Fractured Bolts Research Articles

Fracture failure analysis of high-strength bolts in power plant steel structures

The high-strength bolts of a power plant steel structure fractured and failed in many places. To find the cause of the failure, this paper analyzed the failure reasons of fractured bolts through macroscopic inspection, chemical composition analysis, mechanical performance test, metallographic inspection, and fracture microscopic analysis. The results show that the main cause of bolt fracture is that improper heat treatment leads to a carburized layer on the surface of the bolt, which makes the surface of the bolt sensitive to hydrogen. At the same time, there is a certain amount of hydrogen in the material matrix, which leads to hydrogen accumulation and cracks in the bolt at the stress concentration. Eventually, the cracks propagate and lead to fracture failure.

CATASTROPHIC FAILURE INVESTIGATION OF THE OIL FILTER MACHINE BOLTS

An assessment of the fractured bolt of a cooking oil filter machine was performed. The objective of this assessment is to reveal the root cause of the failure. A series of necessary attempts were carried out for this analysis, i.e., site visit, fracture surface examination of the fractured bolt, collecting the maintenance and operation data, mechanical properties testing, chemical composition testing, and stress analysis. The fracture surface examination showed a catastrophic failure, especially at a repaired bolt. Meanwhile, according to the site visit investigation, some evident data indicated a sudden high-pressure load subjected to the filter machine. Most fractured filter bolts were repaired by welding. The design review shows that the critical operating pressure is around 14.5 bar, which is far above the reported operating pressure during the accident, which is 6 bar. This indicated that the strength of the genuine bolt is sufficient to ensure the operating pressure of the filter machine, which is normally 1 to 3.5 bar. Finally, it can be concluded that the root cause of the failure of the filter machine bolt is decreasing strength of the bolts because of improper repairment indicated by the welding process and assisted by sudden high load due to increasing pressure inside the filter vessel.

In-service failure of a support structure for traffic signaling

Traffic signals and their supporting structures are an essential complement in highways and local roadways all around the world. Their accurate operation is today a well-recognized issue by the respective transportation authorities, especially when located on major roadways because of exposure to fatigue cracking by wind-induced large-amplitude vibrations and severe environmental conditions or wind gusts resulting from the passing trucks. The present paper describes the research undertaken to analyze the fatigue failure of a bolted joint in an overhead porticoed structure for traffic signaling located close to a highway entrance to a tunnel. The structure had been in use for more than 25 years and the failure was preventively detected, during a maintenance inspection. The research is a forensic engineering analysis of failure involving optical and scanning electron microscopy observations of the in-service fractured bolts together with the experimental characterization of the fatigue behavior of the bolt steel by means of non-standardized fatigue tests. The failure analysis pointed to the existence of environmentally-induced fatigue initiators as the failure origin, the galvanized condition of the bolts and environment aggressivity gave rise to local dissolution and microcracking of the Zn-coating, damaging and weakening specific zones at the steel interface that acted as fatigue initiators.

Computer Vision‐Based Detection for Delayed Fracture of Bolts in Steel Bridges

The delayed fracture of high‐strength bolts occurs frequently in the bolt connections of long‐span steel bridges. This phenomenon can threaten the safety of structures and even lead to serious accidents in certain cases. However, the manual inspection commonly used in engineering to detect the fractured bolts is time‐consuming and inconvenient. Therefore, a computer vision‐based inspection approach is proposed in this paper to rapidly and automatically detect the fractured bolts. The proposed approach is realized by a convolutional neural network‐ (CNN‐) based deep learning algorithm, the third version of You Only Look Once (YOLOv3). A challenge for the detector training using YOLOv3 is that only limited amounts of images of the fractured bolts are available in practice. To address this challenge, five data augmentation methods are introduced to produce more labeled images, including brightness transformation, Gaussian blur, flipping, perspective transformation, and scaling. Six YOLOv3 neural networks are trained using six different augmented training sets, and then, the performance of each detector is tested on the same testing set to compare the effectiveness of different augmentation methods. The highest average precision (AP) of the trained detectors is 89.14% when the intersection over union (IOU) threshold is set to 0.5. The practicality and robustness of the proposed method are further demonstrated on images that were never used in the training and testing of the detector. The results demonstrate that the proposed method can quickly and automatically detect the delayed fracture of high‐strength bolts.

