一种Al-Cu-Mg-Zr合金的疲劳裂纹扩展行为研究

研究热处理状态、应力比和试样取向等对含Zr的Al-Cu-Mg系铝合金疲劳裂纹扩展行为的影响.结果表明,T3、T4两种热处理状态试样的疲劳裂纹扩展速率没有明显差别;L-T方向和T-L方向的试样也有大致相同的疲劳裂纹扩展速率;应力比R对疲劳裂纹扩展行为有明显影响,高应力比下的疲劳裂纹扩展速率明显快于低应力比条件下的扩展速率,并且高应力比下的疲劳裂纹扩展在较小的△K值下进入快速扩展阶段,并很快断裂.在较低的△K水平下应力比的影响与裂纹闭合效应有关.     

Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint

对AZ31B 镁合金焊接接头和热影响区的疲劳裂纹扩展行为进行研究,分析了焊接接头[L-T(W)]和热影响区的紧凑拉伸试验[C(T)],其中热影响区的C(T)试验包括焊缝平行于挤压方向[T-L(H)] 和垂直于挤压方向 [L-T(H)]两种。结果表明:对于L-T(W) 试样,裂纹沿挤压方向扩展,裂纹扩展经历先快后慢的扩展过程;T-L(H) 试样裂纹平行于缺口方向扩展,L-T(H)试样裂纹为平行于缺口方向和与缺口成一定角度两种扩展方向,裂纹扩展经历先慢后快的扩展过程。裂纹尖端扩展为穿晶和沿晶的混合模式,疲劳断口为准解理特征的脆性断口。     

2050铝合金的疲劳裂纹扩展行为研究

研究取样方向、应力比以及微观组织对2050铝合金疲劳裂纹扩展速率的影响。结果表明:取样方向和应力比对疲劳裂纹扩展速率在近门槛区和快速扩展区的影响显著,而在稳态扩展区的影响则相对较小;L-T取样方向的速率最低,T-L取样方向的速率次之,S-L取样方向的速率最高,这主要与合金的晶粒取向、织构取向和第二相粒子取向有关;应力比为0.5的裂纹扩展速率高于应力比为0.1的速率,这在近门槛区主要与裂纹闭合效应有关,在快速扩展区主要受最大应力场强度因子Kmax的影响。     

The effect of roll-processing orientation on stress corrosion cracking of warm-rolled 304L stainless steel in oxygenated and deoxygenated high temperature pure water

Stress corrosion crack growth rates of 304L stainless steel after one-directionally warm-rolled along the longitudinal (L) direction were measured in oxygenated and deoxygenated pure water at 288 °C. The fracture mode is mainly intergranular in the Transverse-longitudinal (T-L) orientation specimen, while it is occasionally intergranular in the Longitudinal-transverse (L-T) orientation specimen. The crack growth rates in the T-L orientation are higher than those in the L-T orientation in both oxygenated and deoxygenated environments, which is related to the effective cracking growth path as the result of microstructural anisotropy and applied loading direction.     

Determination of Some Parameters for Fatigue Life in Welded Joints Using Fracture Mechanics Method

In this work, the parameters stress intensity factor (SIF), initial and final crack lengths (a _i and a _f), crack growth parameters (C and m), and fatigue strength (FAT) are investigated. The determination of initial crack length seems to be the most serious factor in fatigue life and strength calculations for welded joints. A fracture mechanics approach was used in these calculations based on SIF which was calculated with the finite element method (FEM). The weld toe crack was determined to be equal to 0.1 mm, whereas the weld root crack’s length was varied depending on the degree of the weld penetration. These initial crack length values are applicable for all types of joints which have the same crack phenomenon. As based on the above calculated parameters, the new limits of FAT for new geometries which are not listed yet in recommendations can be calculated according to the current approach.     

