Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

Experimental and numerical determination of critical stress intensity factor of aluminum curved thin sheets under tensile stress

We determined the fracture toughness of aluminum curved thin sheets using tensile stress tests and finite element method. We applied Linear elastic fracture mechanics (LEFM) and Feddersen procedure to evaluate stress intensity factor of the samples with central wire-cut cracks and fatigue cracks with different lengths to investigate the notch radius effect. Special fixture design was utilized to establish uniform stress distribution at the crack zone. Less than 9 % difference was found between the wire-cut and the fatigue cracked samples. Since generating central fatigue crack with different lengths required so much effort, wire-cut cracked samples were used to determine critical stress intensity factor. Finite element analysis was also performed on one-quarter of the specimen using both the singular Borsum elements and the regular isoparametric elements to further investigate fracture toughness of the samples. It was observed that the singular elements presented better results than the isoparametric ones. A slight difference was also found between the results obtained from finite element method using singular elements and the experimental results.     

Stress and fracture analyses of nuclear power plant LP turbine rotor discs

Fracture phenomenon has been reported on blades, rotors, connections and rotor discs of LP turbines of nuclear power plants, which is caused by fatigue, stress corrosion and erosion. In this study, as a tool of reliability evaluation, a number of stress and fracture analyses were performed on the defected area under various operating conditions using the finite element method. Possible defects on key-way and rotor disc were assumed to be two-dimensional cracks and centrifugal force, temperature distribution and shrink-fit effect were included as external loads. From stress analysis results, stress intensity factors were obtained and these values can be utilized to evaluate reliability and predict remaining lifetime of the turbine discs.     

Static and dynamic fracture analysis for the interface crack of isotropic-orthotropic bimaterial

In the present study, interfacial cracks between an isotropic and orthotropic material, subjected to static far field tensile loading are analyzed using the technique of photoelasticity. The fracture parameters are extracted from the full-field isochromatic data and the same are compared with that obtained using boundary collocation method. Dynamic photoelasticity combined with high-speed digital photography is employed for capturing the isochromatics in the case of propagating interfacial cracks. The normalized stress intensity factors for static cracks are greater when α=90° (fibers perpendicular to the interface) than when α=0° (fibers parallel to the interface), and those when α=90° are similar to ones of isotropic material. The dynamic stress intensity factors for interfacial propagating cracks are greater when α=0° than α=90°. For the velocity ranges (0.1<c/c _s1 <0.7) observed in this study, the complex dynamic stress intensity factor |K _D |, I increases with crack speedc, however, the rate of increase of |K _D | with crack speed is not as drastic as that reported for homogeneous materials.     

发动机悬置软垫强度分析

建立发动机悬置的有限元分析模型,考虑橡胶块与钢板之间的接触和橡胶材料的超弹性特性。计算得到结构应力强度分布规律,据此确定的结构疲劳源,与实际疲劳裂纹初始发生位置相符,进而判断结构的断裂面。对结构交变应力强度的计算进一步确定结构疲劳失效发生的位置。计算结果为将三维结构简化为二维结构进行断裂计算提供依据。     

Stress intensity factors for elliptical arc through cracks in mechanical joints by virtual crack closure technique

The reliable stress intensity factor analysis is required for fracture mechanics design or safety evaluation of mechanical joints at which cracks often initiate and grow. It has been reported that cracks in mechanical joints usually nucleate as corner cracks at the faying surface of joints and grow as elliptical arc through cracks, In this paper, three dimensional finite element analyses are performed for elliptical arc through cracks in mechanical joints. Thereafter stress intensity factors along elliptical crack front including two surface points are determined by the virtual crack closure technique. Virtual crack closure technique is a method to calculate stress intensity factor using the finite element analysis and can be applied to non-orthogonal mesh. As a result, the effects of clearance on the stress intensity factor are investigated and crack shape are then predicted.     

Approximation method for the calculation of stress intensity factors for the semi-elliptical surface flaws on thin-walled cylinder

A simple approximation method for the stress intensity factor at the tip of the axial semi-elliptical cracks on the cylindrical vessel is developed. The approximation methods, incorporated in VINTIN (Vessel INTegrity analysis-INner flaws), utilizes the influence coefficients to calculate the stress intensity factor at the crack tip. This method has been compared with other solution methods including 3-D finite element analysis for internal pressure, cooldown, and pressurized thermal shock loading conditions. For these, 3-D finite-element analyses are performed to obtain the stress intensity factors for various surface cracks witht/R=0.1. The approximation solutions are within ±2.5% of the those of finite element analysis using symmetric model of one-forth of a vessel under pressure loading, and 1–3% higher under pressurized thermal shock condition. The analysis results confirm that the approximation method provides sufficiently accurate stress intensity factor values for the axial semi-elliptical flaws on the surface of the reactor pressure vessel.     

