Fatigue crack growth behavior in ultrafine grained low carbon steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase of R ratio. The R ratio effect on crack growth rates and ΔK_th was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.     

Creep-fatigue crack growth behavior in GH4169 superalloy

This study aims to examine the crack growth behavior of turbine disc GH4169 superalloy under creep-fatigue loading. Crack growth experiments were performed on compact tension specimens using trapezoidal waveform with dwell time at the maximum load at 650 °C. The crack growth rate of GH4169 superalloy significantly increased with dwell time. The grain boundaries oxidize during the dwell process, thereby inducing an intergranular creep-fatigue fracture mode. In addition, testing data under the same dwell time showed scattering at the crack growth rate. Consequently, a modified model based on the Saxena equation was proposed by introducing a distribution factor for the crack growth rate. Microstructural observation confirmed that the small grain size and high volume fraction of the d phase led to a fast creep-fatigue crack growth rate at 650 °C, thus indicating that two factors, namely, fine grain and presence of the d phase at the grain boundary, increased the amount of weakened interface at high temperature, in which intergranular cracks may form and propagate.     

Fatigue crack growth behavior of Mod.9Cr-1Mo steel at elevated temperatures: Effect of temperature,loading frequency and R ratio

The fatigue crack growth behavior of Mod.9Cr-1Mo steel was investigated as a function of temperature, loading frequency and R ratios in the Paris regime. The relationship between fatigue crack growth rate and stress intensity factor range was acquired for each test condition. The results revealed that crack growth rate was accelerated with increasing temperature and decreasing loading frequency. The influence of the R ratio on crack growth rate was only pronounced at the low loading frequency condition. In order to understand the crack growth mechanism, activation energy analysis and normalized ΔK analysis were performed. This study suggests that oxidation and the degradation of mechanical properties promote crack growth behavior.     

Discrepancies of fatigue crack growth behaviour of API X65 steel

This paper discusses the fatigue life behaviour of the API X65 steel tested under constant amplitude loading in ambient temperature. The influence of repeated loading at a particular stress ratio would favour the initiation of fatigue cracks that would consequently affect the fatigue crack growth. The main objective of this paper is to evaluate the load ratio effects on fatigue crack growth rate, taking into account their statistical characteristics. The fatigue crack growth test was carried out using compact-tension specimen at different load ratios of 0.1, 0.4 and 0.7. The experimental results showed the crack growth rate was dependent on the applied load. It was observed that the load ratio effect was less significant in stable crack growth regions. The result was consistent with the multiple regression test obtained by the least square method at a significance level of 0.05. The empirical model of Paris and Walker was utilised to evaluate the effects of load ratio on the fatigue crack growth rates. The approximation of fatigue life lies between 10-25 % of error using conservative model and 6 % error using the Walker model. The experimental data was scattered within a factor-of-2 correlation line suggesting that the accuracy of the experimental data towards the estimated values was high.     

不同应力比下U75V轨道钢疲劳裂纹扩展行为研究

对高速铁路常用轨道钢U75V进行了不同应力比下的疲劳裂纹扩展门槛值测试和疲劳裂纹扩展试验。试验结果表明,应力比对U75V轨道钢的裂纹扩展行为有着显著影响。利用分级降载法测得的疲劳裂纹扩展门槛值随应力比的增大而减小,裂纹扩展速率随应力比的增大而增大。对裂纹断口微观形貌进行了细致观察分析,发现整个疲劳断口分布有凸条纹棱线。疲劳门槛值附近出现沿晶体学平面的穿晶小刻面;低扩展速率区凸条纹棱线平行排列,棱线方向与珠光体片层方向一致;高扩展速率区的棱线出现弯曲及碎裂,同时伴有解理断面及二次裂纹。     

蠕墨铸铁疲劳特性的研究

对在四种热处理方法下的蠕墨铸铁试样进行门槛值和裂纹扩展速率的测试,并对其进行分析和比较.其结果表明,不同的基体组织对蠕墨铸铁的门槛值和低速区的裂纹扩展速率有比较明显的影响,门槛值随相对韧度增大而增大.不同的石墨形态对门槛值的变化影响不大,且仍然可以维持在较高的水平.不同载荷比下的低速区裂纹扩展速率区别较大,但随着平均载荷的升高,载荷比的影响有减小的趋势,中速区载荷比的影响较小.文中给出不同载荷比下的蠕墨铸铁门槛值和裂纹扩展速率经验公式.     