00Cr17Ni14Mo2钢在仿人体液中电化学特性与疲劳行为研究

对００Ｃｒ１７Ｎｉ１４Ｍｏ２不锈钢（３１６Ｌ钢）在Ｈａｎｋ’ｓ溶液中，不同外加电位下疲劳前期电化学特性和疲劳裂纹萌生间的关系进行了研究，表明疲劳过程改变了试样表面形貌与性质，提高了试样电化学活性。静态与疲劳过程中测得的极化曲线有较大的不同。通过瞬时电流曲线可以比较不同电位下疲劳裂纹萌生及长大情况。     

Kinetics of γ’ precipitation and its influence on fatigue crack growth behavior of a new single-crystal nickel- based superalloy (cmsx-4g) at room temperature

An investigation was carried out to determine the growth kinetics of γ precipitates in a newly developed single-crystal nickel-base superalloy containing rhenium (CMSX- 4G). The investigation also examined the influence of γ’ precipitates (size and distribution) on fatigue crack growth behavior of the material in a room-temperature ambient atmosphere. The influence of load ratio on fatigue threshold of the material and crack growth mechanisms in fatigue was also studied. Compact tension specimens were prepared from a single-crystal nickel-base superalloy, CMSX-4G, with the (001) crystallographic direction. These specimens were given two different heat treatments to produce two different γ’ size precipitates. Fatigue crack growth behavior of these materials was studied at three different load ratios (R = 0.10, 0.50, and 0.90) in room-temperature ambient atmosphere. The results of the present investigation demonstrate that rhenium additions in CMSX-4G substantially lowers the γ coarsening kinetics of this alloy. The smaller γ’ precipitate size was found to be beneficial for fatigue resistance and has resulted in a higher fatigue threshold and lower fatigue crack growth rate in the threshold region. The fatigue threshold was found to decrease with an increase in load ratio. The crack growth mechanism in the threshold region was found to occur by a combination of microvoid coalescence and striations.     

Relationship between banding and susceptibility to environment-assisted cracking in a quenched and tempered low alloy steel

The effect of a banded microstructure on sustained-load environment-assisted cracking in D6ac steel has been investigated. In material quenched and tempered at 290°C, sub-critical crack propagation is predominantly intergranular and independent of the banded dendritic segregation. After tempering for 2 h at 550°C, sub-critical crack propagation, while still intergranular, is now confined to the solute-rich bands. This anisotropic cracking behaviour is reflected in a ten-fold greater crack-growth rate and a 50% lower K_Th for the S-L orientation compared with the T-L orientation. At longer tempering times (200 h), however, susceptibility to intergranular sub-critical cracking in the T-L orientation is eliminated resulting in a hundred-fold reduction in crack-growth rate and a 100% increase in K_Th.The different tendencies for the solute-rich and solute-depleted bands to crack intergranularly in the 550°C temper steel are explained in terms of the driving force present within these bands to induce segregation of solute and impurity elements to grain boundaries.     

Effects of stress ratio on fatigue crack propagation properties of submicron-thick free-standing copper films

The dominant mechanics and mechanisms of fatigue crack propagation in ca. 500 nm thick free-standing copper films were evaluated at the submicron level using fatigue crack propagation experiments at three stress ratios, R = 0.1, 0.5 and 0.8. Fatigue cracking initiated at the notch root and propagated stably under cyclic loading. The fatigue crack propagation rate (da/dN) vs. stress intensity factor range (ΔK) relation was dependent on the stress ratio R;da/dN, increases with increasing R. Plots of da/dN vs. the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max ? 4.5 MPa m^1/2) irrespective of R, indicating that K_max is the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode irrespective of the R value. The region ahead of the fatigue crack tip is plastically stretched by tensile deformation, causing necking deformation in the thickness direction and consequent chisel-point fracture. In contrast, in the low-K_max region (K_max < 4.5 MPa m^1/2), the da/dN vs. K_max function assumes higher values with decreasing R; in this region, the fracture mechanism depends on R. At the higher R value (R = 0.8), the fatigue crack propagates in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appears: within several grains, intrusions/extrusions form ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagates preferentially through the intrusions/extrusions.     

