Thread Root Research Articles

Effects of triaxial rolling on the microstructure and installation characteristics of reactor pressure vessel studs

Reactor pressure vessel (RPV) studs are key components of nuclear reactors, and their connection with flange ensures the sealing of the RPV under high-pressure and high-temperature conditions. In the present work, the external threads of the RPV stud were prepared by triaxial rolling, and the texture evolution of the external thread root material of an RPV stud was predicted by finite element analysis coupled with viscoplastic self-consistent simulations. The microstructure of the external thread root material of RPV stud was characterized by scanning electron microscope and electron back-scattered diffraction. The installation characteristics of the turned and rolled parts of the RPV stud were tested by installation and pretightening tests. It was found that the dynamic recrystallization at the external thread root formed ultrafine tempered sorbite grains, high-angle grain boundaries (47%), and strong {111} <110> and {111} <112> textures. In the installation and pretightening test, the residual elongation of rolled parts was reduced by 6% under the same loading pressure. The triaxial rolling process distributed the microstructure of the external thread root of the RPV stud in a gradient manner, resulting in improved stud installation characteristics.

Crack Detection Method for Wind Turbine Tower Bolts Using Ultrasonic Spiral Phased Array.

High-strength bolts are crucial load-bearing components of wind turbine towers. They are highly susceptible to fatigue cracks over long-term service and require timely detection. However, due to the structural complexity and hidden nature of the cracks in wind turbine tower bolts, the small size of the cracks, and their variable propagation directions, detection signals carrying crack information are often drowned out by dense thread signals. Existing non-destructive testing methods are unable to quickly and accurately characterize small cracks at the thread roots. Therefore, we propose an ultrasonic phased array element arrangement method based on the Fermat spiral array. This method can greatly increase the fill rate of the phased array with small element spacing while reducing the effects of grating and sidelobes, thereby achieving high-energy excitation and accurate imaging with the ultrasonic phased array. This has significant theoretical and engineering application value for ensuring the safe and reliable service of key wind turbine components and for promoting the technological development of the wind power industry.

Fracture in control rods of helicopters: Same failures but different reasons

This paper presents two incidences of failures of collective control rods which resulted in accidents to a particular type of helicopter. The control rods were made of 2024 Al-alloy tubes fitted with fork at one end and a spherical bearing at the other end (eye-end). In terms of physical appearance, the two failures in the control rods were identical to each other wherein there was fracture in the Al-alloy tube at the eye-end. Fractography study confirmed that in both the cases, multiple fatigue cracks had initiated over a sector at the internal thread roots on the Al-alloy tubes corresponding to 1st/2nd thread on the eye-end stud. After initiation, the cracks had propagated progressively into the tube-wall and then along the circumferential direction. As the crack propagation progressed, the load bearing capacity of the tubes diminished resulting in final overload fracture under the operational load at the time of accidents. Although failures in both the control rods looked alike in terms of failure location, fracture pattern and fracture mechanism, the primary reasons of failures were found to be different. Investigation revealed that premature fatigue crack initiation was due to non-uniform stress distribution on the internal threads of the control tube with increased level of stress over a sector. In one case, the non-uniform stress distribution resulted from thread manufacturing deficiency, and in the other case, it was due to incorrect assembly at the eye-end.

Load-carrying capacity of hole threads in TOBs bolted curved T-stubs to circular steel tube connections

This study investigates the load-carrying mechanism and proposes precise strength prediction methods for hole threads in Thread-fixed One-side Bolts bolted curved T-stubs to circular steel tube connections. A series of parametric analysis was performed with an advanced 3D finite element model considering critical parameters including tube wall thickness, bolt angle and bolt size. The axial load distribution along the hole threads and the shear stress distribution at the thread root were analyzed in detail. It was found that there were three typical load distribution patterns along hole threads, which were (1) linear pattern; (2) concave parabola pattern; and (3) convex parabola pattern, and the ratio of bolt stiffness to tube wall stiffness was identified as the pivotal factor affecting the axial load distribution. Meanwhile, the analysis results indicated that the bending moment on the TOBs did not affect the load distribution along hole threads, but led to a non-uniform stress distribution along the thread in the same turn. In the end, the prediction formulas for the carrying capacity of hole threads are presented by modifying Yamamoto’s equation based on the analysis findings. The formulas are validated against the finite element analysis results and previous test results. The prediction can reach the highest accuracy when adopting the proposed failure criterion 2, with average errors of − 0.001 and − 0.002, respectively, demonstrating the accuracy and reliability.

