Bolt Fracture Research Articles

Fracture simulation of a demountable steel-concrete bolted connector in push-out tests

Many bolted connectors were proposed for use in steel-concrete composite structures over the past few decades. This is because using bolted connectors could make the assembly and disassembly of steel-concrete composite structures more convenient and reusing the dismantled structural components could also improve structural sustainability. Lots of static and fatigue experimental tests on steel-concrete bolted connectors were conducted but detailed finite element simulations including fracture of bolted connectors were rarely reported. This paper presents a series of simulations of a demountable steel-concrete bolted connector in push-out tests, which was proposed and reported by the authors of this paper before. Damage models of bolt and concrete materials are incorporated in the simulations to better understand the behaviour of the bolted connector and the failure mode of the push-out tests. Direct tension tests on bolts are simulated to calibrate the stress-strain relationship of bolt material and assumed pure shear tests on bolts are modelled to validate the fracture criterion used in the simulations. Results of modelling push-out tests indicate that the friction force at the steel-concrete interface accounts for a part of shear resistance of the bolted connector in push-out tests conducted. The load-slip curves and the fracture of bolts in push-out tests can be approximately predicted by incorporating damage models of bolt and concrete materials and considering an appropriate friction coefficient. The effects of concrete damage model, clearance in bolt hole, and pretension of short bolt on the shear performance of the bolted connector are discussed at last.

Fire resistance behavior of T-stub joint components under transient heat transfer conditions

The fire resistance behavior of T-stub joint components at elevated temperatures was experimentally and numerically investigated. To start, twelve T-stub specimens with different geometric properties were tested to failure under the transient heat transfer condition. The failure modes of these specimens were classified into three categories: complete yielding of the flange, pure bolt fracture failure, and the combination of the flange yielding and bolt fracture. The displacement-temperature curves obtained from the experiment were idealized as three segments, and the associated critical temperature, ultimate temperature, and deformation capacity were evaluated. Then the finite-element (FE) models of T-stubs were developed to simulate their responses under the transient heat transfer condition using ABAQUS (version 6.18). The comparison of critical temperature, failure temperature, and deformation capacity indicated that the FE analysis was in good agreement with the experimental results. A comprehensive parametric study was subsequently carried out to quantify the effects of various geometric parameters and load ratios on the response of the T-stubs under fire conditions. Meanwhile, a comparative assessment between the transient and steady analyses proves the reliability of the transient test method. Based on the data collected from the experiment and FE analysis, three analytical models (including the Spyrou’s model, the Heidarpour’s model, and the one recommended by Eurocode 3-1-8) were critically examined to reveal their performance in estimating the ultimate temperature and deformation capacity of T-stubs.

Evaluation of the capacity of steel special bolted moment frame connections subjected to simulated column removal

The eight bolted stiffened extended endplate (8ES) and bolted flange plate (BFP) moment connection are two common rigid joints that are widely used in special steel moment frames (moment resisting frames with high ductility). In this study, to investigate these connections' behaviour and capacity, first, verification of finite element assumptions was carried out using the Abaqus software. Then, based on the verification models, by changing different parameters, 17 specimens were designed following the AISC Manual using two beams and three columns to determine the maximum capacity values in a column loss scenario. The finite element analysis represents that the 8ES connection's failure begins near the joint area and at the vicinity of the rib stiffener, and the BFP connections provided no warning of failure before brittle fracture of bolts at the large displacement due to catenary action mode. The plastic equivalent strain, Von-Mises stress, and middle column force–displacement curve were discussed. Finally, a formula is presented based on the numerical results to determine the displacement at the maximum capacity of these connections, a formula is presented based on the numerical results using the Least Square Method (LSM). Finally, the modification factors are presented for the theoretical formula to determine these connections' maximum capacity under column removal demands.

Push-out tests on novel enhanced C-channels in steel-concrete-steel sandwich structures using ultra-high performance concrete

This paper firstly developed steel-UHPC-steel sandwich structures (SUSSSs) with new enhanced C-channels (ECs). Shear behaviours of ECs embedded in ultra-high performance concrete (UHPC) were firstly investigated by a testing program with 14 specimens. The test results primarily reported the failure mode, shear strength, and shear-slip behaviours of ECs in UHPC. The ECs in UHPC exhibited a successive shear fracture of external bolts and C-channels, and a ductile shear-slip behaviour with a maximum slip capacity exceeding 6 mm. These test results also revealed the effects of length, height, and layout of the C-channel, and concrete core strength on shear behaviour of ECs. Theoretical models were also developed to describe the load-slip behaviour and predict ultimate shear resistances of novel EC connectors in UHPC. The validations against reported tests exhibited that the modified code equations greatly overestimated ultimate shear resistances of ECs, and the developed theoretical models predicted well the shear-slip behaviour and ultimate shear resistances of ECs in UHPC. The average prediction discrepancy for the developed theoretical model is only 5% with a small standard deviation of 0.06 for the 12 reported tests.

