A new ultrasonic technique for detection and sizing of small cracks in studs and bolts

If small cracks in stud bolts are not detected early enough, they can grow rapidly and cause catastrophic disasters in industrial facilities such as nuclear power plants. Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts. This study shows a method of detecting the existence and determining the size of a small crack in a root between two crests in the bolt threads using ultrasound. The Rayleigh wave propagating from the tip of a crack to the opening of the same crack is utilized. A delayed pulse, due to the Rayleigh wave, is detected between regularly spaced pulses from the threads, with the delay time being proportional to the size of the crack. Theoretical explanation is presented and experimental results demonstrating detection of cracks as small as 0.5 mm are shown.     

Metallurgical failure analysis of fasteners in an impeller assembly

This paper examines a failure analysis of the bolts from a failed joint between an impeller blade and a rotating assembly unit. The bolts failed due to poor thread manufacture and installation. Sharpened thread roots led to high stress concentrations that favored crack initiation. An oddly shaped thread profile allowed friction between mismatched thread surfaces. Poor installation procedures allowed for the possibility of overtightening to nucleate cracks in the head-to-shank interface (which had a smaller radius and therefore a higher stress concentration) and possibly also in the thread roots. Each of these influences contributed to crack initiation in the bolts. After cracks had formed, bending fatigue then propagated the nucleated cracks to final fracture. The failure analysis also recommended using bolts with rolled threads, which allow a more complete fit between mating male and female threads, and assuring that an appropriate preload is placed on bolts during installation.     

Failure analysis of a SAE 4340 steel locking bolt

Several SAE 4340 steel locking bolts used to assemble speed reducer housings fractured after a few hours of operation. Micrographic and macrographic analyses, scanning electron microscopy techniques, tensile, impact and hardness testing were used to fully characterize the component and material properties. Stress calculations were performed using both Neuber analysis and Finite Element Analysis (FEA) and the results were compared. Cracks nucleated at the root of the last engaged thread due to a combination of high local stresses in this region, surface defects, non-uniformity of the thread root and low toughness of the material. After nucleation, the crack propagated by fatigue until the catastrophic failure.     

Failure Investigation of Counterweight Separation from Aircraft Propeller

The counterweight of a propeller in a turboprop aircraft was separated during an engine run-up inspection. If this separation occurs in-flight, it may result in an accident involving serious damage or injury. In this investigation, the failed counterweight clamping assembly was studied to determine the root cause of failure. Both experimental and computational investigations were performed to explore and confirm the effects of experimentally observed anomalies on potential clamping assembly failure. Dimensional measurement of the failed clamping thread area by X-ray CT scanning revealed significant deviation from requirements in the major diameter of the thread. Fractographic and microscopic examination along with chemical analysis confirmed that the clamping bolts were pulled out due to overload stripping failure of the internal threads. Detailed computational fracture modeling utilizing the XFEM crack simulation technique provided further insight proving that thread engagement length had a significant effect on the clamping assembly failure. Based on these observations, it was concluded that the main root cause of the stripping failure was the dimensional nonconformance of the internal thread from the requirements in standard 7/16-20UNF-3B that resulted in the loss of thread engagement length.     

The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts

Many aeronautical fastners are exposed to cyclic stresses during service. Therefore, such parts are usually designed for limited fatigue lifetime. Various combinations of process type and sequence may be employed to produce threads, each resulting in different fatigue properties. Specifications of aircraft bolts often require production of threads by heat treatment followed by rolling, in order to improve the fatigue properties. Unfortunately, these specifications are not always followed to the letter. Therefore, for either quality assurance or failure analysis purposes, it is important to be able to determine unambigiously the process by which threads were produced. The objectives of this work were to study the effect of varied thread manufacturing process type and sequence on the mechanical properties of AISI 4340 stud bolts, and to develop a laboratory procedure for distinguishing between them. Threads were produced on heat-treated and non-heat-treated stud bolts either by machining or cold-rolling. The non-heat-treated bolts were subsequently heat-treated. All bolts were then subjected to mechanical testing (static tensile, dynamic fatigue, hardness and microhardness tests), metallographic and fractographic examinations. While the fatigue properties were significantly affected by the manufacturing process used, no effects on the tensile strength of the bolt were observed. Metallographic inspection and microhardness testing, but not fractographic inspection, were found to be effective for distinguishing between different manufacturing procedures.     

