1Cr18Ni9Ti焊缝的物理疲劳短裂纹扩展实验

应用复型技术研究了1Cr18Ni9Ti管道焊缝金属的物理疲劳短裂纹行为。结果表明,依照“有效短裂纹准则”,在物理短裂纹（PSC）阶段,主导有效短裂纹（DESFC）扩展和有效短裂纹（ESFCs）的扩展及合并对疲劳损伤做出直接贡献,在DESFC裂尖前沿区域的ESFCs密度通过影响DESFC扩展,对疲劳损伤做出问接贡献。DESFC受到减弱的微观结构条件影响,逐渐演化为单条长裂纹扩展行为。ESFCs密度在微观结构短裂纹（MSC）阶段反映DESFC萌生区域、在PSC阶段反映其裂尖前沿的微观结构扩展条件,其统计演化特征与DESFC扩展率的统计演化特征一致。这说明了这一微观结构扩展条件的差异和演化,是随机疲劳性能与演化特性的本质原因。疲劳损伤是一个由初始混沌状态演化到MSC与PSC两阶段交界处独立无关的随机状态,然后到史相关随机状态的过程。     

1Cr18Ni9Ti管道焊缝金属疲劳短裂纹萌生与早期扩展

复型研究了1Cr8N9Ti管道焊缝金属的疲劳短裂纹萌生与早期扩展特点，结果表明，有效短裂纹萌生于与管道内外表面一致的试样表面局部区域中delta铁素体与基质的界面，早期扩展具有显著微观结构效应特征，最大障碍尺度约为40μm，与焊缝金属特征微观组织结构――柱状晶中富delta铁素体带距离一致，该尺度范围。由于强微观结构约束，短裂纹密度较高并逐渐增加；尺度不规则扩展，直到若干较长短裂纹发生合并形成导致试样失效的主导短裂纹。这一过程通常称为“微观结构短裂纹（MSC）“阶段，主导短裂纹及其发路径上的短裂纹对疲劳损伤做出直接贡献，可分别称为“主导有效短裂纹（DESFC）“和“有效短裂纹（ESFCs）“，为描述该阶段不规则行为，有必要引入尺度等于ESFCs尺度均值的虚裂纹即MSC阶段DESFC的概念，该阶段DESFC尺度恒定增加，扩展率恒减小。     

1Cr18Ni9Ti焊缝的疲劳短裂纹扩展方程

基于“有效短裂纹准则”,研究了1Cr18Ni9Ti焊缝的疲劳短裂纹行为描述方法。与疲劳群体短裂纹演化行为机制一致,考虑了3个重要因素。首先,主导有效短裂纹（DESFC）行为是群体短裂纹交互作用与影响的结果,因而适于表征短裂纹扩展行为。其次,显著的微观结构短裂纹（MSC）和物理短裂纹（PSC）两阶段行为特征,说明扩展律方程应当揭示这一现象。第三,强烈的不规则扩展行为,说明单一循环应变或应力参量不宜表征疲劳短裂纹扩展行为。因此,以DESFC局部萌生区域（MSC阶段）及其裂尖前沿区域（PSC阶段）的局部应变能密度为驱动力,发展了以包容上述区域的应变能密度为驱动力计算参数的新短裂纹扩展律,试验结果分析验证了方法的有效性。     

16MnR缺口件疲劳启裂寿命的理论分析与试验研究

为了研究缺口形貌和加载条件对16MnR材料疲劳寿命的影响,进行了光滑圆棒试样和缺口圆棒试样的疲劳试验,得到了缺口件的疲劳启裂寿命和疲劳断裂寿命。通过用户子程序UMAT将16MnR材料的精确循环塑性本构关系嵌入到有限元件软件ABAQUS中,并结合多轴疲劳损伤准则,确定出1个加载循环在临界材料面上产生的疲劳损伤,进而通过理论模型预测缺口件的疲劳启裂寿命。疲劳启裂寿命的理论预测结果与试验数据相吻合,表明多轴应力状态下的缺口件疲劳启裂是所有不为0的应力、应变分量共同作用的结果。     

稳压器接管安全端堆焊结构裂纹扩展计算分析

针对核电厂稳压器接管安全端堆焊设计结构,按照稳压器设计载荷,对假设的安全端焊接区域的裂纹开展疲劳和应力腐蚀引起的裂纹扩展分析,获得裂纹扩展尺寸,并进行安全评估,结果表明,异种金属焊缝区域在周期末的最大环向裂纹深度扩展量为0.4×10~(-3) mm,最大轴向裂纹深度扩展量为23.6×10~(-3) mm;不锈钢焊缝区域在周期末的最大环向裂纹深度扩展量为12.4×10~(-3) mm,最大轴向裂纹深度扩展量为0,裂纹扩展量均满足堆焊设计要求,为稳压器接管安全端堆焊结构设计和评定提供参考。     