DETERMINATION OF THE CAUSES OF DESTRUCTION OF HIGH-STRENGTH FASTENING ELEMENTS

Intensively operated modern technology requires the use of high-strength fasteners. The article presents the results of studies of the structure and physical and mechanical properties of the material of fastening elements (threaded hardware) in order to determine the causes of their destruction during operation. The fractographic analysis of fractures of fractured bolts revealed pronounced centers of crack initiation with traces of delayed brittle fracture, which is typical for high-strength hardware that are under high load for a long time, close to the yield point. Mechanical tests of the studied fastening elements showed increased values of ultimate strength. The performed relaxation tests of the material of broken bolts also revealed an increased value of the microplasticity limit, which is responsible for the onset of plastic deformation processes in microvolumes of the material, and for the material of whole hardware, the value of the limit is included in the permissible interval. At the same time, the average hardness of the material of all products meets the regulatory requirements. The maximum spread of hardness 70 HB is fixed on the destroyed bolt. Using the experimental values of microhardness, theoretical estimates of the coefficient of plasticity, which characterize the ability of a material to perceive elastic and plastic deformations, were carried out. To ensure a sufficient level of ductility of a long-term working metal material, the ductility coefficient must be at least 0.8. The average value of the hardness of the material of the fastening elements satisfies this criterion for assessing ductility. According to the results of a step-by-step analysis of the microstructure of the samples, the inhomogeneity of the distribution of the carbide phase over the cross-section of the bolts was established. The two-phase composition (a-Fe + Fe3C) of steel was established by X-ray structural analysis, which confirms the results of microstructural studies. It should be noted that the phase composition of all studied samples is identical. Based on the results of the studies carried out, it was found that the destruction occurred due to the reduced strength and increased fragility of the material due to the presence of microdefects. The material of the whole bolts in terms of structure and mechanical properties comply with regulatory requirements.

The Interface Friction in The Friction-Type Bolted Joint of Steel Truss Bridge: Case Study

The friction-type bolted joint transfers the internal forces in the structural members by interface friction, but noticeable seasonal temperature and bolt fracture cause the redistribution of interface friction and threaten the joint safety. Therefore, this study carried out finite element analysis on the interface friction considering the influence of seasonal temperature and bolt fracture. Through finite element analysis, the simulation of interface friction under seasonal temperature revealed the distribution of temperature induced interface friction in different areas and locations. Further simulation of fractured bolts revealed the influence of quantity and location of fractured bolts on the redistribution of interface friction. Finally, the interface frictions in the bolted joint were evaluated using limit state equations. The results showed that: 1) the quantity and location of fractured bolts cause obvious redistribution of interface friction in the bolt-fractured areas; 2) the quantity and location of fractured bolts have slight effect on the total interface friction in the whole splice plate; 3) the reduced interface friction in the bolt-fractured areas was transferred to the areas without bolt fracture, producing little change in the total interface friction; 4) all the splice plates had abundant safety margin after analysis of their limit state equations.

The redistribution of temperature-induced interface friction in friction-type high-strength bolted joint after bolt fracture

For the friction-type high-strength bolted joint, the bolt fracture can cause redistribution of interface friction, which will seriously endanger the connection safety of structural members. However, current study scarcely focuses on the redistribution of interface friction after bolt fracture. Therefore, this paper will specifically carry out finite element analysis on the redistribution of interface friction caused by fractured bolts. Firstly, the refined finite element model of friction-type high-strength bolted joint is used to investigate the variation of interface friction with uniform temperature and the distribution of interface friction in different areas. Furthermore, stochastic finite element method is introduced to explain how the quantity and location of fractured bolts influence the redistribution of interface friction. Finally, the mathematical model of friction redistribution is built to describe the redistribution of temperature-induced interface friction in the friction-type high-strength bolted joint after bolt fracture. After validation, the mathematical model can well describe the redistribution of interface friction caused by fractured bolts.

Fracture analysis of the circulating pump bolts

The microstructure, micrograph of the fracture sections and the alloy elements distribution of the fractured flange-connecting bolts of 3# circulating pump in a power plant were analyzed by the scanning electron microscopy, the optical microscopy and the electron probe microanalysis. The results showed that the failed bolts’ material is not the design material, and failure of the bolts is due to the stress corrosion crack propagation of the bolts’ austenitic material by chloride ions in seawater.

Failure analysis study on a fractured bolt

The aim of this work is the determination of the cause of a bolt fracture. The group was formed by a screw and a castellated self-braking nut with a plastic brake. Chemical, mechanical and microstructural material characterizations were carried out, fulfilling all material specifications. A microscopic observation revealed fatigue striations, which are typical signs of a progressive fracture. The fracture has been produced in the penultimate thread, one of the most critical zones of the screw. Hydrogen embrittlement also could have occurred, decreasing the fatigue resistance of the screw, but also its life of service.