Effect of single overload on fatigue behavior in the welded haz of low alloy steel

Room temperature fatigue crack growth behavior was obtained for 4140 parent steel, parent heat treated (same as PWHT), as-welded HAZ and PWHT HAZ material under R≈O constant amplitude loading and single tensile overloads with an over load ratio (OLR: P_over/P_max) of 2.5. Double pass automatic submerged arc welding with AWS EM2 electrode was used. PWHT was performed at 650°C for one hour. Constant amplitude fatigue crack growth behavior was very similar for all four material conditions in the log-log linear Paris region. All material conditions responded favorably to the single tensile overloads with fatigue crack growth retardation ranging from 2.5×10⁵ to 4.5×10⁵ cycles which corresponded to life increases of 250 to 400 percent. SEM analysis indicated many similarities on the fatigue fracture surfaces with predominant ductile quasi-striation morphology.     

An experimental investigation into load relaxation in aluminium (HE30TB) and mild steel (EN1A)

Short-time room-temperature tensile load relaxation tests were conducted on aluminium alloy (HE30TB) and mild steel (EN1A) specimens using a closed-loop, electrohydraulic servo-controlled testing machine under strain control by means of an extensometer mounted directly on the parallel section of the tested specimens. The relaxation periods (usually 60 sec.) were interruptions at chosen points in constant strain rate tensile loadings. The loading strain rates in the different tests varied from 5 × 10⁻⁴ to 10⁻² sec. The transient relaxation behaviour was investigated for the purpose of testing the applicability of the most widely assumed viscoplastic constitutive models. This was achieved by comparing the plastic strain rate just after the beginning of load relaxation at constant total strain to the plastic strain rate during the tensile loading just before the start of the relaxation interval. All common viscoplastic theories predict that the plastic strain rate ratio should be unity. The experimental results for both materials indicate, however, that the plastic strain rate ratio varies from almost zero for relaxation periods early in the loading, to a maximum of around 0·2 for some relaxation periods beginning at relatively high loads and strains. This agrees with previously reported results on pure aluminium, which is not very rate sensitive, but the results for the more rate-sensitive mild steel may be surprising. Only if the actual relaxation rate drops by a factor of about 100 in 0.2 sec, could the findings of this experimental programme be reconciled with predictions of the usual viscoplastic theories. The experimental programme also included constant strain-rate tests at several rates and jump tests, in which the rate was switched back and forth between 10⁻⁴ and 10⁻²/sec.     

航空发动机涡轮盘用GH4133B合金疲劳裂纹扩展行为研究

材料的疲劳寿命由裂纹形成寿命和扩展寿命两部分组成。针对航空发动机涡轮盘用GH4133B合金,进行室温下不同应力比的疲劳裂纹扩展试验,测试疲劳裂纹扩展门槛值。Paris公式回归分析结果表明,裂纹扩展速率随应力强度因子和应力比的增大而增大,含门槛值的修正Paris公式能精确描述疲劳裂纹扩展行为。利用光学显微镜在线观测裂纹扩展路径,并利用扫描电镜考察试样断口微观形貌。结果发现,随应力强度因子增大,裂纹扩展路径由平直变得曲折。在疲劳裂纹萌生区、稳定扩展区和快速扩展区,断裂表面依次呈现为解理断裂、疲劳条带和沿晶韧窝混合断裂模式。基于断口反推理论反推载荷和裂纹扩展方程,结果表明,利用反推方程预测疲劳裂纹的扩展,可有效防范疲劳断裂的发生。     

The effects of artificial asperities on crack closing behavior in quenched and tempered medium carbon steel

To better understand the effect of an asperity on crack closure behavior, K-CMOD relations were examined using artificial asperity/wedge, inserted into the fatigue crack in a three point bending specimen made of a hardened medium carbon steel. Experimental results revealed that the unloading phase of the K vs. CMOD curve exhibited a concave shape if soft artificial asperity (Al alloy) was inserted, signifying acceleration in the CMOD decrease at zero applied load. This was mainly related to elastic and plastic deformation in the wedge material during the unloading process. On the contrary, the linear unloading portion of K vs. CMOD was obtained as hard asperity (high carbon steel) was employed, which specified deceleration in the CMOD decrease at zero applied load, where the only elastic deformation in the asperity was affected. From their unloading curves, the severity of crack closure or ??K _eff value was found to be related to the strength of the asperity material. The values of ??K _eff were examined in two different ways, e.g., (i) the remote displacement method and (ii) the adjusted compliance ratio method (ACR). The ??K _eff value, measured using both approaches, decreases with increasing wedge strength, such as hardness and yield strength. The rate of reduction in ??K _eff was, however, changed depending on the manner of ??K _eff examination, in which the ??K _eff decreased at a higher rate for the compliance ratio method and at a lower rate for the remote displacement method. The reason for this is discussed in the present work.     

