Structural Bolts Research Articles

Automated Structural Bolt Micro Looseness Monitoring Method Using Deep Learning.

The detection of bolt loosening in key components of aircraft engines faces problems such as complex and difficult-to-establish bolt loosening mechanism models, difficulty in identifying early loosening, and difficulty in extracting signal features with nonlinear and non-stationary characteristics. Therefore, the automated structural bolt micro looseness monitoring method using deep learning was proposed. Specifically, the addition of batch normalization methods enables the established Batch Normalized Stacked Autoencoders (BNSAEs) model to converge quickly and effectively, making the model easy to build and effective. Additionally, using characterization functions preprocess the original response signal not only simplifies the data structure but also ensures the integrity of features, which is beneficial for network training and reduces time costs. Finally, the effectiveness of the proposed method was verified by taking the bolted connection structures of two key components of aircraft engines, namely bolt connection structures and flange connection structures, as examples.

Identification of Bolt Loosening Damage of Steel Truss Structure Based on MFCC-WPES and Optimized Random Forest

In the field of bolt loosening detection, although some progress has been made, there are still challenges such as high operational complexity, single feature extraction methods, and insufficient analysis model performance, especially in large steel truss structures, where there is a lack of efficient and accurate bolt loosening identification solutions. In response to these shortcomings, this article proposes an innovative bolt loosening damage recognition method based on sound signals. This method integrates feature extraction techniques of Mel frequency cepstral coefficients (MFCCs) and wavelet packet energy spectra (WPES), and comprehensively characterizes sound signals by constructing MFCC-WPES combined features. Subsequently, the random forest (RF) algorithm optimized by genetic algorithm was used for feature selection and model training, aiming to improve recognition accuracy and robustness. The experimental results show that this method can not only accurately identify bolt loosening signals in steel truss structure bolt loosening detection, but also has strong identification ability for environmental noise. Compared with traditional methods, the proposed solution in this article shows significant improvements in both performance and practicality, providing a new perspective and solution for the technological advancement of bolt loosening detection in steel truss structures.

Navigating the depths: Experimental study on the influence of ground fissures on metro shield tunnels considering different intersection angles

Ground fissures are a typical slowly changing geological hazard that poses great safety risks to metro shield tunnels crossing ground fissures. To thoroughly analyze the deformation and failure of shield tunnels under the action of ground fissures with different intersection angles, this study reviews the existing research and carries out model tests on metro shield tunnels crossing ground fissures considering different intersection angles. Based on the experimental results and numerical simulations under multiple conditions, the deformation and failure characteristics of shield tunnels are systematically analyzed for the first time, which confirms that the activity of ground fissures can lead to a series of problems, including segment dislocation, lining cracking, structural elliptical deformation, bolt yield, joint opening and closing, void areas at the tunnel bottom in the hanging wall and tunnel top in the footwall, as well as resulting waterproof failure and internal gauge changes. In addition, as the intersection angle between tunnel and ground fissure increases, the failure mode of the structure gradually transitions from torsional failure to shear failure, with a decrease in the number of segment rings under bias pressure and an increase in the number of segment rings under shear effect. Based on the deformation characteristics, failure positions, and the changes of structural state of shield tunnels, some reflections on countermeasures are discussed, which can provide guidance for the design and maintenance of metro projects in ground fissure sites.

Mechanism and control of preload force loss of precast structural joint bolts under freeze–Thaw environments

Bolt-connected precast buildings are increasingly prevalent, especially due to their efficient force transfer and accelerated construction processes. However, in cold climates, special attention must be directed towards the effects of freeze-thaw (F-T) damage on concrete at bolt connection sites. This study conducted an accelerated F-t-test to investigate how concrete F-T damage influences the preload force of the connection bolts. The changes in bolt preload forces across F-T cycles were monitored using advanced fiber optic sensing technology. To mitigate preload force loss, silicone rubber sleeves were introduced into the bolts. Results indicate a correlation between initial preload force and subsequent preload loss, with lower initial preloads leading to greater losses. Additionally, the concrete's frost resistance significantly affects preload force loss, with less resistant specimens showing marked preload decreases. A notable risk of brittle damage arises after 3 to 6 F-T cycles if a 4 mm gap between the bolt and concrete exists, attributed to water freezing and expansion within the gap. Concrete specimens subjected to our proposed mitigation strategies effectively withstand over 140 F-T cycles, substantially lowering crack damage risks.

