疲劳短裂纹特性研究

以缺口件为研究对象,对缺口根部短裂纹的萌生与扩展特性进行了研究.结果发现,缺口根部的表面裂纹扩展,是造成缺口试样破坏的主要原因,其扩展特性强烈地受微观结构的影响,而穿透裂纹的早期扩展,却明显地具有奇异性.     

疲劳裂纹萌生尺寸确定方法研究

以缺口件为研究对象 ,根据J积分理论 ,对Dowling法进行了改进 ,并根据疲劳非扩展裂纹的性质 ,给出裂纹萌生尺寸的定义 ,介绍疲劳裂纹萌生尺寸的确定方法     

粉末冶金构件疲劳裂纹的扩展寿命

 针对航空发动机粉末冶金旋转盘件的疲劳裂纹扩展特性,以标准紧凑拉伸试样裂纹扩展数据为基础,基于Paris公式获得了材料典型温度下的裂纹扩展参数,并利用简单件对所采用的裂纹扩展计算方法进行了验证。进行了粉末冶金旋转构件的裂纹扩展试验,继而进行断口分析,通过测量疲劳条带获得了裂纹扩展寿命。同时,采用有限元方法计算出粉末冶金构件的裂纹扩展寿命,与实测值吻合较好,验证了裂纹扩展寿命分析方法的有效性及实用性。     

复杂应力状态2.25Cr-1Mo钢的高温低周疲劳裂纹扩展规律

用带预裂纹的缺口试件研究2.25Cr-1Mo钢高温低周疲劳裂纹扩展规律,通过疲劳试验观察裂纹扩展寿命,应用ANSYS计算裂纹尖端应力应变分量和当量弹、塑性应变范围,利用当量J积分范围表征2.25Cr-1Mo钢在复杂应力状态下的低周疲劳裂纹扩展速率.结果表明:疲劳裂纹扩展速率da/dN与当量J积分范围△Jf的关系不受试件缺口型式和加载应变范围的影响,用当量J积分来评价2.25Cr-1Mo钢的高温低周疲劳裂纹扩展规律是有效的.     

基于晶体相场法研究缺口形貌对微裂纹扩展连通的影响

采用晶体相场方法研究金属微互连结构界面微裂纹在两种不同初始缺口区域形貌情况下的扩展连通过程。研究表明：微裂纹在两种初始缺口区域形貌下呈“树枝状”扩展;扩展过程中伴随着二次微裂纹和三次微裂纹的生长;微裂纹扩展面积占总面积的百分比随着演化时间的增加逐渐增大,主裂纹扩展面积占总扩展面积的百分比随着演化时间的增加而增大;初始缺口区域形貌为正方形的微裂纹扩展面积占总面积的百分比和主裂纹扩展面积占总扩展面积的百分比均高于初始缺口区域形貌为矩形的微裂纹扩展;初始缺口区域形貌的改变并不影响原子间距和原子波动周期。     

船体结构钢低周疲劳表面裂纹扩展规律

采用悬臂弯曲加载方式,以总应变范围（ΔεＴ）作为受检参数和控制参数,对船体结构钢低周疲劳表面裂纹扩展行为进行了研究,得到表面裂纹扩展速率ｄ（２ｃ）／ｄＮ与ΔεＴ的关系,提出了结构疲劳寿命指标的估算方法和表面裂纹低周疲劳扩展机制。     

疲劳裂纹的闭合效应

疲劳裂纹的闭合现象改变了传统的疲劳裂纹扩展理论，虽然它的迟滞机制并没有引起材料疲劳断裂抗力任何本质的改善，而仅仅提高了表观的或非本质的失效抗力，但只要很好地理解并应用它，将会给准确地设计工程寿命提供很大的帮助。     

Ⅰ—Ⅲ复合型临界裂纹长度及剩余寿命的估算

以考虑裂纹面扭转的应力强度因子代替不考虑纹面扭转的应力强度因子,在小范围了条件下的最大应变准则的基础上,估算了Ⅰ－Ⅲ复合型裂纹的临界裂工度,从而估算了Ⅰ－Ⅲ复合型裂纹的剩余寿命,试验表明,此方法可以应用到工程上。     

