Fatigue failure of weld joint of afterburner fuel manifold of a jet engine

A fighter aircraft met with an incident during a routine flight operation wherein pilot noticed a thin trail of smoke coming out from the tail end of the aircraft. Post landing inspection revealed that there was fire in the afterburner section of the aircraft. On investigation, it was discovered that there was a fatigue crack in one of the weld joint of the afterburner fuel manifold which resulted in fuel leakage in the form of streaks. The localized over temperature thus generated was responsible for melting of diffuser casing, adjustable nozzle casing, heat shield and hydraulic pipelines. Through detailed investigation, the sequence of failure in the engine was established. The contributing factors for fatigue cracking of the weld of the fuel manifold were identified and remedial measures suggested for prevention of similar failures.     

不锈钢直型加压接骨板断裂分析

对在患者体内断裂的不锈钢直型加压接骨板进行了化学成分、硬度、光学显微组织、断口扫描电镜及ＥＰＭＡ分析。结果表明,断裂在近接骨端的螺钉孔表面有６～７μｍ的变形层,变形层中的孔洞是疲劳断裂的萌生处,该接骨板属疲劳断裂失效。提出了在接骨板生产和使用中应改进和注意的问题。     

Failure Analysis and Design Optimization of the Steady Rest Hanger Rod Pipe Assembly

This article investigates the failure of the steady rest hanger rod pipe assembly weld joints of an automotive exhaust system. Rig testing of the exhaust system showed the presence of crack at the steady rest hanger rod and brace weld joints. Metallurgical investigation was performed in order to determine the root case of failure and contribution factors. Metallurgical analysis methods included visual examination, thickness measurements, optical and scanning electron microscopy, chemical analysis of the material and weld evaluation. A CAE analysis was performed to simulate the rig test. Finite element simulation of the system also showed high damage at the steady rest hanger and brace weld locations. A DOE study was conducted to identify the design variables that could impact the dynamic response of the system like the thickness of the parts, the weld characteristics of joints, etc. Design changes were proposed; to improve the fatigue life of steady rest hanger rod pipe assembly based on the results of DOE-based study. The new design was analyzed using finite element analysis and compared with the original design for fatigue life, which showed a considerable improvement in the durability of the joint.     

Circumferential inner fatigue crack growth and penetration behaviour in pipe subjected to a bending moment

Surface defects in most pipe components under cyclic stress tend to be origins of fatigue crack growth, which may cause serious failure of the whole structure. An analysis of crack growth and penetration behaviour from these defects may therefore be one of the most important subjects on the reliability of the LBB (leak before break) design philosophy. However, studies on fatigue crack growth and COD (crack opening displacement) behaviour in a complex through-wall crack are relatively rare. To that purpose, a series of fatigue tests has been carried out at room temperature on carbon steel pipe which has an inner surface crack. Before crack penetration, leak-life, fatigue crack growth behaviour, e.g. aspect ratios and d a /d N versus Δ K were evaluated successfully by using a proposed plate model. After crack penetration, a new formula to evaluate K is proposed. For the complex crack created after through-wall penetration, fatigue life and fatigue crack growth behaviour were evaluated quantitatively by using the new formula and the model proposed in this study.     

AlSi10Mg(Cu)铸铝合金的热疲劳裂纹萌生及早期扩展行为

微观观察AlSi10Mg(Cu)铸铝合金在热疲劳裂纹的萌生和早期扩展过程,重点研究共晶硅粒子对热疲劳裂纹行为的影响。结果表明:热疲劳裂纹萌生于脱粘共晶硅粒子与铝基体间的开裂界面,原因是共晶硅粒子与铝基体的热膨胀系数不同,引起热循环过程中两相热应变不协调,从而在两相界面处产生循环应力而引起疲劳破坏。热疲劳裂纹的扩展在长度和宽度上同时进行,具有良好塑性的铝枝晶对疲劳裂纹的扩展起阻碍作用。对热疲劳过程中共晶硅粒子周围应力场的模拟分析进一步解释了实验现象。     

Thermal striping fatigue damage in multiple edge-cracked geometries

Thermal fatigue striping damage may be caused when incompletely mixed hot and cold fluid streams pass over the surface of a component or structure containing a defect. Stress intensity factor (SIF) fluctuations are developed in response to the surface temperature fluctuations. An existing methodology for the analysis of striping damage in geometries containing a single edge‐crack geometry is extended to such an analysis of multiple edge cracks. SIFs are calculated as functions of crack depth, when an edge‐cracked plate and semi‐infinite solid, each containing multiple cracks, are subjected to thermal striping. The effect of various restraint conditions and striping frequencies on the SIF values for a stainless steel plate is examined. The degree of conservatism is shown when an assessment of thermal fatigue striping damage is based on a single, rather than multiple, crack analysis. Accurate curve fits are developed resulting in practical weight functions for an edge‐cracked plate and semi‐infinite solid.     