Failure Mechanism of Rear Drive Shaft in a Modified Pickup Truck

This paper investigates the failure mechanism of the rear drive shaft in a modified pickup truck which had operated for about 3000 km. The investigation included macroscopic and microscopic evaluation to document the morphologies of the fracture surface, measurement of the material composition, metallographic preparation and examination, mechanical testing, and finite element modelling and calculations. The results obtained suggest that rotation-bending fatigue was the primary cause of the drive shaft failure. The crack initiation is located in the root of the machined threads on the drive shaft surface and expanded along the side of the machining line surface. The main cause of fatigue cracks is attributable to a high stress concentration owing to a large unilateral bending impact under overload. Meanwhile, the bidirectional torsional force also produces a higher stress concentration and thus accelerates the fatigue crack to expand radially along the surface. Furthermore, the hardness of the central section of the drive shaft was marginally below standard. This deficiency results in harm to the bearings and other mechanical components, as well as expediting the enlargement of cracks. Finite element analysis revealed significant contact stress between the bearing and drive shaft, with stress levels exceeding the fatigue limit stress of the parent material. This highlights the need for reevaluation of the heat treatment process and vehicle loading quality to enhance the drive shaft’s longevity.

Is fatigue mechanism implicated in intraoral fracture of narrow dental implants? A thorough retrieval analysis of two failed implant fixtures retrieved from a single patient

PurposeThis study aimed to perform a thorough failure analysis of two fractured narrow dental implants after medium-term in vivo use. Materials and methodsThe top parts of two fractured Narrow Dental Implant (NDI) fixtures were retrieved from two different locations at two different times from the same patient. The NDI-specimen-1 was 12-months in service while the NDI-specimen-2 was 17-months in service. In both cases, the top parts of the fractured NDI fixtures that were attached to prosthetic components were retrieved and subjected to thorough, non-destructive and destructive testing. ResultsLight Microscopy (LM) and Scanning Electron Microscopy (SEM) revealed that both the retrieved fractured NDIs failed because of fatigue, characterized by beach and ratchet marks. Macroscopic examination revealed that fatigue cracks initiated at the internal thread surfaces of the implants and propagated around them until final fracture. Both samples fractured near the end of the retaining screw and followed the root of the internal thread. Optical and SEM analyses revealed a uniform distribution of irregularly shaped grains (diameter = 2 to 5 μm). X -ray Energy Dispersive Spectroscopy (EDS) analysis showed that the NDI-specimen-1 was made using Ti-14%Zr with a Vickers Hardens (HV) of 288 ± 5. ConclusionSince the fracture occurred by a fatigue; thus, an increase in fatigue resistance will be beneficial for the longevity of NDI.

Numerical and experimental failure analysis of wind turbine blade fastener

The objective of this study is to conduct a root cause failure analysis on wind turbine fasteners constructed from carbon steel Grade 10.9. The presence of corrosion pits at the thread root, as well as beach and ratchet marks on the fracture surface, suggests that the fastener failed due to a corrosion-fatigue mechanism. The microstructure of the failed fastener predominantly consisted of 98 % tempered martensite, with some bainite and a small amount of chromium carbides, which is characteristic of quenched and tempered low alloy steel. Magnetic particle testing revealed similar crack marks on other fasteners in similar locations, indicating the points of maximum stress in these fasteners. Furthermore, finite element analysis using Abaqus software demonstrated that for a corrosion pit to grow, it required a minimum pit length and depth of 0.5 mm and 0.6 mm, respectively.

Study on Erosion of Small-Size At-bit Azimuthal Gamma Geosteering Tool

Small-size at-bit azimuthal gamma geosteering tool is an important tool used in the process of old well sidetracking. It is significant to carry out numerical simulation analysis of erosion and life prediction to reduce development costs and excavate the remaining reservoir in old wells. By using the finite element software, we calculate and analyze the erosion characteristic of the drilling fluid flow and the sand content ratio of the geosteering tool. These results demonstrate that the water hole erosion rate is positively correlated to the flow rate and sand ratio respectively, and the maximal erosion rate all happens at the root of the female thread. When the drilling fluid flow rate is 14 L/s and the sand content ratio is 4%, its erosion life is about 743.2 h which satisfies the service life of downhole tools. The research results can provide theoretical guidance for configuring drilling fluid on-site, adjusting pump pressure, and prolonging the service life of tools.