氧化物夹杂与Ni-Cr-Mo-V钢多层焊缝低温冲击韧性变化规律的关系

测试了反应堆压力容器用Ni-Cr-Mo-V钢焊缝的冲击韧性，实验结果表明，当实验温度较低时，同一实验温度下沿盖面焊到焊根方向不同层焊缝样品的冲击吸收能呈下降趋势。通过低温实验样品断口观察到韧窝底部、起裂源、解离断刻面裂纹源存在直径为0.3～2.0 μm的球形氧化物夹杂，导致试样在变形过程中氧化物夹杂与基体分离形成微裂纹，并发展为试样的韧窝、起裂源和解离断刻面裂纹源，由此推断氧化物夹杂是造成焊缝低温失效的主要原因。同时氧化物夹杂的数量沿盖面焊到焊根方向逐渐增多，使得微裂纹形核率逐渐增加，造成焊缝低温冲击韧性沿盖面焊到焊根方向逐渐变差。     

核压力容器异种金属焊接接头延性断裂行为数值研究

使用GTN损伤模型与有限元计算相结合的方法,对核电安全端异种金属焊接接头试样裂纹扩展路径及J-R阻力曲线进行数值研究.结果表明:异种金属接头中的堆焊层及焊缝中心裂纹扩展路径和均质材料基本相同,而界面及近界面裂纹的扩展路径显著地偏离初始裂纹位置.界面裂纹及近界面裂纹倾向于向屈服强度低的软材料一侧扩展,这是由裂尖前区域不同材料的强度失配所致.不同初始位置和不同尺寸的裂纹具有不同的材料拘束和几何拘束,其J-R裂纹扩展阻力曲线不同.宏观断裂力学无法预测这种裂纹的扩展路径和扩展阻力,基于GTN损伤模型的数值计算方法可以预测这种裂纹的扩展路径及扩展阻力,可望用于安全端焊接结构的完整性评定和破前漏分析.     

含双边轴向裂纹N18锆合金薄壁管的蠕变裂纹扩展行为研究

通过对含双边轴向裂纹管(DEAT)试样及其加载装置进行设计,基于能量等效和载荷分离原理获得了DEAT试样的能量率回路积分(C*积分)表达式,从而建立了含轴向裂纹薄壁管的蠕变裂纹扩展速率测试方法。基于此方法,采用DEAT试样完成了N18锆合金薄壁管在350℃不同载荷水平下的蠕变裂纹扩展试验。结果表明,蠕变载荷会显著影响N18锆合金的蠕变裂纹扩展速率;蠕变裂纹扩展可分为稳态扩展和快速扩展2个阶段;蠕变裂纹扩展速率(da/dt)与C*积分存在良好的幂律关系,可用于预测N18锆合金管蠕变裂纹扩展行为。     

基于微焦点CT的三维裂纹扩展分析

采用高频疲劳试验机进行了2A50锻铝缺口试样三点弯曲试验,对不同扩展阶段的表面裂纹进行了SEM对比分析。通过微焦点CT对裂纹进行了三维重建,三维可视化结果直观显示了裂纹的尺寸、位置和形态分布。通过相邻两个阶段二维CT图像的灰度、裂纹前沿位置分析裂纹扩展行为特征,结果表明,裂纹扩展不规则。分别将两个阶段的裂纹体进行投影得到了投影图像,对比后可观察到新的裂纹扩展区域,并得到了两个阶段的裂纹扩展前沿分布曲线。通过两个阶段的裂纹体三维分析得到了裂纹前沿扩展增量三维分布。     

核级不锈钢管座焊接区与母材区疲劳裂纹扩展性能对比研究

焊接接头广泛应用于核电站管座处,而疲劳裂纹扩展是导致焊接接头失效的重要原因之一。因此,研究焊接区材料的疲劳裂纹扩展和寿命预测方法对准确预测焊接接头的寿命具有重要意义。本文以核电厂常用的304L不锈钢焊缝材料为对象,研究不同载荷比、不同取样方向对疲劳裂纹扩展速率的影响;基于试验数据建立焊缝材料的疲劳裂纹扩展速率模型,并与美国机械工程师协会（ASME）标准中奥氏体钢进行对比。结果表明:不同取样方向对焊缝疲劳裂纹扩展速率的影响不大,但载荷比对其有较大影响,较低载荷比下,焊缝的疲劳裂纹扩展速率在某个应力强度因子幅值（?K）前高于母材的疲劳裂纹扩展速率,在其之后则低于母材,而较高载荷比下则恰恰相反。      