Growth behavior of short surface fatigue cracks in 2 1/4 Cr-1 Mo steel

Fatigue tests by axial loading (R-0.05) were carried out to investigate short fatigue crack growth behavior in 2 1/4 Cr-1 Mo steel at room temperature using smooth and a small notched flat specimen. All the data of the fatigue crack growth rate in the present tests were analyzed as a function of the stress intensity factor equation in conjunction with crack closure behavior. Analysis was performed accounting for the relation of surface effective stress range,Ua and depth effective stress range,Ub. In the case of isotropic crack growth properties,Ub=(ΔK_ta/ΔK_tb) ·Ua. By use ofUb obtained from the analysis, crack growth rates to surface direction coincide with those of depth direction.     

Lubricant-Enhanced Fatigue Crack Growth in Silicon Nitride

Effect of a fluorinated lubricant on the subcritical crack growth behavior of a Si₃N₄ ceramic was examined under cyclic loading condition. The fatigue lifetime of the specimens tested in the oil was shorter, particularly in the low stress regime, compared to that in air. The growth rate of the surface crack was measured in the oil and compared with that in humid air. Crack growth rate in the oil was twice that in the air. XPS analysis of the fatigue fracture surfaces revealed that F had reacted with yttria-containing grain boundary phase, while the fractographical examination indicated that the oil had caused the dissolution of the grain boundary phase. The dissolution was shown to reduce the extent of crack bridging behind the crack tip, resulting in a higher fatigue crack growth rate in the oil.     

Corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe

In order to investigate the corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe, corrosion fatigue tests were performed under the various stress ratios and 3.5 wt% NaCl solution at room temperature. The corrosion fatigue characteristic curves were represented using crack closure concept. The obtained results are as follows ; when the load frequency is 1.0 Hz, the crack opening point is transited in the region of K_max=20-32 MPa-msu1/2. In the low stress intensity factor range, the crack opening point is higher than that in air. However, in the high stress intensity factor range, it is lower than that in air. In the cases of 0.1 Hz and 0.01 Hz, the crack opening point gradually decreases to K(min) with K_max increase.     

硫化氢环境中低周疲劳裂纹扩展速率的研究

针对高强钢在硫化氢环境中腐蚀疲劳数据极为缺乏的现状,研究高压气瓶材料4130X在硫化氢腐蚀介质中的疲劳裂纹扩展速率.结合气瓶实际运行的环境和应力状态,应用改进型WOL(wedge-opening-loading)试样,在自行改造的专用低周腐蚀疲劳试验机上,完成0.006 7 Hz超低频率下饱和H_2S溶液、中等浓度H2S溶液和空气三种环境下的腐蚀疲劳试验,并用Paris公式进行两段式拟合,得出da/dN-ΔK的数学表达式.将试样微观断口的变化与宏观应力强度因子K的变化进行对比研究,给出不同环境中三个阶段K值的定量结果.结果表明:相同条件下,H_2S环境中的疲劳裂纹扩展速率比空气环境中大20倍以上;但当H_2S浓度达到一定范围后,对da/dN影响并不按比例增长,浓度相差11倍时,da/dN相差2.4倍,H_2S腐蚀介质的存在加速了疲劳破坏.     

基于两步七点拟合法ADB610钢概率疲劳裂纹扩展研究

介绍了两步七点递增多项式拟合方法(简称两步七点拟合法)计算一系列相同裂纹长度下成组多个试样的裂纹扩展速率,然后计算一系列相同裂纹长度在不同存活率下的裂纹扩展速率P-a -da/dN和基于Paris公式的裂纹扩展速率表达式P-da/dN-△K.并采用两步七点拟合法计算了广泛应用于压力容器的ADB610钢7个CT试样在一系列相同裂纹长度下的裂纹扩展速率,以及计算了ADB610钢在存活率分别为50％,90％,95％,99％和99.9％的裂纹扩展速率和基于Paris公式的裂纹扩展速率表达式,有助于ADB610钢抗疲劳断裂可靠性的分析研究.     

