高强度无取向电工钢疲劳性能及断裂机制

测试了高强无取向电工钢的S-N曲线,并借助光学显微镜、扫描电子显微镜、透射电子显微镜分析了实验钢组织,疲劳断口形貌和位错结构.结果 表明:室温条件下,频率为20 Hz,应力比R为0.1,循环107周次时,实验钢的疲劳强度为360 MPa,疲劳裂纹萌生于实验钢的次表面,裂纹萌生点附近有沿晶开裂现象,疲劳裂纹扩展区域有解理台阶与疲劳条纹,瞬间断裂区是韧性断裂,有大量韧窝.实验钢在循环应力作用下基体中产生了大量位错,并有驻留滑移带终止在晶界位置.     

二氧化硫复合盐雾环境下2024-T351铝合金应力腐蚀开裂

通过SO₂复合盐雾试验模拟工业污染海洋大气环境,结合有限元模拟分析、扫描电镜/能谱仪、光电子能谱分析等技术研究2024-T351铝合金在弹性应力区间的应力腐蚀开裂行为.结果表明:应力腐蚀开裂行为优先发生在2024-T351铝合金C型环的顶部应力集中区域;疏松的腐蚀产物层的形貌经历了由细棒状、团絮状到板块状的变化;试验6 h就可以监测到裂纹,进行到480 h的时候有贯穿裂纹形成,720 h的时候试样完全断裂;裂纹为穿晶和沿晶混合机制,主裂纹以穿晶机制沿C型环法线扩展,二次裂纹沿晶界扩展.      

梯度结构硬质合金的疲劳断裂

采用三点抗弯的方法研究WC-6Co梯度结构硬质合金和均质WC-6Co硬质合金的疲劳行为,探讨疲劳断口形貌与破坏机制的关系。结果表明:梯度结构硬质合金的疲劳裂纹在亚表面萌生;梯度结构硬质合金表层Co相发生明显塑性变形,WC相以沿晶断裂为主;中间层Co相变形也很明显,WC相解理断裂增加;内层Co相塑性变形很少,WC、η相以解理断裂为主;均质硬质合金Co相塑性变形明显,WC以沿晶、解理断裂为主,各部位断口形貌接近;梯度结构硬质合金的疲劳极限比均质硬质合金高约100 MPa;梯度结构硬质合金中疲劳裂纹沿垂直于试样下表面、平行于Co相梯度的方向形核,而均质硬质合金的疲劳裂纹沿平行于试样外表面方向形核。在应力集中效应、循环应力的作用下,Co相的马氏体相变是裂纹在亚表面萌生的主要原因;马氏体相变使Co相成为裂纹形核的快速通道,裂纹沿Co相梯度方向形核。     

TC4-DT钛合金高周疲劳行为研究

对TC4-DT钛合金的高周疲劳性能及断口形貌进行了研究。结果表明：TC4-DT钛合金的S-Ⅳ曲线（应力比R=-1）不出现呈水平线的疲劳极限,10^7次不被破坏的条件疲劳极限为550MPa,置信度为95％;疲劳裂纹源均出现在试样的表面,疲劳裂纹扩展区较大说明材料具有较高的断裂韧度;疲劳裂纹扩展区由许多解理小刻面组成,解理面上可见疲劳条带及二次裂纹,以解理断裂为主;断裂区断口表面由许多互相连接的凹坑所组成,主要表现为韧窝断裂。     

GH4169合金高温疲劳裂纹扩展的微观损伤机制

空气环境对高温合金在高温下的损伤行为有显著影响.为了研究标准热处理态GH4169合金在高温疲劳裂纹扩展过程中的微观损伤机制,在空气环境中进行650℃、初始应力强度因子幅ΔK=30 MPa·m1/2和应力比R=0.05的低周疲劳裂纹扩展试验.使用扫描电镜(SEM)及能谱(EDS)对试样的断口、外表面和剖面进行观察和分析.实验结果表明:疲劳主裂纹以沿晶方式萌生并扩展,随后沿晶二次裂纹出现,并且其数量和长度沿主裂纹方向逐渐增加,进入快速扩展阶段后,断口呈现韧窝组织形貌;在裂纹扩展过程中,δ相与基体的界面发生氧化,使得沿晶二次裂纹沿界面扩展并产生偏折,从而起到阻碍二次裂纹扩展的作用;试样外表面的主裂纹周围出现晶界氧化损伤区,其尺寸和晶界开裂程度沿主裂纹扩展方向逐渐增大.      

