Hybrid formulation solution for stress analysis of curved pipes with welded bending joints

A method is presented for evaluating the stress intensity factor of part-through cracks in a thin pipe elbow. A hybrid formulation solution is used to evaluate the stress field close to the crack area. The stress field values are then inputted into a previously developed method published in the literature to evaluate the stress intensity factor in cylindrical shells. Results from cylindrical shells with part-through cracks are extended to double-curvature pipe configurations that contain the same kind of flaw.     

Failure Analysis of Cracked Reducer Flange

A cracked reducer flange was analyzed for the cause of the failure. The flange was carefully cut to obtain samples for metallographic, X-ray, and scanning electron microscopy (SEM) Examinations. The examinations revealed that the introduction of chloride ions in the operational service led to pitting corrosion in the inner surface of the flange. Chloride ion inclusions were probably the result of chemical contaminations, i.e., cleaning chemicals’ contamination during shutdown of the operation. The introduction of corrosion pits caused unexpected load stress intensification and cracking of the flange. Consequently, stress corrosion cracking emanated from the pits under the influence of chloride attack and operational pressure. Then the cracks propagated in a transgranular manner, in the radial direction of the flange, until the final failure occurred.     

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

火电厂600MW超临界机组用大直径热推A335P91弯头的力学性能探讨

以火电厂600MW超临界机组用大直径热推A335P91弯头为研究对象,对该弯头进行了力学性能试验和相关项目检验。结果表明,该弯头无表面裂纹和晶间裂纹,其几何尺寸、壁厚、圆度等符合DL/T695-1999标准要求,弯头各部位常温和高温拉伸性能符合ASTM A335P91钢的要求。但由于应变硬化和鲍辛格（Bauchinger）效应的影响,使该弯头外弧和内弧力学性能存在明显差异。建议严格控制弯头扩径比,减小弯头推制过程中内外弧的塑性变形量,使弯头各部位获得最佳综合力学性能。     

Failure analysis on weld joints between the elbow and straight pipes of a vacuum evaporator outlet

This study presents an analysis on a weld joint leakage accident between elbow and straight pipes that connect the inlet of a catalyst plant vacuum pump and the outlet of a vacuum evaporator tank. Moreover, the causes and mechanisms of material degradation are investigated by analyzing the chemical compositions and observing the microstructures with the use of a metallographic microscope and a scanning electron microscope. Energy dispersive spectroscopy was used on the test materials and corrosion products to examine the changes in the elements during material degradation. The results show that stress corrosion cracking caused the cracks and the leakage on the joints. The fracture morphologies were dominated by river cleavage and fan-shaped cleavage patterns. The existence of Cl⁻ and residual stress from the installation of the elbow pipe and the heat effect of the welding process contributed to the stress corrosion cracking.     

A method for stress intensity factor calculation of infinite plate containing multiple hole-edge cracks

Multiple site damage is the occurrence of small fatigue cracks at several sites within aging aircraft structures. Focusing on this typical structure, an analytical method for calculating the stress intensity factor of an infinite plate containing multiple hole-edge cracks was introduced in this paper. The properties of complex variable functions are used to evaluate the stress function. The approximate superposition method is applied to solve stress intensity factor problems on multiple holes. The equivalent crack is introduced to modify the method. Some numerical examples of an infinite plate containing two hole-edge cracks are examined by the method. By comparing the analytical and finite element analysis results it was realized that the analytical results are accurate and reliable. This modified analytical method is easier to apply than some traditional analytical methods and can provide stress intensity factor solutions for an infinite plate containing a random distribution of multiple hole-edge cracks.     

42MnM07地质钻杆断裂原因分析

通过化学成分分析、断口分析、金相检验和扫描电镜观察等方法,对42MnMo7铜地质钻杆的断裂原因进行了分析。结果表明：钻杆在交变力作用下,在螺纹根部的应力集中处产生裂纹．此裂纹在过载力和钻杆基体存在带状脆性马氏体的情况下进一步扩展,最终导致钻杆发生断裂。最后对此提出了相应的措施及建议。     

Effect of fine crack width and water cement ratio of SHCC on chloride ingress and rebar corrosion

SHCC (Strain Hardening Cement-based Composite) is a material known for its strain-hardening behavior under tensile and bending stress and its characteristic numerous small cracks. SHCC is expected to show superior durability because of the fineness of the cracks. In this study, chloride ingress through cracks into SHCC and progress of rebar corrosion in three mixtures of SHCC with various water-cement ratios were investigated. Through a chloride solution immersion test, it was confirmed that chloride could penetrate through even very fine cracks. The resistivity of cracked SHCC against chloride ingress is mainly governed by the accumulated crack width and the water cement ratio. Chloride pre-mixed SHCC specimens were left in a high-temperature, high-humidity chamber for 11 months to promote rebar corrosion. While the accumulated crack width and the water cement ratio were both influential to an increase in corrosion area, only the water cement ratio had bearing on corrosion loss.     