Fractographic analysis of fatigue damage in 7000aluminium alloys

In this paper, an attempt is made to correlate the fatigue damage in 7000 aluminium alloys with different impurity contents to the microstructural features and to explain their interdependence through fractographic observations. The Paris constants of these alloys in the form of hot‐forged plates subjected to the overaged T73 temper are evaluated and differences in the fatigue crack growth rate described by striation spacing measurements. Scanning electron microscopy analysis of fatigue fracture surfaces revealed that the type and morphological parameters of coarse intermetallic particles play a critical role in fatigue crack growth behaviour. The elemental distribution determined by means of energy‐dispersive spectroscopy analysis showed that the fractured particles accelerating the crack advances are larger particles of Fe‐rich phases. The fatigue crack growth rate increases considerably with increasing amounts of these particles. The smaller η, S and Mg₂Si particles contribute beneficially to fatigue life.     

Effects of Aryl Phosphate Ester Lubricant Additives on Crack Growth in Soda-Lime Glass

The growth of cone cracks in soda-lime glass during static loading with a spherical indenter shows that crack growth is accelerated at low energy release rates (low crack velocity) by tricresyl phosphate. Other aryl phosphate ester lubricant additive mixtures that contain predominantly larger molecules, as well as a synthetic hydrocarbon oil formulated with one of the mixtures, do not show enhancement of crack growth. Although reactivity in a test with static load can predict reactivity in a cyclically loaded test, lack of reactivity in a static test may not predict lack of reactivity in a cyclic test. Comparative tests with neat hydrocarbon oil and formulated oil under the same cyclically loaded conditions are needed to definitively determine whether the additives that are unreactive in these static tests influence crack growth in a bearing.     

Fatigue Crack Growth Rate of Ti-6Al-4V Considering the Effects of Fracture Toughness and Crack Closure

Fatigue fracture is one of the main failure modes of Ti-6A1-4V alloy,fracture toughness and crack closure have strong effects on the fatigue crack growth(FCG)rate of Ti-6A1-4V alloy.The FCG rate of Ti-6A1-4V is investigated by using experimental and analytical methods.The effects of stress ratio,crack closure and fracture toughness on the FCG rate are studied and discussed.A modified prediction model of the FCG rate is proposed,and the relationship between the fracture toughness and the stress intensity factor(SIF)range is redefined by introducing a correcting coefficient.Notched plate fatigue tests(including the fracture toughness test and the FCG rate test)are conducted to investigate the influence of affecting factors on the FCG rate.Comparisons between the predicted results of the proposed model,the Paris model,the Walker model,the Sadananda model,and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region,and the corresponding calculated fatigue life is also accurate in the same regions.By considering the effects of fracture toughness and crack closure,the novel FCG rate prediction model not only improves the estimating accuracy,but also extends the adaptability of the FCG rate prediction model in engineering.     

Three-Dimensional Simulation of Rolling Contact Fatigue Crack Growth in UIC60 Rails

In this article, a UIC60 rail with accurate geometry is studied by employing the finite element method. For this purpose, a three-dimensional elastic-plastic finite element model is conducted using model. In addition, the stress distribution of wheel-rail operation is acquired, and its effects on fatigue life are specified by damage mechanic methods. In the finite element model, the displacements and stress intensity factors (SIFs) are computed on the crack near the leading edge to calculate crack propagation trajectories and crack growth rate. The modified Paris model is used to estimate fatigue crack growth rates.     

Effect of sigma phase on near-threshold fatigue crack growth behavior and ultrasonic evaluation in AISI 316L stainless steel

In this work, we examined the influence of microstructural changes, such as an intermetallic sigma (??) phase, on the fatigue behavior of high-temperature aged AISI 316L stainless steel. Nondestructive ultrasonic test and fatigue crack growth tests were performed to determine the threshold stress intensity factor of these artificially aged specimens. Ultrasonic test results characterizing the microstructural changes were compared with those of the fatigue tests to propose an empirical formula capable of predicting the threshold stress intensity factor by a nondestructive method. We observed a strong correlation between the increase in the volume fraction of the ?? phase and the decrease of ??K_th. Ultrasonic velocity increased in response to the coarsening behavior of the ?? phase in the vicinity of the grain boundaries.