Failure analysis of connecting bolts and location pins assembled on the plate of main-shaft used in a locomotive turbochanger

Three connecting bolts, three location pins and navel of turbo-disk fractured, which were assembled on the plate of the main-shaft used in a locomotive turbochanger. Detailed fractographic study and metallurgical analysis were focused on the trouble bolts. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the surface decarburization layer in the thread tip and root regions of the three failed bolts make the hardness at the thread regions decrease intensely so that the fatigue cracks initiated form the root at the first engaged thread. Surface damage morphology with cutting, wear and plastic deformation features was found on the working flanks of the engaged threads. Other components fractured in succession after the trouble bolt fractured.     

Failure and Fracture Analysis of Bolt Assembled on the Fan Used in the Internal Combustion Engine

Three connecting bolts fractured, which were assembled on the fan used in the internal combustion engine. Detailed fractographic study and metallurgical analysis were focused on the fractured bolts. Fractographic and metallurgical studies indicate that the fracture surface and the microstructure of three bolts exhibit similar features. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the micro-cracks in the thread tip of the fractured bolts makes the stress at the thread regions increase intensively so that the fatigue cracks initiated from the thread regions. Due to the presence of dynamic load, the bolts were never properly tightened during installation which should be responsible for the formation of the cracks.     

A reconciliation of dynamic crack velocity and Rayleigh wave speed in isotropic brittle solids

Following earlier observations of multiple micro-crack formation accompanying crack propagation under dynamic conditions, the question regarding the discrepancy between the theoretically anticipated maximal crack (Rayleigh wave) speed and those observed typically in amorphous, isotropic solids is examined experimentally. It is shown that if the production of these multiple micro-cracks ahead of the main fracture is suppressed by fabricating a material possessing a thin uniform region of vanishing intrinsic (molecular/atomic) material strength, the crack speed is materially increased to the point of approaching the Rayleigh wave speed. Moreover, it is also shown that the presence of small discreet flaws of sufficient spatial density similarly weakens the material to produce fracture speeds comparable to the Rayleigh wave speed. One deduces, therefore, that for a single crack front the linearized theory of elastodynamics correctly predicts the dynamic crack propagation behavior of a solid with sufficiently low material strength.     

汽轮机高压外缸结合面双头螺栓断裂分析

某电厂汽轮机高压外缸结合面双头20Cr1Mo1VTiB钢螺栓在安装时发生断裂,通过断口分析、化学成分分析、金相检验以及力学性能测试等方法对螺栓的断裂原因进行了分析。结果表明:该螺栓钢存在严重的显微组织缺陷,力学性能指标不符合技术条件要求,减弱了螺栓的承载能力,螺纹处的应力集中效应会加剧螺栓的缺口敏感性,导致螺栓在安装时于螺纹处发生脆性断裂。     

Simulations of dynamic crack propagation in brittle materials using nodal cohesive forces and continuum damage mechanics in the distinct element code LDEC

Experimental data indicates that the limiting crack speed in brittle materials is less than the Rayleigh wave speed. One reason for this is that dynamic instabilities produce surface roughness and microcracks that branch from the main crack. These processes increase dissipation near the crack tip over a range of crack speeds. When the scale of observation (or mesh resolution) becomes much larger than the typical sizes of these features, effective-medium theories are required to predict the coarse-grained fracture dynamics. Two approaches to modeling these phenomena are described and used in numerical simulations. The first approach is based on cohesive elements that utilize a rate-dependent weakening law for the nodal cohesive forces. The second approach uses a continuum damage model which has a weakening effect that lowers the effective Rayleigh wave speed in the material surrounding the crack tip. Simulations in this paper show that while both models are capable of increasing the energy dissipated during fracture when the mesh size is larger than the process zone size, only the continuum damage model is able to limit the crack speed over a range of applied loads. Numerical simulations of straight-running cracks demonstrate good agreement between the theoretical predictions of the combined models and experimental data on dynamic crack propagation in brittle materials. Simulations that model crack branching are also presented.     