Ductile fracture of high-strength bolts under combined actions at elevated temperatures

Steel bolted connections are critical structural components that can initiate progressive collapse at ambient and elevated temperatures. High-strength bolts are highly vulnerable members due to rapid strength degradation at high temperatures. This paper presents a numerical study on the ductile fracture of the frequently used Grade 8.8 and 10.9 structural bolts under tension, shear, and combined forces at 20–700 °C. True stress–strain curves have been defined up to fracture for both grades. The fracture parameters have been calibrated by a total of four experiments: two tensile and shear tests for each grade. The ductile fracture models with proposed fracture parameters have been further validated by pretensioned shear tests at elevated temperatures. The relationship between equivalent strain and triaxiality was investigated based on the numerical results. In general, equivalent plastic strain values become larger with increasing temperature. The fracture analysis with given ductile fracture parameters estimates the test results with reasonable accuracy under all failure modes. The fracture model can be used in numerical analysis to investigate the ductile failure of the high-strength bolted connections at ambient and elevated temperatures. Furthermore, the influence of displacement at fracture values on the results was investigated by a three-bolted inclined shear test.

Effect of Atmospheric Conditions on the Tightening Behaviour of HV Galvanised Bolts in Structural Bolt Sets

The aim of this article is to investigate the effect of atmospheric conditions on the tightening behaviour of HV bolts in structural bolt sets. The article describes the results of experimental tests carried out for bolt sets of a selected type. The parts of the joined sets were stored according to the specified six different conditions for four weeks before assembly. Paper, cardboard boxes and anticorrosion foils were used as protection against atmospheric conditions. The behaviour of unassembled and assembled bolt sets was also taken into account. The variation in the friction coefficient depending on the clamping force and storage conditions of the bolt sets was demonstrated. This applies both to the contact between the joining elements on the threaded surfaces and to the area between the bolt head and the bearing surface of the nut and washer.

Experimental and numerical investigations on shear behavior of high-strength bolted connections after impact

For structures subjected to extreme loading, such as impact, the plastic deformation of structural connections is vital to life safety. Despite its importance, a loss of deformation capacity in connections via fracture of bolts has been repeatedly noted experimentally and in situ. To further investigate the residual strength of high-strength bolted connections after impact damage, this paper presents investigations on shear performance of high-strength bolted connections. This is the first study to report strength and deformation performance for structural bolts after impact damage. A total of 144 specimens were tested for the study, with varying impact height, steel plate thickness, bolt shank diameter, and high-strength bolt grade as the main variables. Experimental results involving failure modes, strain development, and effect of parameters are presented in detail. The observed failure mode is typical bolt shear failure, characterized by extrusion deformation around the bolt hole. The increasement of the thickness of steel plate, the diameter of bolt shank, and grade of bolt would improve the ultimate strength. The shear stiffness is positively correlated with impact height, thickness of steel plate and grade of bolt, but the ductility is decreased with the increment of thickness of steel plate. In addition, the results obtained from the finite element simulation are in agreement with the experimental findings. The simulation analysis shows that the specimens after impact damage are different from those of conventional specimens. Parameters formulae are then proposed with sufficient accuracy for the ultimate strength of high-strength bolted connections with impact damage.