金属/金属层合板疲劳性能研究

以１４２０Ａｌ－Ｌｉ合金。ＬＹ１２爆炸层合板和ＬＦ２１热轧层合板为研究对象。考究了不同结合强度界面对Ｌ－Ｔ取向疲劳裂纹扩展的影响。结果表明：随界面结合强度的减弱分层越来越剧烈。其止裂韧化效果越来越明显。三种实验层合板材的疲劳裂纹扩展都出现了止裂现象。但其机制有所不同。Ａｌ－Ｌｉ合金的止裂主要来自事机制。爆炸层合板和热忆合层板的止裂主要来自分层机制。     

激光冲击TC17钛合金疲劳裂纹扩展试验

为研究激光冲击强化对钛合金试件疲劳性能的影响,在标准试件的裂纹扩展路径上设计了全强化和间隔强化两种不同的强化方案,研究激光冲击强化对试件疲劳寿命和裂纹稳定扩展时速率的影响规律,利用有限元数值模拟和X射线残余应力测试获得了试件的残余应力场分布状态,并对比分析了试件的断口形貌和微观组织特征。结果表明:相比于未强化试件,激光冲击强化后试件的平均疲劳寿命分别提高了2.14倍和1.90倍,两种不同的冲击强化方法分别降低钛合金试件的裂纹扩展速率24%和15%。间隔强化后试件表面产生-512 MPa的最大残余压应力,裂纹扩展的C′值为-7.3,m值为2.6,而强化间隔区引入最大值为82.4 MPa的残余拉应力,裂纹扩展速率急剧升高,C′值减小至-13.6,m值为8.0。当裂纹扩展到强化区时,扩展速率再次降低,激光冲击强化对TC17钛合金疲劳裂纹扩展有显著的抑制作用。     

High-cycle fatigue crack initiation and growth in TIMETAL LCB

This investigation has examined the influence of grain boundary α contiguity on the high-cycle fatigue behavior of aged TIMETAL LCB, with the fatigue performance being evaluated under tension-tension loading conditions atR=0.1 in laboratory air and 25 Hz. Fractographic analysis indicated that fatigue initiation, independent of processing history, involved subsurface crack formation. Serial section studies also indicated that crack initiation occurred at the interface between the triple-point α and the aged β matrix. Further back-scattered electron microscopy examination of the aged microstructures indicates that the observed differences in high-cycle fatigue behavior can be understood by considering the effect of processing history on the connectivity of grain boundary α, with decreased connectivity being associated with enhanced fatigue performance. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held during the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

ADB610钢焊接接头概率疲劳裂纹扩展分析

采用焊条电弧焊方法对ADB610钢进行焊接,对焊接接头进行疲劳裂纹扩展试验,研究其概率疲劳裂纹扩展规律.采用两步七点递增多项式拟合方法,计算了一系列相同裂纹长度下母材区、热影响区和焊缝区的对数裂纹扩展速率方差和存活率分别为50%,90%,95%,99%,99.9%的裂纹扩展速率.结果表明,总体上母材区的裂纹扩展分散性最小,热影响区其次,焊缝区分散性最大;而且总体上存活率相同时,在裂纹扩展的初期,母材区的裂纹扩展速率最快,焊缝区的裂纹扩展速率最慢;在裂纹扩展末期,三区域的裂纹扩展快慢相当;随着存活率增大,三区域裂纹扩展快慢差别不大时所对应的裂纹长度减小.     

The role of environmental exposure in the fatigue behaviour of an aluminium alloy

This paper reports measurements of fatigue crack velocities for an aluminium alloy 7017-T651 in high purity gaseous environments with different values of water vapour pressure and frequency. Also a detailed fractographical analysis by SEM is presented and the fracture surface morphology is correlated with the testing parameters and with the crack stress intensity level. Within a range of environmental exposure (defined as the product between the pressure and the time available for surface reaction during one fatigue cycle) crack propagation rates depend linearly on the exposure. The experimental fatigue crack propagation data are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments.     