Q345R钢中微缺陷的裂纹萌生与扩展

 对压力容器与压力管道用钢Q345R在低周疲劳下微孔（[?]40～200 μm）的裂纹萌生与扩展规律进行了研究。研究表明小裂纹的萌生主要机理为滑移带启裂,并且由剪应力起主导作用。微缺陷的尺寸、应力幅等因素对疲劳寿命均有影响显著,当应力幅值较低时,微孔的尺寸对疲劳寿命有明显影响。当应力幅值水平较高时,小孔直径对疲劳寿命的影响则不敏感。微观缺陷尺寸存在临界值,当缺陷尺寸大于临界值时,疲劳寿命下降很快。在同一应力幅水平下,裂纹萌生寿命与疲劳总寿命的比值[（Nt/Nf）]与微孔尺寸没有关系,本试验的低周疲劳下约为10%～25%。     

柔度法测试车轮轮箍的疲劳裂纹萌生寿命

柔度法测试材料的疲劳裂纹萌生寿命，方法简单，数据可靠。     

Vertical Short Crack Initiation in Medium Carbon Bainitic Steel Under Mild Tractive Rolling Contact

To improve the current grinding procedure of the back-up roll of CVC hot rolling mills so that the back-up roll service life can be extended, the crack initiation and propagation behavior of medium carbon bainitic back-up roll steel was investigated, a kind of asperity-scale, surface originated vertical short cracks occurred at 5 × 10^2 -1 × 10^4 cycles. Theoretical analysis indicated that the maximum tensile stress occurring at the back edge of the contact of asperities keeps at above 1 347. 97 MPa, and ratcheting and cyclic plastic deformation take place at such sites within 1 × 10^4 cycles. The early initiation of the vertical short cracks is caused by the asperity contact. According to the crack initiation mechanism, short crack behavior and preventive grinding strategy, steel consumption can be reduced considerably by decreasing the surface roughness and removing the asperity influenced surface thin layer at about 70%-80% of the surface distress life.     

Simulation of Microstructurally Controlled Short Crack Propagation

A model for the simulation of stage I fatigue crack propagation is presented. The model considers the barrier effect of grain boundaries and phase boundaries on short crack propagation rate and the crack path deflection due to the microstructure. The plastic zones in front of the crack tip are modelled as yield strips. The mechanical boundary value problem is solved numerically by a boundary element method. The method was applied based on a statistical model of duplex microstructures, generated by a special Voronoi‐algorithm, so that the influence of the distribution of phases on crack propagation can be analysed.     

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

Tension-compression fatigue tests were conducted on forged (F-950) and rolled (R-950) Ti-6Al-4V alloys. The role of basal texture, produced by forging, was compared with prismatic texture, produced by rolling, on microstructurally short fatigue crack growth initiation and growth resistance. The fatigue life of R-950 alloy proved to be higher than that of F-950 alloy. Major differences in fatigue crack growth rate were detected in the short fatigue crack region at crack lengths of below 100 µm (F-950 alloy showing low resistance). To quantify the effect of texture alone, the opening/closure behaviour of a microstructurally small crack was analysed from the data obtained by combining an automatic in situ observation system with a digital image correlation technique. This analysis showed the crack opening stress of both alloys to be close to identical in the short fatigue crack growth region, nullifying the effect of crack closure. To study the effect of texture, cross-sectional electron backscatter diffraction analysis was performed at crack initiation sites. The results revealed that in F-950 alloy, short fatigue crack growth proceeded along basal planes that had similarly-oriented grains, whereas in R-950 alloy, short fatigue crack growth did not follow prismatic planes: the orientation varied.

     

Prediction of Fatigue Lifetime under Multilevel Cyclic Loading Based on a Short Crack Growth Model in a Low Carbon Steel SAE 1017

This contribution deals with the initiation and growth of short fatigue cracks in cyclic loaded notched specimens made of the 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed, which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical crack is regarded as damage. This approach allows to calculate fatigue life for constant amplitude tests as well as for multilevel tests and irregular loading. Deviations from Miner's rule, which have been often observed for two‐level tests, are attributed to the varying fraction of crack initiation and propagation phase for different loads. The inaccuracy of Miner's rule deduced from two‐level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elemental version of Miner's rule.     