应力集中和表面完整性对平尾大轴抗疲劳性能的影响

某型飞机平尾轴在进行台架试验时发生断裂,失效分析表明,该轴的断裂性质为疲劳断裂,裂纹起源于大轴筒体内腔变截面过渡圆弧根部的加工刀痕谷底。通过对大轴进行扫描电镜观察分析,发现使用过的旧大轴内表面存在10～16 μm厚的“疏松”层,疏松层内有较多的疲劳微裂纹和孔洞,该“疏松”层是大轴服役中氧化腐蚀和疲劳损伤所形成的。“疏松”层的存在破坏了大轴的表面完整性,降低了大轴材料的抗疲劳性能。有限元建模分析表明,变截面台阶造成的结构应力集中和粗糙加工刀痕形成的附加应力集中是造成大轴疲劳断裂的力学因素,两种应力集中因素的联合作用降低了大轴的疲劳寿命,导致大轴在变截面过渡圆弧根部的加工刀痕谷底萌生疲劳裂纹。综合分析表明,大轴内表面“疏松”层的存在以及变截面台阶造成的结构应力集中和粗糙加工刀痕形成的附加应力集中是大轴发生疲劳断裂的主要原因。     

The role of casting porosity in fatigue properties of Al−Si 319 lost foam cast alloy

The tensile, fracture toughness and fatigue properties of Al−Si 319 lost-foam-cast alloy were determined at room temperature. The fatigue properties of this alloy were also determined at 150°C. Fatigue cracks were always initiated at the largest casting pore. Initial pore sizes were measured using a scanning electron microscope. Surface replication showed that majority of the fatigue life was spent in fatigue crack propagation and permitted the estimation of the constants in the Paris power law and the threshold stress intensity factor (ΔK _th ). The role of internal casting porosity was quantified using a linear elastic fracture mechanics (LEFM) model for fatigue crack growth. The predicted lives agreed with the measured values within a factor of two.     

A study on the determination of closing level for finite element analysis of fatigue crack closure

An elastic-plastic finite element analysis is performed to investigate detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using 4-node isoparametric elements can predict fatigue crack closure behavior. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. The crack opening level for the constant mesh size increases linearly from initial crack growth. The crack opening level for variable mesh size, is almost flat after crack tip has passed the monotonic plastic zone. The prediction of crack opening level using the variable mesh size proportioning the reversed plastic zone size with the opening stress intensity factors presents a good agreement with the experimental data regardless of stress ratios.     

大型隔膜泵曲轴三维裂纹起裂裕度有限元分析

讨论基于断裂力学理论求解应力强度因子的方法。在此基础上介绍利用Ansys有限元分析软件对含有三维裂纹的隔膜泵曲轴应力强度因子进行计算的过程,同时介绍使用Ansys软件自带的宏命令建立含三维裂纹模型的方法,并使用该方法对含三维裂纹曲轴各个截面在12种工况下分别进行分析,得出各个截面存在裂纹时的危险等级。通过有限元分析计算,可以判断曲轴裂纹是否扩展,从而为生产过程中的探伤等工艺提供参考。     

DAMAGE TOLERANCE ANALYSIS ON HOLLOW AXLES OF HIGH SPEED MOTOR TRAINS

According to the rules of UIC515-3, the service loads of the axles are defined, which include some different loads cases as follows： the static loads; the impact loads resulted from running through the rail joints and unevenness rails; the loads through curves and from braking. Through the calculating and analysis, the stress distribution of the hollow axles is obtained for 200 km/h high speed motor trains used in China. At the same time, the fatigue crack growth of hollow axles is studied, and the initial surface cracks of 2 mm depth caused by hard objects strike or the other causes are discussed. On the basis of the linear elastic fracture mechanics theory, the stress intensity factor of the crack of the geometry transition outside the wheel seat is also studied. Associated with fatigue crack propagation equation and the corresponding crack propagation threshold, the crack propagation characteristics under different shapes are calculated. Then the running distances are educed with different shapes propagating to the critical length, and the estimation of the residual lives about hollow axles which are the reference values of examine and repair limit of the hollow axle is given.     

Service life estimation of extrusion dies by numerical simulation of fatigue-crack-growth

In this study, fatigue behavior of cold extrusion dies is investigated analytically. Experimental studies and practical experience have shown that fatigue cracks occur at the inlet radius of die shoulders. The highest stress concentration is found also at the same location. For this reason, effective stress intensity factors at different locations of the die-shoulder inlet of a typical axisymmetric extrusion die have been calculated by using the finite element method. The crack growth has been simulated by applying the Paris/Erdoĝan fatigue law to the computed data. Finally, service life of the extrusion die has been estimated from the crack-growth-rate. Agreement of theoretical estimations with available data is found to be satisfactory within the limits of experimental uncertainties. Furthermore the experimental behavior of the crack growth (stable-unstable-stable growth with final fracture) is simulated correctly. It has been shown that the unexplained behavior is caused by decrease of stress concentration with increasing crack-length.     