309S奥氏体耐热钢的高温性能研究

研究了309 S奥氏体耐热钢的高温瞬时强度、高温持久强度和高温疲劳性能.结果表明:随着变形温度从室温到1000℃,瞬时的屈服强度和抗拉强度显著降低,只有室温的14％和7％;根据应力与持久强度的关系,外推出持久时间1000 h时,800、900和1000℃的持久强度分别为37.98、12.63和7.27 MPa,高温变形断裂以沿晶方式进行;试验条件下900℃的疲劳极限为25 MPa,疲劳循环次数和裂纹扩展时间随着应力水平的增加而减少;疲劳裂纹萌生于试样表面,并以穿晶方式扩展.     

高铌TiAl合金的高温断裂韧性

利用紧凑拉伸试样通过预制疲劳裂纹研究近片层组织Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金和全片层组织Ti-45Al-7Nb-0.2W-0.2Hf-0.3B-0.15C合金在750℃下的断裂韧性,并分析两种组织合金的断口形貌.结果表明,近片层组织和全片层组织高铌TiAl合金750℃时的断裂韧性分别为19.54和31.58 MPa·m1/2,且近片层组织疲劳裂纹开始萌生时的最大疲劳载荷明显低于全片层组织.断口分析表明近片层组织中裂纹主要在等轴γ晶中萌生,裂纹扩展方式包括沿γ晶、穿γ晶及沿片层、穿片层;全片层组织中裂纹主要在垂直于加载方向的片层间萌生,裂纹以沿片层与穿片层的混合方式进行扩展且伴有二次裂纹的萌生.      

高温低循环疲劳条件下影响A286高温合金疲劳裂纹转变的因素

大气环境下 ,在 2 5～ 6 5 0℃的温度范围内对含 Nb的 A2 86高温合金进行了连续低温疲劳循环试验 ,研究了在试验温度和所施加应力的范围内疲劳裂纹的转变。由于高温下因晶界处 η相偏析而产生的晶界疲劳裂纹与低循环疲劳过程中施加的应力有关 ,所以对晶界处的偏析是否提供了晶界空穴点 ,从而导致低循环过程中发生晶界裂纹进行了研究。试验结果表明 ,在所施加应力的范围内 ,形成疲劳裂纹的机理发生了变化 ,从穿晶模式变成晶间模式。这种变化导致滑移带冲击到晶界上 ,使应力集中的点产生裂纹高温低循环疲劳条件下影响A286高温合金疲劳裂纹转…     

一种单晶高温合金不同温度的高周疲劳性能

研究了一种单晶高温合金700 ℃和800 ℃的高周疲劳性能，采用扫描电镜和透射电镜分析了断口和断裂机制.结果表明，随着温度升高，合金的疲劳强度系数降低，Basquin系数增加，高周疲劳极限降低.合金700 ℃与800 ℃具有相同的高周疲劳断口，都有几个{111}面平面组成，为类解理断裂机制.疲劳断口由裂纹源区、扩展区和瞬断区3部分组成.裂纹起源于试样的表面或亚表面，并沿{111}面扩展.扩展区可见河流状花样、滑移带、疲劳弧线和疲劳条带特征.瞬断区可见解理台阶和撕裂棱.断裂后γ′相仍保持立方形状，位错不均匀分布在γ基体通道中.      

