锆合金的低周疲劳行为及其机理

锆合金的低周疲劳寿命遵循Coffin-Manson关系NβfΔεp=C1 .冶金状态、试验条件等皆影响锆合金的低周疲劳寿命.疲劳裂纹易在塑性应变不协调的区域形核,其扩展速率可用Paris公式表示为dα/dN=C2(ΔK)n .循环变形中的流变应力可分为摩擦应力和背应力.锆合金在循环变形中所呈现出的循环硬化、软化以及循环饱和状态主要取决于试验温度和应变幅,其作用机制各不相同.位错线、位错条带、胞结构和迷宫结构是锆合金低周疲劳典型的位错组态.     

N18锆合金的低周疲劳行为

在不同试验温度（室温～500℃）下,对N18合金进行了低周疲劳试验。试验结果表明：室温～300℃温区,合金表现为明显的循环软化;400、450℃时,合金逐渐呈现循环硬化,450℃时其硬化现象更为明显;500℃时则主要表现为循环饱和。随着温度的升高,疲劳寿命先增加后降低,300℃时疲劳寿命最高。低应变幅下,温度对疲劳寿命的影响更明显。通过疲劳断口SEM分析,室温下疲劳起源于单个裂纹源,疲劳裂纹扩展阶段的微观特征主要是疲劳条纹,局部区域出现轮胎状花样。在高温下为多裂纹源,大量二次裂纹的存在是高温疲劳断口的主要特征。     

氢及氢化物对锆合金疲劳裂纹扩展速率的影响及估算

采用紧凑拉伸（CT）试样．研究了不同氢含量的Zr-4及Zr-Sn-Nb合金在室温下疲劳加载裂纹扩展（dα/dN）行为．用扫描电镜观察了断口形貌。结果表明，氢含量对疲劳裂纹扩展速率影响微弱，疲劳断裂受通常的裂纹萌生、稳态扩展和瞬间断裂机制控制。根据疲劳裂纹扩展机理．导出了裂纹扩展门槛值△Kth的关系式．得出了一个描述疲劳裂纹扩展速率油（dα/dN）与材料性能常数之间的关系式，该关系式可用于预测材料的疲劳裂纹扩展速率。用锆合金实验数据对（dα/dN）预测表达式进行验证．结果表明，预测值与实验值吻合较好。     

新锆合金低周疲劳特性研究

采用薄壁管材试样,对f9.50.57mm规格的2种新型锆合金管材进行了5个应变水平下的低周疲劳实验,且对其中的两个应变水平做了统计分析。对实验数据处理后,得到了管材的Det-N关系曲线和疲劳拟合曲线方程。并对管材在不同应变水平的低周疲劳断口做了金相和SEM分析,讨论了新型锆合金薄壁管材的低周疲劳断裂机理。     

Zr-4合金低周疲劳特性研究

研究了再结晶态Zr-4合金板材在室温和400 ℃下的低周疲劳特性.Zr-4合金在室温下的循环变形行为与应变幅有关,当应变幅小于0.8%时,表现为循环软化;应变幅大于0.8%时,表现为循环硬化.在400 ℃下均表现为循环硬化.合金在室温和400 ℃下,均遵循Coffin-Manson关系.在低应变幅下,室温的低周疲劳性能明显优于400 ℃下的低周疲劳性能,随着应变幅的增加,两者寿命趋于接近.用扫描电子显微镜观察分析了合金在室温和400 ℃下的疲劳断口特征.     

国产316LN不锈钢在模拟AP1000一回路水环境中的疲劳行为

一回路水环境下的疲劳性能是核电站主管道设计寿命评估的重要参数。针对国产主管道材料316LN开展了模拟AP1000一回路水环境的低周疲劳试验,分析了疲劳行为和失效机理。研究结果表明:国产316LN峰值应力随应变幅的增大而增大,大应变幅试样在疲劳过程中先后发生了循环硬化、循环软化和失稳,而小应变幅试样在失稳前未发生明显的循环硬化和循环软化;在应变幅由0.2%逐渐增加至1.2%的过程中,疲劳周次从105逐渐降低至102;疲劳断口具有典型的疲劳断口特征,裂纹萌生于试样表面,以穿晶方式垂直于主应力方向扩展,裂纹扩展区具有典型的疲劳辉纹,辉纹上有菱形颗粒状腐蚀产物,环境辅助开裂机制倾向于氢致开裂。     

新锆合金碘致应力腐蚀断口研究

利用扫描电镜和EDXA对新锆合金碘致应力腐蚀的断口进行了形貌观察和成分分析。在断口表面发现腐蚀产物、二次裂纹、沿晶开裂和穿晶准解理开裂等应力腐蚀断裂特征,并观察到锆合金碘致应力腐蚀的“沟槽”特征。起裂区为沿晶开裂,在裂纹扩展阶段,开裂以穿晶为主。断口上腐蚀产物的成分主要是氧和锆,局部准解理开裂区域可检测到碘。去应力退火试样上发现了平行轧面的深沟,且沿晶开裂不明显。试验温度升高,断口上的腐蚀产物增多,而且沿晶开裂更容易。碘分压不仅影响腐蚀产物层的厚度,而且碘分压较高时沿晶开裂容易发生。     