Increasing the fatigue strength of threaded joints of oil and gas equipment by plastic deformation of the thread under high load before make-up

Uneven distribution of stresses in the thread can cause fatigue failure of the joint, usually in the area of the first roots of the thread. This problem is very relevant for shouldered threaded joints of oil and gas equipment, which operate under conditions of significant cyclic loads. One of the ways to equalize the stresses in the thread is plastic deformation of the thread with a high load before make-up. The influence of various methods of such pre-deformation of the tool joint of drill pipes and the connection of sucker rods on the fatigue life in the thread roots of the pin and box has been studied. A method for optimizing the load for pre-deformation was proposed and it was found that the optimal load can increase the fatigue life by 5 times. The study was carried out by finite element analysis of the axisymmetric model in Abaqus/CAE and calculation of the fatigue life from the Brown-Miller dependence using fe-safe.

Metallurgical evaluation and failure analysis of turbo expander Tie-bolt

Failure in a turbo-expander Tie-Bolt (stud) made of superalloy A-286 was investigated. The evaluation of the chemical composition and tensile properties confirmed compliance with the relevant standard. However, the coarsening of γ′ due to over aging condition, the non-uniform grain size and banded coarse MC carbide particles, the segregation of phosphorus to grain boundaries and the growth of carbide particles at the grain boundaries led to a decrease in fatigue strength of the alloy. Fractography studies showed that fracture occurred due to primary axial and secondary bending loads. Multiple ratchets and tire tracks on the fracture surface were consistent with low-cycle fatigue (LCF) failure mechanism. Fatigue striations spacing and micro hardness profile of the threads were used to calculate the stress amplitude and estimate the fatigue strength of Tie-bolt. Neuber’s rule was used to estimate the local stress and strain in the thread root. The calculation results showed that the failure occurred due to the application of cyclic loads at a stress level lower than fatigue strength of the stud. A large amount of sulfur detected at the multiple cracks initiation areas by EDS analysis, indicated the presence of H2S in the environment, which has caused cathodic hydrogen charge and reducedthe steel ductility and accelerated fatigue crack nucleation and growth.

Investigation on Flexural Fracture Behaviour of Bolted Spherical Joints with Crack Propagation in Screw Threads.

Bolted spherical joints, due to their prominent merits in installation, have been widely used in modern spatial structures. Despite significant research, there is a lack of understanding of their flexural fracture behaviour, which is important for the catastrophe prevention of the whole structure. Given the recent development to fill this knowledge gap, it is the objective of this paper to experimentally investigate the flexural bending capacity of the overall fracture section featured by a heightened neutral axis and fracture behaviour related to variable crack depth in screw threads. Accordingly, two full-scale bolted spherical joints with different bolt diameters were evaluated under three-point bending. The fracture behaviour of bolted spherical joints is first revealed with respect to typical stress distribution and fracture mode. A new theoretical flexural bending capacity expression for the fracture section with a heightened neutral axis is proposed and validated. A numerical model is then developed to estimate the stress amplification and stress intensity factors related to the crack opening (mode-I) fracture for the screw threads of these joints. The model is validated against the theoretical solutions of the thread-tooth-root model. The maximum stress of the screw thread is shown to take place at the same location as the test bolted sphere, while its magnitude can be greatly reduced with an increased thread root radius and flank angle. Finally, different design variants related to threads that have influences on the SIFs are compared, and the moderate steepness of the flank thread has been found to be efficient in reducing the joint fracture. The research findings could thus be beneficial for further improving the fracture resistance of bolted spherical joints.

Study of fatigue damage of pumping rods based on finite element simulation

This study performed in-depth analysis of onsite fatigue damage and stress distribution in pumping rods. Two aspects of fatigue damage were analyzed: macroscopic morphology and chemical properties. In terms of chemical properties, the crystalline phase composition and hardness of the product at fatigue damage were analyzed; the stress distribution was analyzed in term so of the rod-body stress and the connection-section stress. The cross-sectional characteristics of the fatigue crack expansion were summarized, and the types of fatigue fracture and the influencing factors of the pumping rod were obtained from these cross-sectional characteristics. Finally, modeling and stress analysis of the pumping rod were performed using SolidWorks and ABAQUS software. By comparing the stress cloud diagrams of different thread root shapes, the factors that cause fracture in the pumping rod and the locations of stress concentrations and dangerous cross-sections of the rod were determined. The highest principal stresses were obtained at the rod body near the upsetting flange of the pumping rod, and fatigue damage was the most likely to occur at this location. The shoulder of the unloading groove and the upsetting flange area were relatively safe because of their large cross-sectional area and less likelihood to produce stress concentrations. The results of this study can provide scientific guidance and reference for the development of pumping rods for efficient oil production and the improvement of oil and gas production efficiency.