铝合金焊接接头组织及疲劳损伤行为研究

为了加深对铝合金焊接接头的特征的了解,帮助铝合金材料实现更好的应用和优化,提出铝合金焊接接头组织及疲劳损伤行为研究。分别研究了其在宏观和微观上的组织特征,以此为基础,分析了其在焊接接头疲劳断裂模式和疲劳断口的特征。结果表明在熔敷金属与铝合金材料间具有较高的熔合度,气孔主要聚集在熔合线处,接头处铝合金在保持轧制纤维状组织结构同时,有不规则的结晶情况发生。低应力疲劳断裂模式下,焊缝处不会发生明显的塑性形变,在焊接接头的断口处,疲劳条带粗且明显,条带间距约为1.5μm,高应力疲劳断裂模式下,焊缝处形成明显裂纹,产生塑性形变,在焊接接头的断口处,疲劳条带细且密集,条带间距约为0.13μm。通过该研究,以期为提高铝合金性能的研究提供有价值的参考。     

Effect of aging treatments on the fatigue crack growth and threshold behavior of AA 2219 aluminium alloy

The fatigue crack growth and threshold behavior of AA2219 Al-alloy has been examined for naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. Significant differences were observed in the fatigue crack growth at low and high stress intensity ranges. The alloy in the NA condition possesses the highest resistance to fatigue crack initiation which can be viewed from the values of achieved threshold stress intensity range, ΔK _TH . In the intermediate ΔK (Paris law) regime, all the four materials fall into a single band. In the high ΔK regime (> 20 MPa√m) PA condition exhibits better crack growth resistance than under aged conditions. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low ΔK. The fracture morphology changes from crystallographic facets in the threshold region to clearly developed ductile striations in the Paris law regime to microvoid coalescence in the high ΔK region.     

Study of fatigue crack propagation behaviour for dual-phase X80 pipeline steel

Load-controlled fatigue tests were conducted on dual-phase X80 pipeline steel to investigate the effects of stress ratio (R-ratio) on the fatigue crack growth behaviour. Dual-phase X80 pipeline steel showed a non-linear relationship between fatigue crack growth rate (da/dN) and the stress intensity factor range (ΔK) at each R-ratio. Fatigue crack propagation curves of X80 pipeline steel were evaluated using the conventional Paris equation and a new exponential equation named αβ model. In addition, the electron back-scattered diffraction technique was used to study the effects of stress ratio on the fatigue crack growth behaviour. The results indicated that the corresponding ΔK of the transition point decreased with the increase of R-ratio. That was attributed to the variation of the crack path and the fracture mode because of the changes in the size of monotonic plastic zone and cyclic plastic zone at crack tip. Compared to the overall fitting, piecewise fitting by Paris equation and αβ model, piecewise fitting was the most accurate method, and αβ model is more convenient and efficient than the conventional Paris-based equations.     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.     

Effects of sic content on fatigue crack growth in

An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant K_max control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.     

Effect of strontium modification on near- threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast com-mercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the mi-crostructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1. Formerly Research Project Engineer, Briggs & Stratton Corporation, Milwaukee, WI 53222     

Fatigue crack micromechanisms in ingot and powder metallurgy 7xxx aluminum alloys in air and vacuum

Fractographic analysis of fatigue crack growth in 7075-T651 ingot and MA-87 powder metallurgy aluminum alloys is combined with high resolution, dynamic observation of crack tip deformation and opening. It is found that below the Paris regime, in both moist air and vacuum, crack growth does not proceed cycle-by-cycle, but is incremental. Crack extension is a two-stage process which produces a striation, but only after a number of cycles during which the crack blunts and accumulates local strain damage near the tip, followed by an increment of “brittle” crack extension. The effect of moist air is to reduce the plasticity required for an increment of crack extension, hence the number of blunting cycles preceding crack extension. Implications for analytical crack growth models based on crack tip micromechanics are discussed.     

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute     

An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys

Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased ?K_th values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions.     

Fatigue Crack Propagation in Friction Stir Welded and Parent AA6082

The fatigue crack propagation characteristics of a friction stir welded Al‐Mg‐Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity δK the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R‐dependence, Forman's law was used.