Finite element analysis on enhanced C-channel connectors in SCS sandwich composite structures

A finite element model (FEM) was firstly developed for new enhanced C-channel connectors (ECs) in steel-concrete-steel sandwich structures (SCSSSs). This developed FEM used the continuum plasticity model (CPM) to simulate the shear fracture of C-channels and bolts used in ECs. Moreover, the FEM also considered the nonlinearities of geometry and mechanical properties of materials as well as complex interactions among different components in the ECs, e.g., channel-concrete, concrete-faceplate, and bolt-faceplate interactions. The FEM was validated by 16 push-out tests on ECs. The validations proved that the developed FEM could simulate well shear behaviours of ECs including shear-slip behaviours, failure modes, and ultimate shear resistances. With this validated FEM, a 51-case parametric study was performed to make an in-depth study on key influencing parameters on shear behaviours of ECs, which included grade and diameter of bolts, gap of bolt hole, prestressing force ratio on bolt, length of C-channel, and grade of concrete. This paper also developed analytical models to estimate the shear strength and shear-slip behaviours of ECs. The validations against test and FE simulations results confirmed the accuracies of these analytical models. Design recommendations were finally given based on these test results and FE parametric studies.

Study on Mechanical Properties and Control Technology of Surrounding Rock in the Fracture Zone of a Roadway

We aim at the problem of the large deformation and difficult control of surrounding rock when passing through a fault fracture zone in the centralized rail transportation lane along the south wing of Xinyi Coal Mine; the stress environment and failure mechanism of surrounding rock are analyzed through field investigation, numerical simulation, and field industrial test. The instability of the surrounding rock in the fault fracture zone was considered to be the result of the joint effect of the surrounding rock fracture development, lithology differences, water gushing occurrences, low strength of the original support, high in situ stress, and fault-related tectonic stress. Rock blocks are collected on site at the fracture zone, and the remoulded samples are prepared for mechanical experiments in the laboratory. The basic mechanical parameters of the roadway passing through silty mudstone, sand-mudstone interlayer, and fine sandstone were analyzed. A three-dimensional model is established to analyze the distributions of the stress, deformation, and plastic area in the surrounding rock mass after the tunnel passes through, considering both a single-rock mass and a multilayer-rock mass. Based on the above analysis, the “closed support + shotcrete + grouting + anchor mesh cable coupling support” is proposed. Three stations were arranged on site to observe the mine pressure, and the field industrial test shows that, within the 100 days of observation, the maximum roof-to-floor convergence is 38 mm, while the maximum horizontal convergence is 56 mm. The overall reinforcement effect of the roadway is good, the surface is smooth, and there is no phenomenon of concrete cracking and bolt fracture.

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.

Fracture Failure Analysis on a Bolt

A bolt used in the production equipment broke during the use. This paper analyzes the root cause of bolt fracture. Through the analysis on the chemical composition, mechanical properties, microstructure, micro morphology and micro area composition of the broken bolt, it is considered that the physical and chemical properties of the bolt meet the requirements of the standard. Because of the existence of machining cracks, the bolt is affected by the stress in the use process, resulting in fatigue fracture.

Fracture simulation of partially threaded bolts under tensile loading

Failure of bolts exposed to tension is generally avoided in the design of bolted connections due to the smaller deformation capacity of bolts than the connected plates. This is one of the reasons why few studies focus on the tensile failure behaviour of bolts. However, failure behaviour of bolts is essential for the advanced finite element analysis especially relevant to the deformation capacity and failure mode of bolted connections. This paper presents a numerical study on the fracture of partially threaded bolts under tension incorporating damage models, with which the failure mechanism of bolts can be better understood. The post-necking stress–strain relation is firstly calibrated to describe the behaviour of bolt threaded parts at large deformation. Then, direct tension tests on partially threaded bolts with different threaded lengths within the grip are modelled using ABAQUS with the explicit solver. Two criteria for fracture are investigated: the void growth model (VGM) and a model proposed by Bao and Wierzbicki (BW). The former is adopted to simulate the tensile fracture of bolts and the latter is used to predict the thread stripping failure. Results indicate that bolt failure modes of tensile fracture and thread stripping can be well predicted by combining the calibrated post-necking stress–strain relation and a suitable fracture criterion in the analysis. It is revealed that the large plastic strain in the threads introduced by thread rolling process is a major reason for the thread stripping failure.