Fracture mechanics analysis of the effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by the accelerated temperature cycling test and thermal mechanical analysis. The mechanism of substrate flexibility on improving solder joint thermal fatigue lifetime was investigated by fracture mechanics methods. Two different methods (crack tip opening displacement, CTOD and virtual crack closure technique, VCCT) are used to determine the crack tip parameters which are considered as the indices of reliability of solder joints, including the strain energy release rate and phase angle for the different crack lengths and temperatures. It was found that the thermal fatigue lifetime of solder joints in flip chip on flex assembly (FCOF) was much longer than that of flip chip on rigid board assembly (FCOB). The flex substrates could dissipate energy that otherwise would be absorbed by solder joints, that is, substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during thermal cycling.     

基于应力强度因子评估WCP形状对WCP/Fe复合材料热疲劳裂纹扩展行为的影响

应力强度因子在断裂力学中广泛应用于预测由远程载荷或残余应力引起的裂缝尖端附近应力状态。本文基于平面应力条件下应力强度因子建立WC_P形状与其尖端应力之间的规律,利用有限元分析软件对含不同形状WC_P的WC_P/Fe复合材料的热应力进行模拟仿真,研究WC_P形状对WC_P/Fe复合材料热疲劳裂纹扩展行为的影响。研究结果表明,WC_P的形状显著影响应力强度因子,进一步影响WC_P/Fe复合材料的热疲劳裂纹扩展行为。含球状和不规则状WC_P的WC_P/Fe复合材料的极限抗压强度分别约为460 MPa和370 MPa。含不规则状WC_P的WC_P/Fe复合材料因应力集中而容易产生脆性开裂现象。通过热震实验进行验证,发现实验结果与模拟仿真结果相近,说明有限元法的准确性,同时为WC_P/Fe复合材料的热疲劳裂纹扩展行为研究提供科学依据和理论基础。      

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.     

Bending overload and thermal fatigue fractures in a cast exhaust manifold

The left exhaust manifold of a Ford Y-Block engine failed at two different locations after nearly fifty years of service. A study was conducted in order to determine if the main causes and the sequence of failure of this component were related to its operation. A detailed analysis comprising materials characterization, fractography and a thorough finite element modeling was carried out. The results showed that the normal operation of the engine induced several thermal fatigue cracks, which were primarily located in the outermost runners of the component. One of these cracks propagated unstably during the operation, producing one of the fractures. The other one was generated during an overhaul, when a bending load and an improper assembly condition propagated a preexisting crack. In addition, the presence of a mixed graphite distribution comprising A, B and D flake graphite compromised the mechanical properties of the exhaust manifold, and aided in the process of crack propagation.     

Experimental and numerical investigation on thermal fatigue behaviour of 9Cr 1Mo steel tubes

Boiler tubing is subjected to alternate cycles of heating and cooling during operation initiating alternate thermal expansion and contraction. Alternate cycles of differential expansion and contraction causes thermal fatigue of the component. Thermal fatigue causes tube failures, significantly reducing the working life of the tubular components. Boilers have matured into super critical design using T91 grade materials thereby increasing the operating efficiency. The studies involving thermal fatigue of tubes of T91 grades for boiler components in operating conditions are thus an important area. A simple experimental set up has been developed to simulate thermal fatigue conditions in the internal diameter (ID) side of the tube. The work involves both experimental and numerical investigations of the thermal fatigue behaviour by creating a simulated environment and a Finite Element Model (FEM). FEM analyses are carried out based on the decoupled thermal and inelastic stress analyses to compute the total plastic strain range experienced by the boiler tubes. The cyclic spray cooling causes thermal fatigue cracks in the T91 tube. The number of cycles to crack initiation has been obtained from the experimentation and number cycles to failure has been calculated using modified Coffin-Manson relation. The study thus presents a reliable fatigue failure analysis of 9Cr 1Mo steel tubes used in boiler industry.     