Failure Analysis of a MW-Class Wind Turbine Blade Root Bolt

Failure analysis of a fractured high strength bolt used in a MW Class wind turbine blade connection system was conducted to clarify the failure cause and to determine the prevention method. Applying detailed mechanical characteristics and metallurgical investigations, the bolts failure cause can be attributed to bending fatigue. Evidenced by the nut polished markings, insufficient tightening was considered as the important reason for the bolt failure. The thread microstructure indicated that the fractured bolt thread was rolled rather than machined. However, rolling induced cracks locating on the thread crest and non-uniformly distributed decarburized layer on the bolt thread root existed. Thus, it can be concluded that the bolt failure was caused by the insufficient tightening and improper manufacturing procedure.

Failure analysis on premature fracture of anchor bolts in seawater booster pump of nuclear power plant

Abstract Three of the four TP321 stainless steel anchor bolts used in an indoor seawater booster pump of a nuclear power plant were encountered with premature fracture during service. A comprehensive analysis was performed to investigate the operational conditions, base materials and macro/micro morphologies of the failed bolts to identify the failure causes. The results showed that a collection of banded inclusions was distributed among the grains by metallographic examination, and typical pitting holes were observed and high levels of chlorides were detected on the surfaces of the failed bolts' fractures by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). In addition, branched cracks were found on the axial sections of the fractured bolts. Given all that, the premature fracture of the anchor bolts was determined to be mainly caused by stress corrosion cracking (SCC) from chlorides, which was then proved to be induced by splashing of the seawater during routine maintenance of the pump. Besides, the sequence of fracture for these three anchor bolts was ascertained by fractography from three aspects, including shapes of fractures, morphologies of corrosion products and chemical compositions of corrosion products. Finally, countermeasures were put forward as well.

Failure investigation of in-flight separation of afterburner nozzle flap in a turbojet engine

Abstract A turbojet fighter experienced a separation of the afterburner nozzle flap during flight. Three mounting bolts were intended to fasten the nozzle flap assembly, but complete fracture of these bolts was the primary reason of failure. One of them was lost during the incident and two shank parts of the mounting bolts remained in the engine component. Laboratory investigation of the failed components of the nozzle flap assembly indicated that the mounting bolts were improperly installed during the last maintenance. Detailed fractographic study and metallurgical analysis focused on the fractured bolts and revealed fatigue striations with an overload dimple morphology in all fractured surfaces of the bolts. The investigation established sufficient evidence of improper installation of the primary flap assembly due to deformation of the alignment hole which inevitably resulted in higher operational loads on the bolt. It is concluded that the missing bolt had failed first, and other bolts fractured in succession in the manner of fatigue fracture. In response, preventive measures regarding inspection strategy have been improved to avoid similar failures.

Austenitic Stainless Steel Bolt Failure by Stress Corrosion Cracking

AbstractDuring a minor inspection of a heavy-duty gas turbine engine, the hexagonal head of a bolt used for fixing a metal plate in the air intake was found on the floor of the air intake casing [1, 2]. The respective washer was not found. It is believed to have been ingested into the engine, having caused moderate compressor damage. The inspection was started after 83.500 operating hours. Only one fractured bolt was found. The subject bolt fractured due to stress corrosion cracking (SCC), induced by pitting corrosion and possibly assisted by crevice corrosion.

Material analysis of fractured bolts in Ayeyarwady Bridge (Yadanarbon), Myanmar

Material analysis of fractured bolts in Ayeyarwady Bridge (Yadanarbon), Myanmar

Fracture Failure of Bolts Assembled in Tuning-Wheel of a Marine Diesel Engine

The bolts assembled on a tuning-wheel of a marine diesel engine were found fractured and loose in service of only 70 days. The damage and fractography features of the bolts were investigated to identify the failure sequence and the failure causes or modes. The crack origins were from the diameter surface of the bolts, and reverse bending and rotating bending fatigue fractures were the dominant failure modes. Various manufacturing defects, such as tight radii of transition fillet between the shanks and the threaded sections of bolts, poor surface roughness, and machined threads rather than cold-rolled threads, were found on the fractured bolts, which created a high stress concentration. A few bolts were loose in the first place during service, the overall stress state of the bolts changed to lead to a non-uniform distribution of load on the bolts. Some bolts had to sustain a great load, which was sufficient to cause fatigue crack initiation at the defective regions of bolts in such a short time period. Unsuitable installation procedures allowed for the possibility of low preload on the bolts leading to bolt loosening when subjected to vibrational loads during service. Additionally, bolt loosening could occur due to severe loading conditions. The bolts on the wheel failed in succession due to poor bolt manufacture and installation.