Effect of biaxial loading on the fracture behaviour of a ferritic steel component

The effect of biaxial loading on the ductile behaviour of a through-wall crack in a ferritic steel structure under contained yield is of particular interest to the structural integrity argument for reactor pressure vessels. This results from the fact that there are many instances in practice (for example a crack in a circumferential weld), where a significant applied stress is present in the direction parallel to the crack as well as in the perpendicular direction. Two large plate ductile tearing tests have been performed on centre through-crack specimens (75 mm by 2 m by 2 m) manufactured from a ferritic steel. The first test specimen was loaded in uniaxial tension and the second test specimen was loaded biaxially. This paper presents experimental details and results of the two plate tests and describes the analysis work undertaken to interpret the experiments satisfactorily.     

Random cyclic stress–strain responses of a stainless steel pipe-weld metal II — a modeling

This paper pays special attention to an issue that there is a significant scatter of the stress–strain responses of a nuclear engineering material, 1Cr18Ni9Ti stainless steel pipe-weld metal. Efforts are made to reveal the random fatigue damage character by fracture surface observations and to model the random responses by introducing probability-based stress–strain curves of Ramberg–Osgood relation and its modified form. Results reveal that the fatigue damage is subjected to, 3-D interacting and involved microcracks. The three stages, namely microstructural short cracks (MSC), physical short cracks (PSC) and long cracks (LC) subdivided by Miller and de los Rios, can give a good characterization of the damage process. Both micro- and macro-behaviour of the material have the character of three stages. The 3-D effects are strong in the MSC stage, tend to a gradual decrease in the PSC stage, and then show saturation after going to the LC stage. Intrinsic causes of the random behaviour are the difference and evolution of the microstructural conditions ahead of the dominant crack tips. The ‘effectively short fatigue crack criterion’ introduced by Zhao et al. in observing the material surface short crack behaviour could facilitate an understanding of the mechanism of interaction and evolution. Based on the previous obtained appropriate assumed distribution, normal model, for the cyclic stress amplitude, the probability-based curves are approximated by the mean value and standard deviation cyclic stress–strain curves. Then, fatigue analysis at arbitrarily given reliability can be conveniently made according to the normal distribution function. To estimate these curves, a maximum likelihood method is developed. The analysis reveals that the curves could give a good modeling of the random responses of material.     

Fracture criterion of a mixed-mode crack based on CED (Crack Energy Density)

The CED (Crack Energy Density) in an arbitrary direction, , is defined as the quantity which has the meaning of strain energy area density in the plane containing a crack front line without any restriction on constitutive equation. We may expect that, as far as the opening type fracture occurs, a mixed mode crack begins to grow in the direction where (mode I contribution of ) takes the maximum value, , when reaches a critical value peculiar to a material (we call this criterion criterion). In this paper, the applicability of this criterion to elastic-plastic fractures is studied based on the fracture experiments on specimens of Aluminum alloy with a crack inclined to the loading axis and it is demonstrated that the onset and the direction of crack growth can be estimated by criterion in either case of fractures with relatively small yielding region and those with large yielding region. The applicability of criterion to elastic (quasi-elastic) fractures is also shown through the comparison of the result by criterion with the results by other criteria and experimental results.     

Crack arrest in high cycle thermal fatigue crazing

Thermal crazing in high cycle thermal fatigue due to thermal fluctuation in residual heat removal (RHR) system of some nuclear power plants is explained by crack arrest in the depth due to a decreasing stress intensity factor. This is related to high frequencies of thermal loading. An attempt has been made through a parametric study to acquire some knowledge about the loading, knowing the crack depth. For this purpose, analytical as well as finite element simulations of crack propagation in 2D- and 3D-semi-elliptical cracks have been performed. In periodic loading, bounds for the number of cycles to fatigue life are proposed. Moreover, it is shown that in the absence of mean stress, fatigue damage in RHR may be produced in the macroscopic elastic-plastic regime. Finally, it is shown by FE simulations that for a semi-elliptical crack, a small error on stress intensity factor may result in significant error on crack length at high number of cycles, due to error accumulation cycle by cycle. Moreover in this paper is given the reason as to why shielding effect has not been taken into account in the study of crack arrest in RHR.     