Study of polyethylene pipe resins by a fatigue test that simulates crack propagation in a real pipe

A fatigue test that simulates the step-wise crack propagation found in pipes in the field, and uses a standard compact-tension specimen, was employed to study and rank crack resistance of various pipe resins. The thermal history during compression moulding of the test specimens strongly affected fracture kinetics. It was found that crack-resistant properties of in-service pipe were best reproduced if compression-moulded plaques were fast cooled under load. This procedure was used to prepare specimens from candidate pipe resins for fatigue testing. The resins were compared in terms of discontinuous crack growth kinetics. The ranking based on resistance to fatigue crack propagation correlated with results of a standard PENT creep test. However, fatigue failure times were an order of magnitude less than the standard creep times. After comparing the initiation and failure times of the resins with detailed kinetics of step-wisse crack propagation, a simplified and rapid procedure is proposed which calls for evaluating only the first jump after initiation.     

发动机振动对排气歧管低周疲劳寿命影响研究

发动机排气歧管在动态热负荷与整机振动载荷耦合作用的恶劣环境中工作,在热负荷单独作用时其部分区域就已经发生塑性变形,而整机振动载荷的耦合作用将使其疲劳失效问题更为严峻。为量化整机振动载荷对排气歧管低周疲劳损伤的影响,以某三缸增压发动机为研究对象,首先,基于流固耦合方法获得了排气歧管在标定工况和怠速工况下内外流场的换热边界,并联合增压器、催化器等部件温度和换热边界对两工况下排气系统的温度场进行计算。然后,根据温度场计算结果,耦合螺栓预紧力的作用,对怠速工况下的弹性应力场以及标定工况下的弹性和弹塑性应力场进行了计算,并基于标定工况下的弹塑性应力场,应用模态瞬态动力学对标定工况下的整机振动载荷作用下的动应力进行分析。最后,依据标定和怠速工况下的弹性应力场、标定工况下的动应力场结果,参照发动机低周疲劳试验标准分别建立了排气歧管常规高温低周疲劳与整机振动-热耦合低周疲劳分析模型,引入Neuber准则对两者的载荷谱进行应力-应变修正后采用主应变法进行疲劳寿命评估。结果表明：排气歧管疲劳破坏风险点主要位于高温拉应力区域,叠加振动载荷会使整体疲劳寿命下降接近25.2%,部分区域下降幅度甚至高达57%。研究结果为排气歧管整机瞬态振动-热耦合低周疲劳寿命预测提供了一定的理论依据和参考。     

Study of Thermal Fatigue Resistance of a Composite Coating Made by a Vacuum Fusion Sintering Method

Thermal fatigue behavior of a Ni-base alloy chromium carbide composite coating made by a vacuum fusion sintering method are dicussed,Results show that thermal fatigue behavior is associated with cyclic upper temperature and coating thickness,As the thickness of the coating decreases ,the ther-mal fatigue resistance increases,The thermal fatigue resistance cuts down with the thermal cyclic upper temperature rising,The crack growth rate decreases with the increase in cyclic number until crack arrests.Thermal fatigue failure was not found along the interface of the coating/matrix.The tract of thermal fatigue crack cracks along the interfaces of phases.     

Industrial experiences of bending fatigue strength in table liner for cement mill

A table liner for the vertical roller mill has been used to grind natural limestone. Unexpected fatigue failure accidents have occurred during portland cement manufacturing process. The design life of a table liner is 4 × 10⁷ cycles, but the actual fatigue life of a table liner is 2 × 10⁶ to 8 × 10⁶ cycles. The fatigue crack of a table liner initiates from the outside edge of the grinding path of the limestone. When such a crack occurs, the table liner has to be replaced, and this requires 30% of the total maintenance cost of the vertical roller mill. Therefore, this study examines the fatigue failure of a table liner by plane‐bending fatigue test, stress measurement test, finite element analysis and fatigue fracture analysis.     