周向拉伸条件下的N36锆合金包壳管碘致应力腐蚀开裂

350℃下,对N36锆合金包壳管环形试样施加周向拉伸载荷,绘制碘浓度分别为102、103、104 Pa下的应力-延伸率曲线,研究包壳材料在碘气体(I2)环境下的碘致应力腐蚀开裂(I-SCC)情况。应力-延伸率曲线能够衡量锆合金包壳管发生I-SCC开裂的敏感性:当I-SCC发生时,在曲线中表现为没有明显的颈缩阶段。在应力-延伸率曲线上可以找出I-SCC裂纹萌生、扩展的相应发生阶段。I-SCC降低材料的延伸率和断裂能量,降低程度均随着碘浓度的升高而增加。I-SCC环形试样在周向拉伸过程中产生平行于断口的裂纹沟槽,裂纹起始阶段均为沿晶扩展形式。     

Ti-4Al-2V合金高周疲劳性能研究

评价了钛合金Ti-4Al-2V棒材的室温高周疲劳性能。采用旋转弯曲的加载方式完成了Ti-4Al-2V棒材在室温空气中的成组常规疲劳试验和升降法试验,对试验数据进行了分析,绘制了S-N曲线和P-S-N曲线,对疲劳断口进行了扫描电子显微镜(SEM)分析。结果表明,钛合金Ti-4Al-2V棒材的疲劳极限σ_(-1(10~7))与经验公式计算值吻合,Ti-4Al-2V棒材具有较好的抗高周疲劳的性能,疲劳断口呈现裂纹萌生、扩展和断裂的特征。     

新锆合金碘致应力腐蚀性能研究

研究了再结晶状态的相同成分，不同织构取向的两块新锆合金板材在不同的实验条件下的抗碘致应力腐蚀性能。用直流电位降法动态监测裂纹的长度并对试样进行了织构的测定、第二相的观察和断口分析结果表明，在实验条件下，N18-2(L-T)抗碘致应力腐蚀的能力优于N18-1(T—L)断口形貌表明，在应力腐蚀裂纹的初始扩展阶段，断口沿晶开裂；在裂纹的稳态扩展阶段，以穿晶准解理扩展为主。     

用迭代函数系统提取两相流不稳定性的分形码

文章提出了将小波包分析数据压缩和消噪并与迭代函数系统（IFS）分形理论相结合用以研究汽液两相流动不稳定性的一种新方法。首先对实验得到的汽液两相流动不稳定性的振动曲线进行小波包数据压缩和消噪，得到用主要的小波低频和高频系数重新表示的信号；然后对新得到的信号提取IFS分形码，分别得到信号的高频和低频IFS分形码。这一新方法为深入研究汽液两相流动不稳定性的物理本质奠定基础。     

热流密度对汽水两相流压力波动特性的影响

实验研究了螺旋管内绝热和沸腾汽液两相流动时压力波动特性。分析了热流密度对压力波动的影响规律。结果表明沸腾时波动的均方根和分形参数（分维数、关联维数和Kolmogorov熵）与绝热两相流明显不同,加热热流密度的大小影响各特征参数的大小及其随干度的变化规律。未考虑热流密度影响的流型图或流型转变准则只能在小热流密度时使用。     

The frequency effect on the fatigue crack growth rate of 304 stainless steel

Under cyclic loading condition, the fatigue crack growth (FCG) rate governed by stress intensity factor and stress ratio is well known; Walker’s equation, Forman’s equation and Elber’s equation are typical formulae to describe the fatigue crack growth rate. However, the loading frequency effect on the fatigue crack growth rate has yet to be explored. Recently, studies have focused on the loading frequency effect on some visco-elastic materials, and have provided a clearer understanding of the frequency effect on the fatigue crack growth rate. In a physical sense, knowledge about the loading frequency effect on the fatigue crack growth rate for 304 stainless steel is still lacking. James conducted a lot of experiments, and through data analysis, he concluded an evaluation equation which is based upon the experimental illustration. In this study, the physical properties of the material are used to illustrate the modification of fatigue crack growth rate, and a new formula which is based upon the modified Forman’s equation, is provided.     

