具有内表面裂纹高压厚壁圆管的疲劳寿命分析

从KI的基本表达式出发,利用Schmitt-Keim影响系数并考虑压力介质的影响,对两支具有初始裂纹的模拟管进行了局计算.计算KI达到KIC的裂纹深度,与实验一致.本文将Paris方程作为厚壁圆管内表面裂纹扩展速率模型并给出方程系数,计算结果与实验结果相吻合.在此基础上计算了实验模拟管的疲劳寿命,给出了压力.疲劳寿命关系曲线.不失一般性,给出具有内表面裂纹高压厚壁圆管的疲劳寿命保守计算.     

Surface small crack growth behavior in low-cycle fatigue at elevated temperature and application limit of macroscopic crack growth law

Surface small crack growth behavior of Type 304 stainless steel in low cycle fatigue under fast-fast and slow-fast cyclings was investigated at a temperature of 873 K, by means of the smooth specimens with various grain sizes. It was shown that once the small cracks had grown up to a few grains size, they predominantly propagated with strain cycling, while most of small cracks stopped propagating when they grew up to one grain size. It was also shown that the small crack growth rate significantly slowed down where the crack length was integral multiple of the grain size. Above behavior resulted from the grain boundaries temporarily impeding the small crack growth. The crack length below which the grain boundaries affected the small crack growth rate was also given as function of the relative length to the grain size. Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fast-fast cycling, the small crack growth rate was about ten times as large as as the macroscopic crack growth one where its length was comparable to the grain size. Based on the results thus obtained, the application limit of macroscopic crack growth law to the surface small crack growth was discussed. The macroscopic crack growth law was not applicable to the small crack growth, until the crack length was about ten times average grain size in fast-fast and slow-fast cyclings.     

Low cycle fatigue properties of CLAM steel at room temperature

The low cycle fatigue (LCF) properties and the fracture behavior of China Low Activation Martensitic (CLAM) steel have been studied over a range of total strain amplitudes from 0.2 to 2.0%. The specimens were cycled using tension-compression loading under total strain amplitude control. The CLAM steel displayed initial hardening followed by continuous softening to failure at room temperature in air. The relationship between strain and fatigue life was predicted using the parameters obtained from fatigue test. The factors effecting on low cycle fatigue of CLAM steel consisted of initial state of matrix dislocation arrangement, magnitude of cyclic stress, magnitude of total strain amplitude and microstructure. The potential mechanisms controlling the stress response, cyclic strain resistance and low cycle fatigue life have been evaluated.     

Effect of notch dimension on the fatigue life of V-notched structure

The stress singularity degree associated to a V-notch has a great influence on the fatigue life of V-notched structure. The growth rate of the crack initiated at the tip of a V-notch depends on the stress singularity of the V-notch. The fatigue life accompanying with this small crack will represent a large amount of the total fatigue life. In this work, boundary element method (BEM) is used to study the propagation of the crack emanating from a V-notch tip under fatigue loading. A comparison of the fatigue life between the crack initiated from V-notch tip and a lateral crack is done by a crack propagation law until these two cracks have the same stress intensity factors (SIFs). The effect of initial crack length, notch opening angle and notch depth on the crack extension and propagation is analyzed. As an example of engineering application, the fatigue life of a welded joint is investigated by the present method. The influence of weld toe angle and initial crack length on the fatigue life of the welded structure is studied. Some suggestions are given as an attempt to improve the fatigue life of welded structures at the end.     

Fatigue cracks in Eurofer 97 steel: Part I. Nucleation and small crack growth kinetics

Fatigue crack nucleation and growth were studied in the Eurofer 97 ferritic-martensitic steel at room temperature. Cylindrical specimens with a shallow notch and no artificial crack starters were used. The constant strain amplitude cycling was adopted. First fatigue cracks nucleate at about 5% of the fatigue life along the surface slip bands. If a crack overcome the barrier of the first high angle boundary, its growth is regular and an exponential growth law is observed. This law may be used for the residual fatigue life prediction based on the small crack growth kinetics.     

Effect of ratcheting deformation on fatigue and creep-fatigue life of 316FR stainless steel

Components of fast breeder reactor (FBR) plants will be subjected to large thermal load, and progressive deformation with loading cycles (ratcheting) and creep-fatigue damage should be considered in their design. To clarify the effect of ratcheting on fatigue and creep-fatigue life, a series of fatigue and creep-fatigue tests coupled with strain progress were carried out for 316FR stainless steel. It was found that tensile ratcheting decreases the failure life to a large extent at small strain range, while compressive ratcheting does not decrease the failure life. Measurement of striation intervals on fracture surface showed small influence of strain increment on the crack propagation rate, suggesting that the main cause of the life reduction is the decrease in the crack initiation life. It was also found that failure life in various conditions is correlated well with a product of strain range and tensile peak stress.     

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.     