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.

Failure analysis of submersible pump system collapse caused by assembly bolt crack propagation by stress corrosion cracking

Mechanically fastened joints using bolts are critical components in submersible pump systems. These bolts are subjected to aggressive environments in oil wells. Tightening torque preload and motor's weight are the principal loads that bolts support mechanically plus an occasional pipe flexure. Additionally, a corrosive sulfide-rich water environment presents an extremely demanding chemical condition. A better understanding of those failure mechanisms affecting such components could provide more safety, as well as costs and time saving during the operation of pump systems in wells.An assembly bolt from a submersible pump was fractured during service. Failure was the result of stress corrosion cracking (SCC) originated by pit corrosion. Mechanical and optical tests were performed to identify property changes. SEM with EDS, XRF and OES analyses were used to characterize the material and crack propagation. The fractured bolt material specification was medium carbon steel, while the material specified by the manufacturer was Ni–Cu alloy. The origin of the crack was located on a stress concentration region, but its nucleation was a result of high corrosive conditions.

Magnetic Memory Testing and Intensity Failure Analysis of Fractured Bolt of Cylinder Head on Hydrogen Gas Compressor

Along with the wide application of pressure energy, large scale reciprocating compressor unit is becoming one of the necessary key equipments in petrochemical industry. The reciprocating compressor has complicated structure and it is very difficult to carry out measurement & analysis to the operation status of various components. Above all, as the most important fastener of the compressor, once fracture of cylinder head bolt of the reciprocating compressor occurs, major disaster will occur. Metal magnetic memory testing technology is a nondestructive examination technology based on magneto-mechanical effect principle. Specific change of leakage magnetic field will occur in stress & deformation concentrated region under combined action of alternate work load and geomagnetic field, macroscopic cracking can be detected and stress concentration and early state damage can be evaluated by utilizing this specific change, We carried out on-site examination on the fractured bolt of cylinder head of hydrogen gas compressor unit in refinery of certain petrochemical corporation with latest metal magnetic memory testing technology. On the basis of intensity analysis and dynamic load calculation to cylinder head bolt of the reciprocating compressor, we analyzed the cause of bolt fracture by using metal magnetic memory testing technology and drew out the magnetic memory signal distribution characteristics of the bolt and its fracture. We verified the correctness of testing result of the metal magnetic memory technology on cylinder head bolt and its fracture with metallographic examination technology; furthermore, we studied the reliability of bolt fatigue crack & fracture analysis and investigation of accident cause by using magnetic memory characteristic distribution signal.

Failure analysis of a pressure vessel bolt in a nuclear fuel fretting wear simulator

This paper presents a failure analysis on a pressure vessel bolt of a fretting wear simulator. After 500 h tests, in an upper pressure vessel of a fretting wear simulator, one bolt among eight was fractured near the bolt neck regions. The fracture surface was examined by using a scanning electron microscope (SEM) to determine the failure initiation and failure mode. The result indicates that the fracture surface shows intergranular fracture features. Based on the mechanical property data of a bolt material, it is concluded that the exerted stress on the bolt applied by an internal pressure of the pressure vessel has a negligible effect on the major failure causes. In order to verify the mechanical properties of the fractured bolt, tensile test has been performed and its result was compared with material specification. As a result, it is thought that both excess heat treatment during the surface hardening procedure and loose parts in the thread hole have significant effects on the pressure vessel bolt failure. In this paper, the reasons for this failure were discussed by using metallographic studies of the failure surface, mechanical tests with the failed bolt and the stress distribution of the contact regions with considering loose parts by using FE analysis.

Failure analysis and risk management of a collapsed large wind turbine tower

Developing renewable energy is crucial as nations face the twin threats of global warming and a reduction in energy supplies. Wind turbines are one of the most promising sources of renewable energy in Taiwan. However, on September 28, 2008, Typhoon Jangmi struck Taiwan, bringing strong winds and heavy rainfall that collapsed a wind turbine tower located on the shore of Taichung Harbor. This study provides significant insights into, and lessons learned from, post-disaster inspection into the causes of tower failure during this typhoon. This event represented the first time that a wind turbine in Taiwan that had to be reconstructed after collapsing. To prevent similar accidents, the likely causal mechanisms are examined from the risk management perspective. Data for case analysis are collected from original tower design reports, the tower design code, construction records and documents, historical wind-speed data, structural tower analysis, and intact and fractured bolt material tests. Furthermore, similar accidents in other countries and their causes are reviewed to identify potential risk factors affecting the lifecycle of wind turbines.