Study on Force Characteristics and Safety of Segment Structure and Bolts with and without Cavity behind Lining with Multi-Field Coupling

In this study, a refined three-dimensional stratigraphic–structural model is established based on ABAQUS finite element software, and the basis for determining pneumatic and vibration loads is explained in detail. From this, the force characteristics of the segment and bolts with and without a cavity behind the lining under the action of multi-field coupling were analyzed, and the force law and corresponding safety of the segment structure and high-strength bolts were determined. The results show that the peak value of the maximum principal stress on the segment structure caused by the surrounding rock pressure was 92.7 times greater than the variation in the peak value of the maximum principal stress caused by additional loads (pneumatic and vibration loads). Despite this, the safety factor of the segment structure satisfied the code requirements. Compared to the situation with no cavity behind the lining, when the cavity behind the lining was small the stresses of the segment structure were large and concentrated, which increased the possibility of crack development in the segment structure. The nodal stresses and strains on the straight and bending bolts exhibited an approximately “W”-shaped distribution with a cavity behind the lining. In addition to the effect of the preload near the nut, the stress and strain at the central measurement point of the bolt rod at the joint face were larger owing to the coupling effect of multiple fields. The high-strength bolt remained in an elastic state and did not yield with damage.

Experimental and numerical investigation on structural bolts under compression

Experimental and numerical investigation on structural bolts under compression

Factors in deformation capacity loss of bolts under singular and consecutive impacts

For structures subjected to extreme loading, such as a blast or impact, the plastic deformation of structural connections is vital to life safety. Despite its importance, a loss of deformation capacity in connections via fracture of bolts has been repeatedly noted experimentally and in situ. To further investigate the dynamic fracture of bolts, this work subjects A325 structural bolts in single shear to impulsive loads using a high-speed hydraulic actuator. This is the first study to report strength and deformation performance for structural bolts fractured via singular and consecutive impacts. Further, this work is the first investigation into the root cause of the deformation capacity loss that has historically coincided with shear bolt fracture. Previous split Hopkinson pressure bar experimentation suggests that rate-induced changes in strain hardening contribute to, but do not fully explain, the loss of deformation capacity in high-strength bolts. Further investigation with scanning electron microscopy suggests that loss of deformation capacity is also due to a rate-induced change in failure mechanism, adiabatic shear banding. These findings impact our understanding of connections that fail via dynamic bolt fracture – behavior changes on the macroscale are the result of underlying rate-dependent fracture mechanisms.

Automatic bolt tightness detection using acoustic emission and deep learning

Bolts are typically deployed to unify disparate components in civil engineering structures, making the surveillance of their tightness critical to ensuring the stability of such structures. The innovation of this paper is the development of an automatic monitoring method for bolt tightness using acoustic emission (AE) and deep learning (DL). Initially, AE sensors are first used to monitor structural bolts, and AE signals are preprocessed by a continuous wavelet transform (CWT) to obtain different time–frequency components, which are then fed into the convolutional neural network (CNN). To reduce the amount of training parameters and enhance the capacity for feature extraction, transfer learning (TL) is introduced to train the model. In the experimental scenario, the bolts exhibit seven categories of tightness. The findings show that the proposed method can accurately classify the bolts’ tightness, and can be used to monitor the early looseness of the bolts online.

Ultimate Strength Study of Structural Bionic CFRP-Sinker Bolt Assemblies Subjected to Preload under Three-Point Bending.

Countersunk head bolted joints are one of the main approaches to joining carbon fiber-reinforced plastics, or CFRP. In this paper, the failure mode and damage evolution of CFRP countersunk bolt components under bending load are studied by imitating water bears, which are born as adult animals and have strong adaptability to life. Based on the Hashin failure criterion, we establish a 3D finite element failure prediction model of a CFRP-countersunk bolted assembly, benchmarked with the experiment. The analysis shows that the simulation results under specified parameters have a good correlation with the experimental results, and can better reflect the three-point bending failure and fracture of the CFRP-countersunk bolted assembly. Based on the specified parameter of the carbon lamina material change, we used the countersunk bolt preload to investigate the stress distribution near the counterbore zone, and to investigate the effect of bolt load on the three-point bending limit load. The results obtained using FEA calculations indicate that the stress distribution around the countersunk hole is related to the laminate direction. The bolt preloading force increasing reduces the load value at the initial damage, and the appropriate preload force will increase the ultimate load of the joint.