Quasi-dynamic visualization of crack propagation and bridging evolution in Si3N4 and SiC ceramics by mechano-luminescence

The propagations of macro-scale cracks were visualized in Si₃N₄ and SiC ceramics using a mechano-luminescence (ML) of SrAl₂O₄:Eu,Dy. The bridging zones behind the crack tip were also clearly detected in the crack path within realistic time frames. The ML made it possible to detect cracks propagating within a speed range of 200 m/s to 250 m/s, thereby realizing so-called quasi-dynamic R-curves. The magnitudes and shapes of the bridging stress distributions were found to change with the advancing cracks, continuing to change as the applied load changes, even after the cessation of crack propagation. The effective toughening then commenced and the applied stress intensity factors increased dramatically up to ∼120 MPa√m in Si₃N₄ and 150 MPa√m in SiC. The K_Tip values expected on the basis of the instantaneous bridging stress distributions obtained from ML observations deviated greatly from those measured using the conventional crack tip lengths; rather, the values support the results obtained by using bridging tips in quasi-dynamic crack propagations.     

Prediction of fatigue crack propagation life in notched members under variable amplitude loading

One of the interesting phenomenon in the study of fatigue crack propagation under variable amplitude load cycling is the crack growth retardation that normally occurs due to the application of a periodic overload. Fatigue crack growth rate under simple variable amplitude loading sequence incorporating period overloads is studied using single edge notched specimens of AISI304 stainless steel. Load interaction effects due to single and multiple overload have been addressed. Substantial retardation of fatigue crack growth rate is observed due to the introduction of periodic tensile overloads. Estimates of fatigue life have been obtained employing Wheeler model (using Paris and modified Paris equations) and Elber’s model. Analytical predictions are compared with experimental results. Results of these analytical fatigue life predictions show good agreement with the experimental fatigue life data. Fatigue crack propagation rates also have been evaluated from the fractographic study of fatigue striations seen on the fracture surface. Good agreement was found between the experimentally observed crack growth rates and the fatigue crack growth rates determined by the fractographic studies.     

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.     

Influence of retained austenite on short fatigue crack growth and wear resistance of case carburized steel

The influence of the amount of retained austenite on short fatigue crack growth and wear resistance in carburized SAE 8620 steel was studied in this article. Different amounts of retained austenite in the microstructure of the carburized case were obtained through different heat treatment routes applied after the carburizing process. The wear tests were carried out using pin on disk equipment. After every 200 turns the weight loss was registered. Four point bend fatigue tests were carried out at room temperature, using three different levels of stress and R=0.1. Crack length versus number of cycles and crack growth rate versus mean crack length curves were analyzed. In both tests the results showed that the test pieces with higher levels of retained austenite in the carburized case exhibited longer fatigue life and better wear resistance.     

Fatigue crack propagation behavior in dual-phase steel

The fatigue crack propagation in dual-phase steel was studied with the objective of developing ferriticmartensitic microstructures via intercritical annealing and thermomechanical processing. It was found that the changes in fatigue crack propagation rates and in the threshold stress intensity range, ΔK _th, resulting from microstructural variations, were directly related to tensile strength in the same manner that was observed in other types of structural steels. It was also observed that the relationship between tensile strength and fatigue crack propagation in intercritically annealed and thermomechanically processed dual-phase steel was much the same as for conventional steels of similar strength level.     

Fatigue crack growth rate in underwater wet welds: out of water evaluation

The characteristics of fatigue crack propagation in welds produced in conventional conditions (welds made out of water) and in underwater repair welding conditions (underwater wet welding) were evaluated out of water. The fatigue crack growth rates da/dN showed considerable dependence on pore density and distribution, factors that vary significantly with the welding process and the environment. Variations in the fatigue crack propagation rate were correlated with analysis of the fracture surface in the zone of stable crack propagation. The results of this study show that the underwater wet welding procedure produces weld metal resistant to fatigue, which is suitable for use at low applied stresses in offshore structures.