A fracture mechanics life prediction methodology applied to dovetail fretting

This work evaluates a fracture mechanics based crack growth life prediction methodology for dovetail fretting fatigue laboratory experiments. The Ti–6Al–4V specimens were configured with angles of 35°, 45° and 55°. Experiments were conducted with constant amplitude loading at R of 0.1 and 0.5 with lives ranging from 100,000 to 10 million cycles. The approach included the contact loads and bulk stress calculated from the finite element method as inputs to the stress and life analysis. Contact stresses were calculated using the contact stress analysis software CAPRI. These stresses were input into a stress intensity factor calculation at the edge of contact. Crack propagation life was calculated from an assumed initial crack size. Analysis showed that propagation consumes a majority of the total life and is insensitive to a large range of initial crack sizes.     

含裂纹螺接件应力强度因子三维有限元分析

对飞机结构螺接件来说,最主要的强度问题之一,是在疲劳载荷下会产生裂纹并扩展。而目前对含裂纹螺接件的应力强度因子(Stress intensity factor,SIF)分析多以简化的二维有限元模型为主。为此,在考虑螺栓与孔壁间的非线性接触、摩擦和螺栓预紧力等因素影响的基础上,利用非线性有限元软件MSC.Marc构建含裂纹螺接件的三维有限元分析模型,通过分析得到不同损伤模式下的SIF与裂纹长度沿板厚度方向的变化曲线。结果表明,虽然螺接件孔边裂纹是复合型裂纹,但Ⅰ型SIF起主导作用,Ⅱ型和Ⅲ型SIF可忽略不计;在裂纹长度一定时,接触面处的SIF最大,外表面处最小,孔边双侧裂纹对称时的SIF大于孔边单侧裂纹情形;裂纹长度相同时,含沉孔螺接件接触面上的SIF大于直孔情形时相应的SIF,而外表面的SIF趋势则恰恰相反。     

某型主战坦克扭力轴的疲劳断裂分析与寿命计算

应用断裂力学理论和有限元分析方法对某型主战坦克扭力轴进行疲劳断裂计算和分析，论述裂纹尖端的奇异三角形单元划分技术，在有限元法计算的基础上求解裂纹尖端应力强度因子和裂纹形状系数，并结合Paris公式对某坦克扭力轴的疲劳寿命进行计算。     

基于子模型与网格随移技术的叶轮三维裂纹应力强度因子分析

基于断裂力学理论和有限元数值分析方法,针对压气机叶轮由于铸造缺陷和疲劳引起的三维裂纹,应用子模型技术和ParaMesh网格随移技术,给出了压气机叶轮轴孔三维裂纹前沿应力强度因子的求解方法及途径,并对压气机叶轮轴孔三维裂纹的扩展方向和扩展速率进行了分析.压气机叶轮轴孔三维裂纹应力强度因子求解结果表明,叶轮轴孔三维裂纹前沿...     

Application of modified mapping-collocation method to cracks emanating from a circular hole in an orthotropic finite plate

A modified mapping-collocation method is applied to the analysis of cracks emanating from a circular hole in an orthotropic finite plate under uniform stress. To check the effectiveness of this procedure, we present the various results for comparison with references. Then, the stress intensity factors are presented for several plate configurations of [0 _n /90 _m ] _s laminates. The results show that the modified mapping-collocation method is effectively applicable to analyzing such cracks in an orthotropic finite plate. The results also show that the stress intensity factors depend on the material orthotropy and geometry.     

腐蚀结构损伤评估方法

针对腐蚀环境下老龄化飞机结构表面存在广布随机的腐蚀坑,而使其强度降低这一问题,先采用有限元方法并结合仿真软件ANSYS的参数化分析语言APDL,得到不同形状腐蚀坑底部的有效应力集中系数,然后结合局部应力应变法求解各个腐蚀坑底部在疲劳载荷作用下产生初始裂纹的萌生寿命值,再通过蒙特卡罗方法求解数值积分和非线性方程,得到和腐蚀坑具有相同寿命等效裂纹的尺寸,最后采用神经网络技术建立腐蚀坑各种几何尺寸与等效裂纹深度间的多变量非线性映射关系.为工程实践中用广布裂纹方法处理含随机分布腐蚀坑的老龄化结构提供可能.     

地铁车辆轮缘润滑装置吊架断裂机理和试验研究

针对地铁车辆服役过程中出现的轮缘润滑装置吊架断裂问题,采用材料和动应力试验与有限元仿真相结合的方法开展断裂机理研究,提出了一种新型结构设计方案。通过对吊架断口微/宏观的全面分析与表征,从材料角度确定疲劳断裂的机理成因;结合动应力试验和累计损伤理论,计算吊架断口关键位置的疲劳损伤值,从线路试验角度确定吊架断裂的结构成因;建立吊架的有限元模型,基于EN13749标准对其进行静强度和疲劳强度仿真,从理论角度判别结构设计的合理性,并与材料和线路试验分析结果进行对比分析;最后,提出吊架结构的改进方案,并进行仿真和动应力测试验证。结果表明：吊架断裂成因为低应力高周疲劳,疲劳源位于加强筋角焊缝内部,且加强筋角焊缝的累计损伤值和材料利用率均大于1;吊架断裂的主要原因为,结构不合理导致加强筋角焊缝承受较为频繁的交变载荷且焊缝存在未熔合缺陷;通过改变吊架结构和材质,提高了焊接质量,达到了轮缘润滑装置在线运行的标准要求。