Atmospheric corrosion performance of structural steel bolts in the Wenchang marine environment, South China

The atmospheric corrosion performance of three different grades of hexagonal head structural steel bolts with distinctive surface finishes (304 stainless steel (plain), galvanized steel (Zn coated), and 40Cr steel (black coated)) was examined in this study using surface analysis and characterization techniques. The bolts were exposed in the Wenchang marine environment for two years. Results showed that a combination of factors such as alloying content, microstructure, surface finishing, and environmental conditions impacted the corrosion performance of the examined bolts. The corrosion mechanisms and patterns varied between the different bolt materials; 304SS bolts exhibited localized corrosion, while galvanized steel and 40Cr steel bolts exhibited uniform corrosion. In terms of coating performance in the marine environment, the zinc coating provided minimal protection to the underlying steel substrate when compared to the black oxide coating which failed prematurely, rendering it unsuitable for use in the marine atmosphere. Furthermore, despite the formation of a dense rust layer on the galvanized and 40Cr steel substrates after 2 years of marine atmospheric exposure, the fastener degradation was critical, suggesting that the rust layer is not beneficial in the threaded fasteners due to the high susceptibility to exfoliation that may eventually lead to thread stripping. This poses a serious risk to bolted assemblies.

3D vision-based bolt loosening assessment using photogrammetry, deep neural networks, and 3D point-cloud processing

Structural bolts are essential structural elements. Detection of structural bolt loosening is of great importance to provide earlier warnings of structural damages and prevent catastrophic system-level collapse. Most existing studies about bolt loosening assessment were built in 2D computer vision, where the assessment may be restricted based on the camera views. In this paper, a novel 3D vision-based methodology is proposed for autonomous bolt loosening assessment. First, a 3D point cloud of bolted connection is created using readily available 2D images. Second, a new convolutional neural network (CNN)-based method is developed to localize structural bolts in the 3D point cloud. Further, a 3D point cloud processing algorithm is developed to recognize and quantify bolt loosening. Parameter studies were conducted to investigate the effectiveness of the proposed pipeline. Finally, a real-world implementation has been conducted to quantify bolt loosening on a steel column base connection with bolts. The results indicate that the proposed bolt loosening assessment methodology can effectively localize and quantify bolt loosening at high accuracy and low cost.

Multi-axial fatigue behavior of high-strength structural bolts

In this study, the fatigue failure of high-strength bolts (galvanized and regular) was experimentally and analytically explored under different tension-to-shear cyclic loading ratios. The observed cracking mechanisms were found to be a function of the bolt types and loading conditions. Though the cracks in the galvanized bolts followed the helical shape of the threads, the cracks for the regular bolts were parallel to the shear plan. The predicted fatigue lives under the combined tension and shear loads using the analytical approach matched the experimental results. Utilizing Eurocode SN curves for independent tension and shear demands overestimated the fatigue life of bolts under multi-axial stresses. The resulting SN curves obtained from this study can aid engineers in designing connections where bolts are subjected to multi-axial fatigue load.

Behaviour and Design of Innovative Connections of Prefabricated CFST Columns under Tension

This paper investigates the tensile behaviour of prefabricated concrete-filled steel tube (PCFST) columns with bolted connections. Innovative bolted column-column (BCC) connections are developed using standard structural bolts to simplify the construction process for the connection of two PCFST columns, especially for the corner, edge, and interior columns. The behaviour of BCC connections in PCFST columns under tension has been investigated, adopting the finite element (FE) modelling approach. Parametric studies are carried out to understand the influence of bolt arrangements (TB = 4, 6, 7, 8), base plate thickness (tbp = 8, 10, 14, and 18 mm), bolt diameters (db = 16, 18, 20, 24 mm), vertical stiffeners (ths = 4, 6, 8, 10 mm), horizontal stiffeners (ths = 10, 12, 13, 15 mm), and yield strength of steel tube (fy,t = 380, 450, and 550 MPa) on the behaviour of PCFST columns with developed BCC connections. The results show that the PCFST columns with the developed BCC connections can attain sufficient tensile strength and satisfy the tensile strength requirements recommended in AS5100 and the robustness requirements in AS1170. The outcome of this paper will be useful to practising structural engineers to design prefabricated CFST columns with BCC connections under tension.