Impact of technological and operational deviations of the thread flank angle on the fatigue life of the tool joint

The total effect of technological and operational factors can lead to various types of deviations of the flank angle of the tool joint thread profile, which are uneven along the axis. It is shown that uneven deviations of the flank angle appear when tapered threading with a lathe tool with a non-zero rake angle. To analyse the effect of these deviations on the fatigue strength of the joint, a parametric axisymmetric finite element model of the ZN-80 tool joint was developed in Abaqus/CAE. In the fe-safe program, dependences of the fatigue life in the thread roots on the deviations of the flank angle of the pin thread were obtained. The box thread has not been changed. To increase the fatigue strength of the pin, in most of the cases considered, negative deviations of the flank angle of the short flank of the pin thread should be avoided, and preference should be given to positive ones. Negative relative deviations of both flanks usually worsen the fatigue strength of the pin due to the "wedge" effect. Types of deviations have been found that significantly increase fatigue strength and have an “anti-wedge” effect or equalize the load in the thread. They need detailed studies and can be the basis for optimizing the values of flank angle deviations on each turn.

Failure analysis of fractured dental implants

The success and predictability of titanium implants over long periods of time are well established, and there has been a tremendous increase in implant popularity among patients and clinicians over the last four decades. However, complications can occur, resulting in the loss of both the implant and the prosthesis. Dental implant fracture is uncommon; however, implants or abutment screws can fracture and cause significant problems for both the clinician and the patient. Improper design, overload, fatigue, and corrosion are all potential causes of implant fracture. Six retrieved fractured dental implants of varying diameter and thread design were collected on a regular basis to characterize their fracture behavior by SEM and assess the fracture mechanism. The majority of the implants were fractured as a result of fatigue crack initiation and propagation from the thread roots.

Prediction method based on strain monitoring for thread root crack in high pressure hydraulic fracturing manifold

There is a high stress concentration at the connection threads of high-pressure manifolds. Under the action of internal pressure(Pint) and rapid vibration, the root of the thread is prone to fatigue cracking. To study the variation in the damage of a high-pressure manifold with connection threading, this paper first analyzed the strain of the manifold with different Pint and with or without defects at different positions on the outer wall shaft. The potential fatigue crack damage of a high-pressure manifold of a hydraulic fracturing truck was monitored and analyzed by strain testing technology. The variation of the strain signals of defected and defect-free high-pressure pipe fittings were compared. The damage stress concentration point of the connection thread of the high-pressure manifold was analyzed. When there is a crack at the root of the thread, results show that there is a large difference in strain in the axial direction of the crack compared to the strain in threading thread without a crack. Meanwhile, the strain concentration coefficients of different stress concentration points change regularly. In this paper, the damage state of the thread is analyzed by strain monitoring. The results can provide a theoretical and practical basis for the prediction of root cracks in connection threads.

Loosening and fracture behavior of hybrid titanium-to-steel threaded connection under cyclic loading condition

Hybrid threaded connection involving titanium alloy and conventional steel components is introduced and studied in the face of lightweight metals application trend in modern machinery design. Pre-tightening procedure and loosening test were conducted based on specially designed specimen regarding cyclic pulling load condition. The loosening and fracture behavior were intentionally investigated through both mechanical analysis and related microstructure observation. The loosening evolution tendency along with the corresponding low cycle fatigue process was obtained, which was then further related with the examined structural mechanical responses and thread wear mechanism. Results have shown that in the early stage of fatigue process the pretension would experience a rapid and mass reduction accompanied with a high rate cyclic hardening response of structure. The cyclic load induced thread wear occurred mainly in the titanium nut component, which involved abrasive wear, fatigue wear and oxidative wear mechanism. Besides, the thread root turn closest to the bearing surface was demonstrated as major position of stress concentration and strain accumulation, making it critical source for loosening initiation and fatigue fracture, which should be properly optimized in both designing and fabricating process.