Development of fatigue life criteria for experimental fusion reactor first-wall structures

An approach to the rational design of fusion reactor first-wall structures against fatigue crack growth is proposed. The approach is motivated by microstructural observations of fatigue crack growth enhancement in unirradiated materials due to volumetric damage ahead of a propagating crack. Examples are cited that illustrate the effect of mean stress on void nucleation and coalescence, which represent the dominant form of volumetric damage at low temperature, and of grain boundary sliding and creep cavitation, which are the dominant volumetric damage mechanisms at high temperature. The analogy is then drawn between these forms of fatigue crack growth enhancement and those promoted by irradiation exposure in the fusion reactor environment, such as helium embrittlement and atomic displacement. An enhanced strain range is suggested as a macroscopic measure of the reduction in fatigue life due to the higher fatigue crack growth rates. The enhanced strain range permits a separation of volumetric and cyclic effects, and assists in the assignment of rational design factors to each effect. A series of experiments are outlined which should provide the numerical values of the parameters for the enhanced strain range.     

Fatigue cracks in Eurofer 97 steel: Part II. Comparison of small and long fatigue crack growth

The fatigue crack growth rate in the Eurofer 97 steel at room temperature was measured by two different methodologies. Small crack growth data were obtained using cylindrical specimens with a shallow notch and no artificial crack starters. The growth of semicircular cracks of length between 10-2000 μm was followed in symmetrical cycling with constant strain amplitude (R_ε = −1). Long crack data were measured using standard CT specimen and ASTM methodology, i.e. R = 0.1. The growth of cracks having the length in the range of 10-30 mm was measured. It is shown that the crack growth rates of both types of cracks are in a very good agreement if J-integral representation is used and usual assumptions of the crack closure effects are taken into account.     

Fracture strength and behavior of carbon steel pipes with local wall thinning subjected to cyclic bending load

The power plant piping is designed to withstand seismic events using the design fatigue curve. However, the fatigue strength of a pipe with local wall thinning caused by erosion/corrosion is not clear. To evaluate the fatigue strength of pipes with local wall thinning, low cycle fatigue tests were conducted on 100A carbon steel pipes with local wall thinning. In load controlled tests on these pipes, ratcheting deformation was observed, and the fatigue strength became lower than that of cracked pipes. In displacement controlled tests, the fatigue strength of eroded pipes with 100 mm in eroded axial length, 0.5 in normalized eroded depth and 90° in eroded angle was almost equal to that given by the design fatigue curve in ASME B&PV Code Sec. III. To evaluate the local strain range in the maximum wall thinning area, the finite element analysis was conducted on the eroded pipes in the displacement controlled tests. It is concluded that the Mises strain range in the maximum wall thinning area and the low cycle fatigue curve can be used to conservatively estimate the low cycle fatigue life of an eroded pipe and the validity of estimated results can be confirmed experimentally.     

Fatigue Life and Crack Growth Mechanisms of the Type 316LN Austenitic Stainless Steel in 310°C Deoxygenated Water

The low cycle fatigue tests of the type 316LN stainless steel were conducted to investigate the cracking mechanisms in high-temperature water. The fatigue lives of the specimens tested in 310°C deoxygenated water were considerably shorter than those tested in air. For the specimens tested in 310°C deoxygenated water, the evidences for the metal dissolution such as the stream downed feature, the blunt crack shape, and the wider crack opening were observed but rather weakly. In the same specimens, the evidences for the hydrogen-induced cracking such as the coalescence of microvoids and the decrease of the dislocation spacing at the crack tip were observed rather clearly. Therefore, it is thought that the hydrogen-induced cracking is mainly responsible for the reduction in the fatigue life of the type 316LN stainless steel in 310°C deoxygenated water while the effect of metal dissolution is less significant. The hydrogen-induced cracking is more pronounced in the slower strain rates. This behavior is in accordance with the larger reduction in the fatigue life at the slower strain rates. Furthermore, the fatigue life and the dislocation spacing show the minimum value in the strain rate range from 0.008 to 0.04%/s, which indicates the existence of the critical strain rate.     

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.     

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。     

直流电压降法测量核电结构材料在空气中的疲劳裂纹扩展速率

介绍了直流电压降（DCPD）方法测量材料裂纹扩展的原理,并采用DCPD方法测量了反应堆典型结构材料在空气中的疲劳裂纹扩展速率（CGR）,分析了载荷、频率和载荷比（R＝K_max/K_min, K为应力强度因子）对疲劳裂纹扩展速率的影响。实验结果表明,材料的硬度与疲劳裂纹扩展速率有密切关系,即材料硬度越高,裂纹扩展速率越高。     

Low cycle fatigue behaviors of elbow pipe with local wall thinning

Low cycle fatigue tests were conducted using 100A elbow specimens made of STPT410 carbon steel with local wall thinning. Local wall thinning by erosion/corrosion was simulated by machined pipe wall thinning. The local wall thinning areas were located at three different areas, called extrados, crown and intrados. The elbow specimens were subjected to cyclic in-plane bending under displacement control without internal pressure. The effects of eroded conditions, such as eroded ratio, eroded angle and position, on the low cycle fatigue behavior and fatigue life were discussed by using experimental results and finite element analyses. Also the location of crack initiation and the crack growth direction could be predicted by three dimensional elasto-plastic finite element analyses. In addition, the safety margin of eroded elbows against seismic loading was discussed by comparing the fictitious stress of elbows with the allowable stress limit demanded by the design code.     