双卡簧片失效分析

采用光学金相、扫描电镜、电子探针成分分析和显微硬度测定等方法分析了双卡簧片非正常断裂的原因。在部分双卡簧片折弯处发现有尖锐凹角并有明显微裂纹,裂纹表面有镍和铜,说明裂纹是在成形加工过程中形成。双卡簧片盛开有时弯角较尖锐并有明显裂纹是导致失效的原因。     

Stress intensity factor analysis of an interface crack between dissimilar anisotropic materials under thermal stress using the finite element analysis

New numerical methods were presented for stress intensity factor analyses of two-dimensional interfacial crack between dissimilar anisotropic materials subjected to thermal stress. The virtual crack extension method and the thermal M-integral method for a crack along the interface between two different materials were applied to the thermoelastic interfacial crack in anisotropic bimaterials. The moving least-squares approximation was used to calculate the value of the thermal M-integral. The thermal M-integral in conjunction with the moving least-squares approximation can calculate the stress intensity factors from only nodal displacements obtained by the finite element analysis. The stress intensity factors analyses of double edge cracks in jointed dissimilar isotropic semi-infinite plates subjected to thermal load were demonstrated. Excellent agreement was achieved between the numerical results obtained by the present methods and the exact solution. In addition, the stress intensity factors of double edge cracks in jointed dissimilar anisotropic semi-infinite plates subjected to thermal loads were analyzed. Their results appear reasonable.     

Determination of approximate point load weight functions for embedded elliptical cracks

This paper presents the application of weight function method for the calculation of stress intensity factors in embedded elliptical cracks under complex two-dimensional loading conditions. A new general mathematical form of point load weight function is proposed based on the properties of weight functions and the available weight functions for two-dimensional cracks. The existence of this general weight function form has simplified the determination of point load weight functions significantly. For an embedded elliptical crack of any aspect ratio, the unknown parameters in the general form can be determined from one reference stress intensity factor solution. This method was used to derive the weight functions for embedded elliptical cracks in an infinite body and in a semi-infinite body. The derived weight functions are then validated against available stress intensity factor solutions for several linear and non-linear stress distributions. The derived weight functions are particularly useful for the fatigue crack growth analysis of planer embedded cracks subjected to fluctuating non-linear stress fields resulting from surface treatment (shot peening), stress concentration or welding (residual stress).     

Developments in the analysis of interacting cracks

This paper presents an overview of the finite element alternating technique for the analysis of interacting cracks. To illustrate the ease and accuracy of this method the technique is used to analyse several problems associated with both widespread fatigue and multi-site damage, a problem which is attracting worldwide attention. Whilst this paper presents an overview of the technique for both two- and three-dimensional problems attention is focused on three-dimensional problems. In particular, the interaction effects between two fully embedded elliptical flaws and between two semi-elliptical surface flaws, and the effects of crack proximity and crack aspect ratio on the stress intensity factors are presented. For semi-elliptical surface flaws these results indicate that as the cracks approach each other the position of the point on the crack front with the highest stress intensity factor shifts. This subsequently suggests that surface cracks will tend to grow preferentially towards each other. The same trend is evidenced for fully embedded cracks. However, in this case there is no shift in the position of the maximum stress intensity factor. A discussion of the results in terms of stress intensity magnification factors is also presented.     

Growth of three-dimensional cracks in finite-thickness plates

The stress field in a finite-thickness plate weakened by a three-dimensional crack and subjected to tension in a direction perpendicular to the crack plane is studied. The cases of an embedded elliptical, semi-elliptical and quarter-elliptical surface crack are considered. The stress analysis takes place by a finite element computer program which uses twenty-node isoparametric and fifteen-node enriched elements. The stress intensity factor variations along the periphery of the elliptical cracks are given for various plate thicknesses. The results of the stress analysis are used in conjunction with the strain energy density theory to study the growth characteristics of the cracks. The history of non-self-similar crack growth from initiation to final instability through the intermediate stage of stable growth is analyzed. The increments of crack growth from each point of its front are determined on the basis that the critical element in the direction of crack growth absorbs a critical amount of strain energy density. Crack growth becomes unstable when the last increment from the critical point of the crack front takes a limiting value. Results for the crack growth characteristics are given for the three types of cracks considered and various plate thicknesses.     