Fatigue crack growth behavior of weld heat-affected zone of type 304 stainless steel in high temperature water

The fatigue crack growth behavior of the weld heat-affected zone (HAZ) of type 304 stainless steel in high temperature water which simulates the boiling-water reactor environment was investigated to clarify the effects of welding residual stress, cyclic frequency f and thermal aging on crack growth rate. A lower crack growth rate of the HAZ than of the base metal was observed in both the high temperature water and the ambient air caused by the compressive residual stress. The crack closure point was measured in the high temperature water. The effect of the welding residual stress on the crack growth rate of the HAZ can be evaluated separately from the environmental effect through the crack closure behavior. The high temperature water increased the crack growth rate at a cyclic frequency of 0.0167 Hz but did not affect it much at 3 and 5 Hz. The crack growth behavior of the thermally aged HAZ at 400 °C for 1800 h was almost the same as that of the unaged material tested at 0.0167 and 5 Hz in the high temperature water.     

Corrosion fatigue behavior of low alloy steels under simulated BWR coolant conditions

The corrosion fatigue crack growth behavior of A533 and A508 low alloy steels under simulated boiling water reactor (BWR) coolant conditions was studied. Corrosion fatigue crack growth rates of A533B3 and A508 cl. 3 steels were significantly affected by the steel sulfur content, loading frequency and dissolved oxygen content of water environments. The data points outside the bound of Eason’s model could be attributed to the low frequency, higher steel sulfur content and high dissolved oxygen in water environments. The sulfur dissolved in the water environment from the higher-sulfur steels was sufficiently concentrated to acidify the crack tip chemistry even in the hydrogen water chemistry (HWC). Therefore, nitrogenated or HWC water showed little or no beneficiary effect on the high-sulfur steels. For the steel specimens of the same sulfur level, their corrosion fatigue crack growth rates were comparable in different orientations, which could be related to the exposure of fresh sulfides to the water environment. The percentages of sulfides per unit area, by quantitative metallography, were comparable for the steel specimens of both orientations. When the steel sulfur content was decreased to a critical sulfur content 0.005 wt.%, the crack growth rates decreased remarkably.     

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.     

某花岗岩型铀矿田断裂构造的分形特征

基于分形理论,提出了一种快速测量多区断裂分维值的方法,并获得了研究区断裂分维值的平面等值图.研究结果表明,断裂分维值的大小较好地反映了断裂构造复杂程度的高低,它们之间存在着正相关关系.研究区花岗岩型铀矿田内约有86%的铀矿床位于断裂分维值D偏高(D＞1.18)的区域内,表明断裂分维值越大,越有利于花岗岩型铀矿成矿,为建立该区花岗岩型铀矿新的预测标志提供了参考依据.     

A unified time dependent model for low cycle fatigue and ratchetting failure based on microcrack growth

In this paper, a unified time dependent model for low cycle fatigue and ratchetting failure has been developed based on the microcrack growth. The model utilized fracture mechanics theory using J-integral under creep–fatigue loading and assumed that the microcrack propagation determines the failure life. The microcrack rates are separated into three parts: (1) the time independent fatigue crack growth, (2) the time dependent fatigue crack growth and (3) the time dependent ratchetting crack growth. The cyclic failure criteria under different loading conditions were derived from the microcrack growth. Some simplified models were obtained and they can characterize the time dependent low cycle and ratchetting failure lives with hold time and loading frequency effects.     

PWSCC of thermally treated alloy 600 pulled from a Korean plant

Three tubes of alloy 600 were pulled out from a Korean nuclear power plant. The microstructure was analyzed using an optical microscope and TEM. Information on the crack length and depth was obtained by metallography, and crack detection and evolution were evaluated by analyzing the eddy current data obtained from each in-service-inspection (ISI). The carbon content in the pulled tubes was higher (around 0.03 wt.%) than that (around 0.015 wt.%) of Alloy 600 tubing used in other operating nuclear power plants. Most carbides in the pulled tubing were distributed in the grains rather than along the grain boundaries. The poor microstructure might come from high carbon contents, low temperature annealing, or high residual stresses during tube straightening. Mill annealing temperature should be high enough to dissolve all carbon in order to decorate the grain boundaries with semi-continuous carbide precipitation during 700 °C thermal treatment. Shot peening seemed to suppress the growth of the axial cracks, while it was analyzed to play a role in increasing crack growth in the wall thickness direction.     

Modes of long crack growth under non-stationary temperature fields

The exploitation practice of structures under thermal loads evidences that the final length of a quasistatic crack can be considerably greater than the thermal tension zone, sometimes causing that the structure approaches complete fracture. This occurs in one or several cycles of a gradual crack growth due to the evolution of thermal field in time resulting in that fracture zone follows the moving tension zone. By the extreme example of quasistationary thermal stress field the set of quasistatic crack growth modes and their peculiarities for the case of moving thermal stresses are described here. These are modes developing both in the direction of the thermal stress field propagation and in the opposite direction. The critical condition of each mode is described, and the crack growth rates are estimated. The rational crack growth evaluation procedure is also proposed. The theoretical conclusions are supported by the experiment, which demonstrates the growth of long thermal cracks.