裂尖具线性分布约束应力的运动裂纹模型及其解析解

以恒定速度运动的Griffith裂纹解析解为著名的Yoffe解。静止裂纹的条状屈服模型即Dugdale模型,将其推广到运动裂纹模型时发现,当裂纹运动速度跨越Rayliegh波速时,裂纹张开位移COD趋于(∞,且表现为间断。通过在裂尖引入一个约束应力区及两个速度效应函数,假设约束应力为线性分布,采用复变函数方法,求得动态应力强度因子SIF与裂纹张开位移COD的解析解。新的结果,在Rayleigh波速下裂纹张开位移连续且为有限值。给出裂纹张开位移的一些数值结果,获得了一些有意义的结论。     

The stress intensity factor due to normal impact loading of the faces of a crack

The plane strain problem of a half-plane crack in an unbounded elastic solid is considered. The faces of the crack are subjected to suddenly applied, equal but opposite concentrated normal forces which tend to separate the crack faces. The elastic wave propagation problem, which contains a characteristic length, is solved exactly by linear superposition over a fundamental solution arising from a particular problem in the dynamic theory of elastic dislocations. Attention is focused on the time-dependent stress intensity factor. For an applied load with step function time dependence, the stress intensity factor is negative from the time the first wave arrives at the crack tip until the arrival of the Rayleigh wave. At that instant, it takes on its appropriate static value, which is thereafter maintained. Generalizations are discussed for spatially distributed and/or time-varying normal impact loads.     

CRACK SHAPE EVOLUTION STUDIES IN THREADED CONNECTIONS USING A.C.F.M.

Abstract— Crack shape evolution has been studied in high strength, rolled thread, bolts. It has been found that, using the Crack Microgauge, it is possible to detect and size fatigue cracks, located at the thread root, from depths of less than 0.1 mm up to fracture. This work required a special probe system to be developed and also a theoretical solution for the a.c. field distribution in a thread. Some of the work and results are briefly described in this technical note.     

Theoretical analysis of the transient response for a stationary inplane crack subjected to dynamic impact loading

The problem considered here is the plane strain response of an elastic solid containing a half plane crack subjected to suddenly applied concentrated point forces acting at a finite distance from the crack tip. Attention is focused on the time-dependent full field solutions in the transient process. It was found by Freund that at the instant that the Rayleigh wave reaches the crack tip, the stress intensity factor jumps to the appropriate static value. We find in this study that the full field stresses will approach to the appropriate static value upon arrival of the shear wave diffracted by the Rayleigh wave from the crack tip.     

宏微观双尺度运动裂纹模型面内拉伸下的解析解*

研究裂纹动态扩展中宏微观因素相互作用机制与微观裂尖区的钝化效应。平面拉伸状态下,宏观主裂纹以恒定速度运动。通过一个介观约束应力过渡区,将宏观主裂纹与微观裂尖区相连接,由此建立了一个宏微观双尺度运动裂纹模型。应用弹性动力学与复变函数理论,分别在宏观与微观尺度下对该模型进行解析求解,获得了解析解。通过裂纹张开位移从宏观到微观的连续性条件与宏微观应力场协调条件,将两个不同尺度下的解相耦合,获得了计算宏微观损伤区特征长度的显式表达式。研究表明,运动裂纹的宏观应力场仍具有通常的r&;#61485;1/2的奇异性。由于微观裂尖的钝化,微观应力场奇异性的阶次有所降低,与宏观应力场相比具有弱奇异性。双尺度运动裂纹模型中,可允许裂纹运动速度达到剪切波速,解除了经典运动裂纹理论中裂纹速度不能超过Rayleigh波速的限制。数值结果表明,介观损伤过渡区与裂尖微观损伤区尺寸,及裂纹张开位移等,与裂纹运动速度、材料性质、约束应力比、裂尖钝化角度等因素有关。     

II型III型动态裂纹尖端断裂过程区近似评估方法

动态裂纹尖端断裂过程区轮廓的确定问题仍然是一个没有得到完全解决的问题。基于弹性动力学的理论和复应力函数方法,提出一种伪应力函数方法,用于近似评估动态裂纹尖端应力场。通过与已知应力场计算结果对比,验证了伪应力函数的正确性。利用此近似方法通过Von Mises强度准则和Tresca强度准则,分别确定了不同强度准则条件下、不同裂纹扩展速度下断裂过程区的轮廓。计算结果表明:II型和III型动态裂纹尖端断裂过程区关于裂纹面对称分布,随着裂纹扩展速度增大而增大。当裂纹传播速度接近瑞利波速时,断裂过程区变化加剧。利用Tresca强度准则计算得到的动态裂纹尖端断裂过程区面积比利用Von Mises强度准则计算得到的断裂过程区的面积大。     