Investigation and implementation for forming lead screw by through-feed rolling process with active rotation

A through-feed rolling process with active rotation for manufacturing lead screw has been developed, where the speed matching and stable motion between workpiece and the rolling die during the whole rolling process can be ensured. Its process control, die structure and rolling equipment are relatively simple, and the accuracy of pitch for rolled lead screw can be effectively ensured. The motion characteristic during through-feed rolling process was analyzed, and then the general design criteria for the die were set up. The feasibility of the through-feed rolling process with active rotation was verified by experiment and finite element method (FEM). The verified results indicated that the error of lead-screw pitch between experimental value and designed value is about 0.125 %, and the error between the FEM value and the experimental and designed values is less than 1 %. According to the deformation characteristics of lead screw rolling and the distribution of field variables, the workpiece can be divided into non-deformation zone, entry-angle zone (incremental forming zone), correction zone, exit-angle zone, and full-formed zone. The same position of workpiece will undergo those five forming states in turn. The FEM results indicated that the effective stress and effective strain both gradually increase from the entry-angel zone to the correction zone, and the strain in the exit-angle zone and full-deformed zone are almost quite; the effective stress and effective strain in thread root are larger, and both gradually decrease along thread flank to thread crest; the distributions of stress and strain have a certain periodicity along the axial direction. The results in this study provide a basis for high-efficiency and precise rolling forming of threaded parts with large diameter such as the lead screw.

Equivalent modeling for a finite element analysis of the mechanical properties of a bolted joint with a superelastic shape memory alloy

In this study, a beam element model with tension–compression asymmetry is constructed by using the improved Brinson model. A comparison between the calculated and experimental results verifies the computational effectiveness of the beam element. Then, a new mechanical property analysis model of a shape memory alloy (SMA) bolted joint based on the beam element model is proposed. Combined with a constructed convergence function, the mechanical equilibrium state of the SMA bolted joint under preload and external load is solved iteratively by the bisection method. Consequently, the stress–strain response curve of the SMA bolt rod and the root of the first thread, the deformation law of each thread and the separation conditions of the contact surface of members are analyzed by the proposed model. It is shown that the new proposed method can greatly improve the calculation efficiency with high calculation precision compared with the traditional calculation method.

In-depth residual stress analysis considering manufacturing process and cyclic loading of bolts

Although bolts are considered to be a well-established structural element, only general statements on the influence of the different manufacturing processes on the fatigue life are available. Detailed investigations of the in-depth residual stress path and the residual stress stability under cyclic loading have not yet been carried out. A challenge in determining the residual stresses is the complex geometry of the thread and the expected residual stress state with steep gradients close to the surface.This paper examines the influence of the manufacturing process of the main two process types on the residual stress state. The focus lies on the residual stress and material state of high tension bolts (HT) rolled before heat-treatment (RBHT) and rolled after heat-treatment (RAHT) both with hot-dip galvanization (tZn). To get detailed information about in-depth residual stresses, neutron diffraction is used. In addition, the residual stress stability under cyclic loading is examined.For the first time, in-depth residual stress distributions are determined on unloaded and loaded RAHT M 36 and RBHT M 24 bolts, tZn, grade 10.9. The results show that RAHT bolts in the unloaded state have compressive residual stresses below the surface in the thread root in the magnitude of −225 MPa. In the case of RBHT bolts, low residual stresses are present at the surface in the magnitude of −160 MPa. The different progression of the residual stresses is associated with the manufacturing process.Loaded RAHT and RBHT bolts exhibit different stress characteristics. In the loaded state, RAHT bolts have residual compressive stresses on the surface, while RBHT bolts have residual tensile stresses. This may be one reason for the significantly longer fatigue life of RAHT bolts compared to RBHT bolts.

Effect of manufacturing defects on fatigue life of high strength steel bolts for wind turbines

This work investigates the effect of manufacturing defects induced by thread rolling on the high-cycle fatigue life of M30 class 10.9 stud bolts. High-cycle fatigue tests of two batches of nominally identical bolts show significant differences in fatigue performance between the batches. Scanning electron microscopy characterisation of defects in the form of rolling-induced microcracks in the thread root reveals a clear correlation between the defect size and the fatigue life of the investigated bolts. It is demonstrated that initial cracks present in the stud threads have a considerable effect on the slope of S–N curves. Numerical fatigue analysis shows good agreement with the experimental data. Electron backscatter diffraction is used to establish a characteristic length of microstructurally short defects in the tempered martensite microstructure. The obtained results shed light on the definition of critical manufacturing defect sizes in bolts as an essential parameter for quality control in manufacturing process.