite is used in high temperature and neutron irradiation environments. Thus the creep coefficient becomes a key factor in stress analysis and lifetime prediction of nuclear graphite. Numerous creep models have been established, including the visco-elastic model, UK model, and Kennedy model. A Fortran code based on user subroutines of MSC.MARC was developed in INET in order to perform three-dimensional finite element analysis of irradiation behavior of the graphite components for HTRs in 2008, and the creep model used is for the visco-elastic model only. Recently the code has been updated and can be applied to two other models—the UK model and the Kennedy model. In the present study, all three models were used for calculations in the temperature range of 280–450 °C and the results are contrasted. The associated constitutive law for the simulation of irradiated graphite covering properties, dimensional changes, and creep is also briefly reviewed in this paper. It is shown that the trends of stresses and life prediction of the three models are the same, but in most cases the Kennedy model gives the most conservative results while the UK model gives the least conservative results. Additionally, the influence of the creep strain ratio is limited, while the absence of primary creep strain leads to a great increase of failure probability.     

Hybrid method for vibration analysis of rectangular plates

This paper presents a semi-analytical approach for the dynamic analysis of rectangular plates. The mathematical model is developed using a hybrid combination of the finite element method and Sanders’ shell theory. The in-plane, membrane displacement components are modelled by bilinear polynomials and the out-of-plane, normal to mid-surface displacement component is modelled by an exponential function that represents a general form of the exact solution of the equations of motion. The displacement functions are obtained by exact solution of the equilibrium equations of the rectangular plates. The mass and stiffness matrices are then determined by exact analytical integration to establish the plate's dynamic equations. The effect of various geometrical parameters and boundary conditions on the dynamic responses of the rectangular plates has been explored in this work. The results are in satisfactory agreement with those of experiments and other theories.     

基于弥散燃料颗粒开裂的金属基体裂纹特征模型

金属基弥散燃料元件在特殊工况下会发生表面起泡失效。燃料颗粒开裂是金属基体开裂的前提条件,只有当金属基体开裂后元件才会发生表面起泡。燃料颗粒开裂后,裂纹宽度和塑性区长度等裂纹特征决定了金属基体开裂行为。基于弹塑性断裂力学和应力平衡条件,建立了基于弥散燃料颗粒开裂的金属基体裂纹特征模型。计算结果表明：裂纹张开位移随退火温度和燃耗深度的升高而增加;裂纹尖端塑性区长度主要与退火温度相关。裂纹张开位移和塑性区长度的计算结果与实验数据均符合较好,验证了金属基体裂纹特征模型的有效性。     

Vibration Compacted UO2 Fuel, (I) Effects of Sinusoidal Vibration on Compacted Density

A systematic study has been made on the effect of differences in conditions of applied vibration on the density obtained with a ternary grained powder of arc-fused and crushed UO₂ powder packed in a stainless steel cladding tube.

Comprehensive analysis is made of the relationships between densities obtained and vibration conditions, defined by combinations of two factors among four-i.e., frequency, displacement, velocity and acceleration. The highest density has been found to be obtained at the highest acceleration, provided that displacement is in the range of 10~30 μm, and this range is found to be the optimum condition for all levels of acceleration.

The process of vibratory compaction is discussed on the basis of the combination of different factors conducive to the compaction, segregation and disintegration of the powder particles.     

温度梯度对金属波纹管力学性能影响分析

高温熔盐调节阀中金属波纹管是保证其正常运行的重要部件,波纹管外侧被熔盐介质包围,承受外压、轴向位移及高温载荷,且波纹管轴向存在较大温度梯度,为调节阀中薄弱元件。本文应用有限元软件ANSYS,材料模型选择理想塑性材料模型,计算单元为热固耦合单元,对比分析了U形和V形波纹管在各设计工况下的应力分布,结果表明,位移载荷是两种波纹管失效的主要原因。位移载荷在两种波纹管中引起的应力大小基本一致,但V形波纹管在设计压力、温度载荷作用下的应力显著小于U形,故调节阀中使用波纹管类型选择为V形。此外,对V形波纹管在多工况下的应力分布、塑形应变及极限位移载荷进行了计算,对比分析了设计温度载荷、阀体存在保温层时温度载荷及常温温度载荷对波纹管的影响,结果显示,高温下波纹管极限位移载荷约为常温的三分之一,但阀体外部添加保温层,虽然使得波纹管温度升高,但对波纹管极限压缩载荷影响并不大。