Numerical and experimental evaluation of SIF for threaded connectors

This paper presents a methodology for fatigue crack growth analysis in tubular threaded connectors. A solution for stress intensity factor for semi-elliptical surface cracks emanating from a thread root in a screw connector is also discussed in the paper. The solution is based on a mixed approach incorporating weight function and finite element methods. The weight functions used are the universal functions for cracks in mode I and these are linked with a thread through-thickness stress distribution obtained from finite element analysis to produce a stress intensity factor for a crack at the critical tooth of a thread. The resulting crack growth data are then validated experimentally.     

Failure analysis on fracture of worm gear connecting bolts

The worm gear connecting bolts of refueling machines of a nuclear power plant, with implementing standard of ANSI/ASME B18.3 and ASTM A574-08 and strength grade of 10.9, fractured at the thread neck position after running for about 10 years, and means such as macro examination, chemical compositions analysis, hardness testing, metallographic examination and fracture analysis, were used to analyze the fracture property and reasons of the bolts. The results show that the fracture of the bolts is due to two-way bending fatigue fracture. Surface decarburization of the bolts and stress concentration at the bolt thread neck decreased the fatigue strength of this position and resulted in the initiation of fatigue cracks. By comprehensive analysis and stress estimating, it was concluded that the main reason for fracture of the bolts is that there was a big gap between the bolts and the bolt holes, which resulted in fatigue fracture of the worm gear connecting bolts.     

层状场地脉动卓越频率对应的理论计算深度研究*

地脉动观测是一种简便有效的场地土层动力特性测试方法。本文依据场地脉动能量中瑞利波为主的特性,基于层状场地瑞利波弥散特征曲线,研究了不同场地条件下脉动卓越频率对应的瑞利波波长与场地自振频率理论计算深度之间的关系。计算结果表明：场地一定深度范围内土层剪切波速差异特性是其主要影响因素。如果土层波速差别较小或者某一波速占主导地位,则理论计算深度与瑞利波波长比值约为1/4a(a为瑞利波波速与剪切波速之比),与均匀场地结果接近;反之,土层波速差异越大,两者比值与1/4a偏离越远,且1/4a为比值上限。     

Failure analysis of vessel propeller bolts under fastening stress and cathode protection environment

This paper describes the failure analysis of propeller blade fastening bolts made from martensitic stainless steel 0Cr16Ni5Mo, which was ruptured under service of cathode protection for 5 years. The general crack pattern of the bolts, fractographic features, hydrogen content determination and slow tensile test results are all exhibiting the characteristics of hydrogen embrittlement. Accordingly, hydrogen diffusion driven by hydrogen concentration gradient and stress concentration was identified by experiment and finite element analysis (FEA). The morphology of the crack was intergranular of initiation from bolt cap root surface, and quasi-cleavage of propagation. The hydrogen distribution indicated that the hydrogen concentration in the bolt was in gradient distribution, and the region farther away from the sea water contains less hydrogen content. This revealed that hydrogen entered the bolt top surface through sea water under cathodic protection, and diffused from top to cap. The hydrogen content of the cap where crack initiated was 7.0 ppm, which was much higher than that in bolt shaft with normal content of 1.1 ppm. Results of low tensile test together with fractographic observation showed that the brittleness of the bolt was enhanced by the effect of hydrogen. Stress distribution calculated by FEA analysis indicated that the maximal stress of the bolt was about 1016 MPa, located at cap root surface which was consistent with crack initiation sites. The stress drove hydrogen to accumulate at root surface until cracking occurred. In a sum, the failure was attributed to the hydrogen diffusion, local high stress, and the martensitic microstructure susceptible to hydrogen embrittlement. Remedial measures such as avoiding over protection potential, that increase tempering temperature were suggested. Methods to optimize stress distribution of the bolt were also suggested based on FEA calculation.