Crack growth determination on laboratory components

The present work outlines the reasoning behind the selection of laboratory component tests for the validation of design and remanent life models governing crack growth behaviour. For the case of creep crack growth a ferritic and an austenitic alloy have been studied and a reference stress based solution used to successfully relate the stress rupture behaviour of internally and externally, axially and circumferentially notched, tubular components to base line creep data. Using the same reference stress based approach, it has been demonstrated that the notched component creep crack growth rates exhibit the same C^* dependence as conventional compact tension specimens. For 316L stainless steel components subjected to thermal fatigue conditions simulative of the fusion reactor first wall, a modified version of the superposition method of Buchalet and Bamford has been applied to estimate the stress intensity range as a function of crack length during the test. By this approach the crack growth rate dependency on stress intensity range for a variety of notch geometries is seen to be broadly in line with the conventional specimen mechanical fatigue data. Recent studies of crack growth under combined creep and thermal fatigue conditions are described and some early results are reported.     

Low cycle fatigue of irradiated and unirradiated Eurofer97 steel at 300 °C

Low cycle fatigue results are reported for unirradiated and irradiated reduced activation ferritic martensitic steel Eurofer97. The neutron irradiation experiment (irradiation at 300 °C to a nominal dose of 2.5 dpa) has been performed in the High Flux Reactor, Petten, the Netherlands. Post-irradiation low cycle fatigue tests have been performed in air at 300 °C at a total strain range of 0.6%, 1.0% and 1.4%. Neutron irradiation at 300 °C resulting in irradiation hardening is found to be beneficial for fatigue life at low strain amplitudes and to be adverse at high strain amplitudes. No effect of the different technological product forms on the fatigue life in Eurofer97 is observed, and fatigue behavior of Eurofer97 steel is found to be similar to that of F82H steel.     

Strain-time effects in low-cycle fatigue of nickel-base heat-resistant alloys at high temperature

A series of strain controlled low-cycle fatigue tests in the simulated high-temperature gas-cooled reactor (HTGR) helium environment were conducted at 900°C on Hastelloy X and its modified version, Hastelloy XR. In those tests the effects of strain rate and hold time on high-temperature low-cycle fatigue behavior were investigated. Decreasing the strain rate led to notable reductions in the fatigue life. In the tests with the trapezoidal strain waveform, the fatigue life was found to be reduced most effectively in tensile hold-time experiments. The tendency was interpreted through the feature of the crack morphology.     

温度及氢化物对Zr-Sn-Nb合金焊缝疲劳裂纹扩展行为的影响研究

采用小尺寸三点弯曲试样完成了渗氢和未渗氢Zr-Sn-Nb合金母材和焊缝在室温和360 ℃下的疲劳裂纹扩展速率试验,研究了温度和氢化物对焊接薄板的疲劳裂纹扩展行为的影响。结果表明,腐蚀吸氢后,在母材和焊缝区均析出了呈水平向分布的片状氢化物。相比母材区,焊缝区析出的氢化物更为致密。在相同温度下,未渗氢母材的抗疲劳裂纹扩展性能均优于未渗氢焊缝。腐蚀吸氢后,母材在相同温度下的抗疲劳裂纹扩展性能也优于焊缝。在室温下,腐蚀吸氢后的母材和焊缝的抗疲劳裂纹扩展性能相比吸氢前明显下降。360 ℃下,渗氢母材和焊缝中的氢化物部分溶解,使得其抗疲劳裂纹扩展性能得到一定程度提升。     

Estimation of crack growth in the creep range during plastic cyclic loading

Crack growth investigations were performed on the creep-resistant steel 13 CrMo 4 4 in the fatigue and the creep fatigue regime, especially regarding the influence of creep damage on crack growth. To this effect, 2% creep strain was applied to the material at a temperature of 560°C. The crack propagation rate was determined as a function of the specimen shape, temperature, test frequency and hold times. In the case of compact tension (CT-)specimens, creep pretreatment does not affect crack growth. For center-cracked tension (CCT-)specimens, however, the creep pretreatment results in a considerable increase in the crack propagation rate. Hold times of 90 minutes at maximum loading cause an increase in da/dN due to further cavity nucleation. The hold time at which cavity nucleation might occur is evaluated. The dependency on frequency of crack growth may be evaluated by means of a linear superposition of creep and fatigue crack growth. The transition frequency above which pure fatigue crack growth occurs is calculated and the regimes of fatigue, creep and creep—fatigue interaction with environmental influences are characterized.