Fracture mechanical analysis of self-fatigue in surface compression strengthened glass plates

This paper presents a fracture mechanical analysis of the static fatigue and spontaneous fragmentation of surface compression-strengthened glass plates in the absence of applied load. It is suggested that if an initial surface crack which is sufficiently deep to penetrate into the tensile zone within the plate interior is introduced into the plate, then static fatigue, and eventually spontaneous fracture may follow. The crack problem for glass plates under various internal stress fields is solved and the stress intensity factor is obtained as a function of the crack depth. Using the fracture toughness and the slow crack growth characterization of the material, the conditions for no crack propagation, crack propagation leading to crack arrest, and that leading to catastrophic failure are established and discussed. The general results obtained are illustrated by means of a numerical example based on a 2 mm thick surface compression-strengthened glass plate exposed to water at 25° C.Such a problem was encountered in relation to an eye-lens during a consulting case by one of the authors (DPHH).     

Fatigue life prediction for boom structure of concrete pump truck

Frequent premature fatigue failure of construction machinery has spurred the demand for fatigue life prediction of such equipment. In this study, fatigue cracks were investigated in a concrete pump truck boom typically a high-strength steel plate welded box girder structure. The focus was two-tip corner cracks which frequently initiate at the weld toe between the top flange plate and the web plate because these are commonly observed in the field. A fatigue crack growth numerical approach for three-dimensional shell problems was proposed to simulate fatigue crack growth in cracked structures. Fatigue experiments were performed on a full-scale cracked boom to validate the effectiveness of this approach. The influence of the initial crack length was examined using the proposed approach. Numerical results revealed that variation in the initial crack length can affect the remaining fatigue life. For engineering convenience, an interpolation method was employed to determine the remaining fatigue life of a cracked boom with an arbitrary initial crack length. The comparisons between interpolation and numerical results demonstrate that the interpolation method can be used to guide repair decisions with reasonable precision.     

低温环境下桥梁钢Q345qD疲劳裂纹扩展行为研究

为了明确在寒冷地区服役桥梁钢的疲劳裂纹扩展行为,以16 mm厚桥梁钢Q345qD为研究对象,完成了室温和低温下的夏比冲击韧性试验、疲劳裂纹扩展速率试验和疲劳裂纹扩展门槛值试验。结果表明,夏比冲击功和试样断口剪切断面率随温度的降低而减少;在应力比0.1、0.2和0.5条件下,疲劳裂纹扩展速率均随温度降低而变缓,该桥梁钢的疲劳韧-脆转变温度点在-60℃以下;在室温~-60℃,其裂纹扩展速率均对应力比的变化不敏感;应力比0.1条件下的疲劳裂纹扩展门槛值随温度的降低有略微增大的趋势。该批次桥梁钢表现出了良好的抵抗低温疲劳裂纹扩展性能,防止低温脆性破坏成为疲劳设计的重点;试验数据能为钢结构桥梁的进一步抗低温疲劳和防低温冷脆断裂设计提供参考。     

GROWTH OF SEMI-ELLIPTICAL SURFACE CRACKS IN FERRITIC STEEL PLATES UNDER CYCLIC THERMAL SHOCK LOADING

Abstract— Investigations of crack growth under thermal fatigue loading are presented. The experiments were performed with a ferritic reactor pressure vessel steel. A plate, containing a semi-elliptical surface crack is heated up to a homogeneous temperature and cyclically cooled down by a jet of cold water. On the basis of linear elastic fracture mechanics, stress-intensity factors are calculated with the weight function method. The prediction of crack growth under thermal fatigue loading using data from standard specimens is compared with the experimental result.     

Evaluation of equi-biaxial fatigue of stainless steel by the pressurized disc fatigue test

The pressurized disc fatigue (PDF) test technique was employed to obtain fatigue lives of Type 316 stainless steel under equi-biaxial stress conditions. In the PDF test, a disc-type specimen was subjected to the cyclic bulge test. The biaxial fatigue lives were successfully obtained by the PDF tests, and they were longer than those obtained by the uni-axial and plate bending fatigue tests under the same equivalent strain range. Observations of crack initiation and growth behavior during the PDF test revealed that the relatively large size of the disc-type specimens had only a minor influence on the fatigue lives. Finite element analysis results showed the PDF test was valid for evaluating the fatigue lives under equi-biaxial conditions. It was concluded that the influence of equi-biaxial condition was not necessary to be considered in the design fatigue curve.