Tensile bearing capacity for bolted spherical joints with different screwing depths of high-strength bolts

Aiming at evaluating the reliability of the high-strength bolts’ fake tightened-up phenomena of the bolted spherical joints in a spatial grid structure, this research studied the tensile strength, stiffness, and failure modes of the bolted spherical joints with different bolt types and screwing depths using a combined method of the lab experiments and numerical simulations. A total of 40 axial tension tests of the bolted spherical joints were conducted with five different bolt types and eight different screwing depths. The test results showed that the bolted spherical joints with the different bolt screwing depths of high strength bolts suffered three failure modes, namely, the thread failure, bolt fracture failure, and sphere rupture failure. Validated by its feasibility and accuracy, the proposed numerical simulation method of the bolted spherical joints can estimate the ultimate strengths and failure modes, as well as the detailed stress distribution and failure development process. According to the test and numerical analysis results, a group of calculation formulas was developed for predicting the tensile strength of bolted spherical joints with different bolt screwing depths.

A Detailed Failure Analysis of Vertically Suspended Diffuser Pump Caused by Hex Head Bolt Fracture: The Marine Service Condition

Vertically suspended diffuser pumps play a significant role in the seawater cooling systems of the petrochemical and refinery industries. The present work deals with providing a detailed analysis of mechanisms involved in the early failure of hex head bolt used in the vertically suspended diffuser pump framework, which has served three years in the marine condition. The morphological and microstructural characteristics of the failed bolt were evaluated by optical microscope, scanning electron microscope (SEM), spark-optical emission spectroscopy (spark-OES), and X-ray diffraction (XRD) methods. Besides, microhardness test was performed to assess the mechanical properties of the samples. The results demonstrated the presence of Cr, Fe, Mn, and Si elements in the microstructure of failed bolt, which coincides with the composition of stainless steel (X5CrMnN17-8). While there exist sulfur and oxide inclusions in the parts close to the surface, no surface-related defects were detected in the central part. Moreover, the microstructural assay showed that the failed sample was composed of austenite-martensite micro-duplex structure. The microhardness values of regions close to the surface of the failed bolt were higher than that of the central part by ≈20 %, since there is higher martensite content. Fractography images revealed a brittle fracture mechanism in the parts close to the surface, while ductile fracture was observed at the center of the bolt. In general, the main factor governing the failure of the bolt can be attributed to the lack of precise control over the chemical composition of the bolt and the application of inappropriate fabrication process.

Field investigation and structural analysis of wind-induced collapse of outdoor single-post billboards

Outdoor single-post billboards, which consist of a supporting steel column and two or three billboard panels, are widely used in public places. However, wind-induced collapse of these billboards has been frequently witnessed, which indicates that improvement to the current design would be worthwhile. To study the wind-induced collapse mechanism of single-post billboards, a field investigation and a dynamic simulation of billboards destroyed by the super-typhoon Mujigae were carried out. The results indicate that the current Chinese design standard might heavily underestimate the actual wind loads and dynamic responses. Moreover, two collapse modes were identified: an abrupt mode caused by fracture of the anchor bolts and a gradual mode caused by severe local buckling of the steel column. A further push-over analysis found that different design combinations of the steel column and the anchor bolts can lead to different collapse modes. Three types of design combinations were defined: elastic, acceptable and unacceptable designs. To avoid the abrupt collapse mode, either elastic or acceptable designs should be adopted in engineering practice. For the gradual collapse mode, local buckling resistance of billboards were evaluated based on the European Standard EN1993-1-6, which proved to be applicable for billboards through this evaluation. Moreover, some instructive suggestions were proposed for the design of single-post billboards.

Analysis of Effect of Slight Pitch Difference on the Fatigue Life of Bolt

In this work fatigue failure is examine for bolt with nut connections, when a slight pitch difference is established between bolt and nut. To improve the fatigue life, there are three types of slight pitch difference processed on the specimens and results are discussed in terms of FEM analysis. Taking into consideration the standard dimension bolt and nut connection having pitch difference of (α) =0 μm, the bolt fracture does not happen at the No. 1 thread by introducing a slight pitch difference of (α) =5 μm and pitch difference (α) =15 μm, is observed. Furthermore, it is having been discovered that the fatigue life of bolt can be increase by introducing some slight pitch differences. The effect of bolt-nut connection with slight pitch difference, on the fatigue failure of bolt is discovered.

Frequency spectrum and fatigue analysis of T-bolt fracture in DT III fastener based on Refined Model.