A study on the evolution of crack networks under thermal fatigue loading

The crack network is a typical cracking morphology caused by thermal fatigue loading. It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow deeply. In this study, the mechanism of evolution of crack network and its influence on crack growth was examined by numerical calculation. First, the stress field near two interacting cracks was investigated. It was shown that there are stress-concentration and stress-shielding zones around interacting cracks, and that cracks can form a network under the bi-axial stress condition. Secondly, a Monte Carlo simulation was developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network. The local stress field formed by pre-existing cracks was evaluated by the body force method and its role in the initiation and growth of cracks was considered. The simulation could simulate the evolution of the crack network and change in number of cracks observed in the experiments. It was revealed that reduction in the stress intensity factor due to stress feature in the depth direction under high cycle thermal fatigue loading plays an important role in the evolution of the crack network and that mechanical interaction between cracks in the network affects initiation rather than growth of cracks. The crack network appears only when the crack growth in the depth direction is interrupted. It was concluded that the emergence of the crack network is preferable for the structural integrity of cracked components.     

Cyclic and transient thermal loading of the HDR reactor pressure vessel with respect to crack initiation and crack propagation

In a series of thermal loading tests at the HDR reactor pressure vessel – thermal stratification, cyclic thermal shock and pressurized thermal shock – the methods applied in safety analysis had to become qualified by a continuous intercomparison of calculated results and experimental data. Above all the complex boundary conditions of the HDR-tests offer a close approximation to the original components, so that they provide a real assessment of the transferability.The results of the thermal mixing tests indicated that during cold water inflow into the RPV longitudinal strains build up in the cylindrical wall which dominate over that in circumferential direction.During the cyclic thermal fatigue tests incipient crack formation in the cladding as well as the behaviour of crack propagation in the cladding and in the base material was analyzed.In the pressurized thermal shock tests, the nozzle region and the cylinder wall in the incipient crack condition were loaded by long cooling streaks. Even in the aggravated loading condition as the result of a routed cold water streak no remarkable indications of crack growth were noticed.In both cases, cyclic and pressurized thermal shock loading, the expected crack propagation was overpredicted by the fracture mechanical methods used.The non-destructive examination methods used were able to locate all of the cracks but they mostly overpredicted the actual crack depth.     

Experimental investigation on ductile stable crack growth emanating from wire-cut notch in AISI 4340 steel

An experimental investigation on the stable crack growth (SCG) behaviour in AISI 4340 using CT type specimen with a sharp slit (0.05 mm) under mode I and mixed modes (I and II) loading is presented. The slit was made in the specimen through wire cutting technique. Different combinations of loading angle and ratio of original crack length to specimen width (a₀/W) are examined. Data concerned with direction of initial crack extension, load–load line displacement (L–LLD) diagrams, initiation and maximum loads, range of stable crack growth, crack tip blunting, crack front geometry, fracture surfaces and their scanning electron micrographs are obtained. A noticeable blunting effect is observed prior to crack initiation. Although the crack initiates from a straight front, a considerable front tunnelling effect occurs as the crack extends. Under mixed mode, the crack extension takes place initially almost along a straight path, inclined with the main crack. The loading angle and initial crack length affect the initiation (P_i) and maximum (P_max) loads significantly, but the ratio between P_max and P_i remains almost constant. The direction of initial stable crack extension due to mixed mode loading is determined throughout an elastic finite element analysis. There is a good agreement between the experimental and predicted results.     

New approaches to improve the RPV materials data base

The models for the local approach to cleavage fracture and to ductile tearing include the effects of specimen geometry and loading situation on the damage development. Thus, these concepts can be used to predict fracture toughness and tearing resistance curves for different sizes of specimens or structures, based on an analysis of tests with small, simple specimens. This advantage is especially valuable if only small pieces of material are available for testing which do not allow for standard fracture specimens. The paper outlines the basic ideas of the Beremin concept for the fracture process in brittle-to-ductile transition regime and of the modified Gurson model for ductile fracture in the upper shelf. A systematic study for the brittle-to-ductile transition regime showed that the Beremin model has to be modified to take into account the effect plastic strain has on the fracture process. For ductile fracture, it was demonstrated that by testing and modeling a smooth bar and a fracture mechanics specimen, one set of material parameters may be found which can be subsequently applied to other structures and, hence, used to extend the material data base.