Approximate weight function method for elliptical arc through cracks in mechanical joints

Mechanical joints such as bolted, riveted or pinned joints are widely used to join the constituent parts of structural components. Reliable stress intensity factor analysis of arbitrary cracks in mechanical joints is required for the safety evaluation or fracture mechanics design. It has been reported that cracks in mechanical joints usually nucleate as the corner crack and grow as the elliptical arc through crack. The weight function method is a useful technique to calculate the stress intensity factor using the appropriate weight function for a cracked body and the stress field of an uncracked body. In this paper, the weight function method for the two surface points of elliptical arc through cracks in mechanical joints is developed to analyze the mixed-mode stress intensity factors. Unknown coefficients included in the weight function are determined using the reference stress intensity factors obtained from finite element analysis.     

The stress intensity factor for a crack perpendicular to the welding bead

An analysis of the stress intensity factor due to the residual stress is made for a crack perpendicular to the welding joint in a large plate. The residual stress distribution is represented by a simple function which is chosen to satisfy the physical requirements for the residual stress and to simulate the commonly observed distribution. The stress intensity factor is obtained using customary method based on the superposition principle. The function chosen for the residual stress distribution leads to an exact expression of the stress intensity factor in a simple closed form. The solution yields somewhat conservative values of the stress intensity factor for large cracks and it may be conveniently used for practical applications.     

盘架管路弯管断裂原因分析

通过断口分析、化学成分分析和金相检验等方法对某盘架管路弯管的断裂原因进行了分析。结果表明:该盘架管路在生产过程中因非正常使用工具校形导致弯管根部存在较大的残余应力;另外,工具校形在弯管表面产生的机械损伤形成了应力集中,促进了裂纹的萌生;加之弯管焊缝热影响区晶粒粗大降低了该处材料的疲劳强度;上述因素综合作用最终导致弯管在耐久振动试验中于焊接热影响区的机械损伤处发生疲劳断裂。     

Universal features of weight functions for cracks in mode I

An analysis of known analytical and numerical weight functions for cracks in mode I has revealed that they all have a similar singular term and that it is possible to approximate them with one universal expression with three unknown parameters. The unknown parameters can be determined directly from reference stress intensity factor expressions without using the crack opening displacement function. The universal weight function expression, with suitable reference stress intensity factors, was used to derive the weight functions for internal and external radial cracks in a thick cylinder. These weight functions were then further used to calculate the stress intensity factors for radial cracks in a cylinder subjected to several nonlinear stress fields and were compared to available numerical data.     

Analysis of elliptical cracks perpendicular to the interface of two joined transversely isotropic solids

This paper presents a boundary element analysis of elliptical cracks in two joined transversely isotropic solids. The boundary element method is developed by incorporating the fundamental singular solution for a concentrated point load in a transversely isotropic bi-material solid of infinite space into the conventional displacement boundary integral equations. The multi-region method is used to analyze the crack problems. The traction-singular elements are employed to capture the singularity around the crack front. The values of stress intensity factors (SIFs) are obtained by using crack opening displacements. The results of the proposed method compare well with the existing exact solutions for an elliptical crack parallel to the isotropic plane of a transversely isotropic solid of infinite extent. Elliptical cracks perpendicular to the interface of transversely isotropic bi-material solids of either infinite extent or occupying a cubic region are further examined in detail. The crack surfaces are subject to the uniform normal tractions. The stress intensity factor values of the elliptical cracks of the two types are analyzed and compared. Numerical results have shown that the stress intensity factors are strongly affected by the anisotropy and the combination of the two joined solids.     

转向器螺杆失效分析

采用化学分析、金相检验和硬度测试等方法对转向器螺杆螺纹椎部的裂纹进行了分析。结果表明，转向器螺杆渗碳处理及淬火后，螺杆表层产生高碳马氏体组织而其心部却未发生马氏体转变，引起零件内的组织应力增大，这是产生裂纹的内在因素，另外，磨削方法不当又产生了二次淬硬层,叠加的应力超过了材料的强度极限时，导致了钢件表面磨削裂纹的产生。     

X-ray microanalysis and acoustic emission studies on the formation mechanism of secondary cracks in PMMA

The formation mechanism of secondary cracks in poly(methyl methacrylate) (PMMA) was investigated using various methods. X-ray element analysis and SEM observations on parabolic markings showed that there exist certain defects such as silicon compound and microcracks at their foci, forming the potential sources for the secondary cracks (a static factor). On the other hand, the fracture process was monitored simultaneously with acoustic emission (AE) and high speed shadow-optical techniques. These observations revealed that density of the parabolic marking has a closer correlation to the amplitude of the stress wave emitted during crack propagation, rather than to the values of crack speed or stress intensity factor (a dynamic factor). The formation mechanism is thus explained by the combination of these static and dynamic factors.