It is observed that T-bolt fracture of DT III fastener affects the safety of railway operations. The present study is aimed at finding solution to this critical issue of DT III T-bolt fracture. The paper is based on outcomes of field studies and analysis of relationship between bolt installation torque and stress. A refined DT III fastener model is established based on test data and nonlinear contact theory as per the actual size of each component. It is being observed that head-shank joint is a critical area during service. With respect to fatigue and stress, when the installation torque is less than 300 KN-mm, the bolt is prevented from yielding during installation. To a certain extent it also improves the fatigue life of bolt. From the frequency spectrum point of view, the vibration sensitive frequency band (1050-1100 Hz, 1230-1270 Hz) of bolt in service is very close to the main frequency of bolt vibration (1200 Hz). It may cause bolt fracture. In order to avoid resonance, the train operational speed should be strictly controlled between 115 km/h and 120 km/h.

Fatigue fracture analysis of brake disc bolts under continuous braking condition

In this research, the bolts fracture problem of brake disc under a section downhill of the Lanzhou-Wulumuqi railway (LW) line has been studied via experimental and fatigue analysis. The physical and chemical properties of the bolts were examined to analyze the fracture causes. In addition, the LW line condition was investigated and fitted to obtain the altitude-velocity histories. The finite element model of the brake disc was established with the thermo-mechanical coupling method to obtain the temperature and stress variation. Results revealed that the material defect was inexistence at the thread root, and the bolt performances meet relevant standards. Due to special geographical conditions in LW line, the train have to brake continually. The bolt fracture of the brake disc is caused mainly by high temperature during the continuous braking.

Significance of Nut Factor in Fastening of Joints in Engineering Structures

Threaded fasteners are category of bolts used in the joint assembling of aerospace structure. Torque is the process of applying a force that works at a circular distance and rotates the threaded fasteners at the bolted joints of these structures. With the aid of this torqueing force the two different parts are clamped together to form the joints. Due to torqueing there arises an opposing force which is known as the frictional force. The frictional force in threaded portion of the bolt transforms the applied Torque into stress. Due to this stress there is a chance of joint loosening or bolt fracture which is considered to be critical. Hence, selection of torqueing force for a desired preload is a major issue in joint deployment. The friction coefficient is a key factor for estimating the torque that is required for the fasteners to be deployed in the aerospace structures. There are many features which influence the friction coefficient for example size, pitch, thread tolerance. Till date only experimental methods and theoretical formulae has been used to estimate the friction coefficient, torque and preload. This paper identified the key features which are most correlated to coefficient of friction using the fisher score filtering technique and ca boost algorithm for both numerical and categorical datatypes. Along with that gives the insight on the torque, preload and coefficient of friction which are associated in fastening of the bolted joint in different non-permanent aerospace structures.

Improvement of 3-Way Valve for Temperature Control of Gas Turbine Lube Oil in CCPP

Lubrication oil, which is supplied to rotating elements in gas turbines of combined-cycle power generation, is used to maintain the efficiency of electricity production. The gas turbines are exposed to high-temperature environment due to the thermal energy obtained from fossil fuel, which degrades the performance of the lubricating oil. A system for controlling the temperature of the oil is essential, but there are a few problems in the case of the temperature control valve in the system, such as the hunting phenomenon, with which the target mass flow rate cannot be achieved, fatigue fracture and leakage. In this study, finite element analysis was performed to overcome these issues. The hunting phenomenon was prevented by redesigning the orifice shape of the valve cage. The fatigue fracture and self-loosening of the clamp bolt were overcome by changing the bolt size and tightening torque. In addition, a proper O-ring was selected to prevent leakage. The performance of a prototype was also tested to verify the reliability of the finite element analysis.

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.

Strength resistance factors for seismic design of exposed based plate connections in special steel moment resisting frames

This study presents a critical assessment of the reliability of current base plate connections in steel special moment resisting frames (SMRFs). Using a probabilistic outlook, this research evaluates the reliability of exposed column base (ECB) connections in SMRFs designed based on the current seismic design provisions; it suggests (and implements) a statistical approach to compute resistance factors for three modes of failure (concrete bearing, base plate yielding at tensile interface, and anchor bolt fracture) of ECB connections to achieve a target reliability index, β, of 4.5. Since ECB connections are limited to short buildings, therefore, this study is conducted on two-story and four-story SMRFs which are analyzed using a suite of 120 ground motions originating from strike-slip and reverse faults. ECB connections for the two-story building are designed to simulate pinned connection, while the bases of the four-story building represent moment connections. Detailed methodology for calculating the β of ECB connections is presented considering the three limit states in a moment–axial load interaction curve. Results indicate that the implementation of current seismic provisions results in β ~ 3.3 for non-moment resisting ECB connections for all tried combinations of resistance factors. For moment resisting ECB connections, however, only the designs based on a resistance factor for concrete bearing failure mode less than the current 0.65 result in an acceptable reliability factor of β > 4.5.