Crack Resistance of Modern Ceramics and Ceramic Composites. Part 1. SEVNB-Method

The crack resistance of monolithic and single-crystal ceramics, as well as laminar and normal composites based on them, are studied by the SEVNB method (bending a specimen with a V-shaped notch) over a wide temperature range. Manual and mechanical procedures for preparing a V-shaped notch and also test procedures for three- and four-point bending are described. Ceramic deformation curves at different temperatures are studied. Results of testing by the methods of SENB (specimen bending with a blunt notch) and SEPB (specimen bending with a sharp crack) as well as micro-Raman spectroscopy data are analyzed. It is established that the ratio of crack resistance values according to the SEPB and SENB methods is about 0.6 only for linearly-elastic monolithic and normal composite ceramics and it exceeds 0.9 for the same inelastic ceramic. Polishing a V-notch does not give rise to a tetragonal-monoclinic phase transition in zirconium dioxide ceramic.     

Crack path in single crystals

A two-dimisnsional analysis of crack propagation in anisotropic single crystal has revealed that the cleavage path depends on the shape of the γ-plot. Consequently, for a given notch plane the fracture path has been predicted to depend on the notch front orientation. The results of this study suggest that the fracture path depends not only on the level of resistance to fracture but also on the change of fracture energy with orientation. Furthermore, when a crack kinks into a minimum energy plane, it does not necessarily exhibit a lower fracture toughness than the case where crack follows a higher energy plane.     

Effect of Environment on Creep Crack Growth in PM/HIP René-95

The creep crack growth rates (CCGR) of PM/HIP René-95 were measured from 10^-9 m per second to 10^-4 m per second in air and in high purity argon at 760°C and 650°C using single edge notched (SEN) specimens. The crack length was monitored by the D.C. potential difference technique. The data were reported asda/dt vs the elastic stress intensity factor,K,, since PM/HIP René-95 is a creep-brittle material. The CCGR were shown to be strongly environment sensitive. The CCGR were up to 1000 times faster in air than in argon for a given value ofK _I . The temperature andK _I dependence of CCGR in air were shown to correlate with a modified formulation of the Larson-Miller parameter. Notched stress rupture (NSR) tests were performed at 650°C in air in order to study the effect of notch root radius on the time to initiate a creep crack. A comparison of the NSR data with SEN data for the same values of initialK, shows that the crack initiation times are a strong function of the notch root radius. It was observed that ninety percent of the rupture time is spent in crack initiation when the notch root radius is finite, while no incubation time was observed for creep crack growth from fatigue precracked specimens. Formerly with Massachusetts Institute of Technology, Cambridge, MA     

复杂应力状态225Cr 1Mo钢的高温低周疲劳裂纹扩展规律

 用带预裂纹的缺口试件研究225Cr 1Mo 钢高温低周疲劳裂纹扩展规律，通过疲劳试验观察裂纹扩展寿命，应用ANSYS计算裂纹尖端应力应变分量和当量弹、塑性应变范围，利用当量J积分范围表征225Cr 1Mo 钢在复杂应力状态下的低周疲劳裂纹扩展速率。结果表明：疲劳裂纹扩展速率da/dN与当量J积分范围ΔJf的关系不受试件缺口型式和加载应变范围的影响，用当量J积分来评价225Cr 1Mo 钢的高温低周疲劳裂纹扩展规律是有效的。     

Theoretical Considerations on Corrosion Fatigue Crack Initiation

Based on corrosion kinetics and fracture mechanics, it has been possible to determine quantitatively the corrosion fatigue life for three different types of corrosion behavior. Under general, active corrosion, corrosion fatigue life is controlled by the corrosion rate and the applied alternating stress range. If pitting corrosion occurs, corrosion fatigue life depends on the incubation time for nucleating a pit, the pit growth kinetics, and a critical pit depth, which is a function of the applied stress range. It has been assumed that under passive corrosion conditions, the passive layer has to be penetrated by slip steps, to form corrosion fatigue cracks. The corrosion fatigue crack initiation in this case is controlled by the repassivation kinetics of the material and also by a critical notch depth, depending on the applied stress range. It has been found that a critical current density exists, below which no corrosion fatigue cracks can initiate. Comparison of the theoretically calculated life times with experimental results showed a quite good correlation. M. MüLLER, formerly Research Engineer with Brown Boveri Research Center, Baden, Switzerland.