Damage Detection of a Pressure Vessel with Smart Sensing and Deep Learning

Structural Health Monitoring plays a crucial role in ensuring the safety and reliability of critical infrastructure, including pressure vessels involved in various applications. This research reports the damage detection of a pressure box employed in space habitat that operates in harsh environment where both structural failure and bolt joint loosening may occur. These failure modes are extremely hard to model based on first principles. We explore proper sensing mechanism and the associated inverse analysis algorithm that can elucidate the health condition of the pressure box. It is identified that piezoelectric impedance based active interrogation can provide necessary information for damage detection in such a system. Concurrently, deep learning technique leveraging spatial convolutional neural network is synthesized to analyze the raw data acquired and identify different types of damage. By training the deep learning model on a dataset of healthy and various damage scenarios, we can achieve high accuracy in identifying the presence of damage and its type. This research provides a data-driven methodology for structural damage detection using deep learning and has the potential to be extended to various systems with different failure modes.

Spatially resolved neutron texture measurements of copper bolts from the Deltebre I (1813) site

Spatially resolved neutron texture analyses were performed on several copper bolts used to fasten different wooden components of the hull of the Deltebre I (1813) shipwreck. Since 2008 this site has been the subject of a thorough archaeological study by the Catalan Centre for Underwater Archaeology of the Archaeological Museum of Catalonia. It has been identified as one of the transport ships of a combined British, Sicilian and Spanish fleet that ran aground in the Ebro delta, off the coast of Catalonia, after an unsuccessful expedition to liberate the city of Tarragona from Napoleon's forces. In this paper, the texture and texture gradients of five different bolts recovered from this site were examined non-destructively, using the TOF neutron scanner ENGIN-X and analysing the data using the NyRTex methodology. The spatially resolved texture results indicate that at least three different methods were probably used to manufacture the bolts: drawing, casting, and forging and/or rolling. These results can both shed light on the processes of manufacture of fastenings of the early 19th century and contribute to identifying the ship. • The texture and texture gradients of five different bolts recovered from the Deltebre I site have been determined non-destructively using TOF neutron diffraction. • The results indicate that at least three different methods were most likely used to manufacture the structural bolts: drawing, casting, and forging and/or rolling.

Study of the Effect of Quenching and Tempering Modes on the Strength Level of Alloyed Structural Steels Used to Produce Fasteners

The development of high-strength fasteners is required due to the modern trend of reducing the weight of engineering structures. At present, structural bolts of property classes 8.8–10.9 are widely used, and higher-strength bolts of property classes 12.9 and 14.9 are being developed. In this paper, we analyzed the possibilities and conditions for manufacturing fasteners of different strength categories from steels of seven grades, containing and not containing boron. The necessary corresponding temperature ranges of processing are determined based on the study of hot-rolled round steel. Tensile strength, yield strength, relative elongation, reduction in area, and Vickers microhardness were measured. An analysis was made of the hardenability and the influence of the holding time during tempering on the mechanical properties of the considered round steel. Using the method of scanning electron microscopy, the effect of carbide precipitation during tempering on the mechanical properties of fasteners was established.

Experimental studies on slip friction device using symmetric friction connections

The use of friction-based energy dissipater is rapidly growing in seismic active zones to avoid interruptions and costs resulted from local or global damage to buildings due to severe earthquakes. Well designed and detailed friction devices using symmetric friction connections (SFCs) are often considered as an efficient way to dissipate energy for not only their stable hysteretic behaviour and low strength degradation but also the comparable assembling cost to that of conventional construction. The sliding commences when the designed sliding shear force is achieved, which relates to the contact surface’s Coefficient of Friction (CoF) and the total amount of clamping force. This paper reports 15 quasi-static testing of a slip friction device utilizing SFC clamped by two grade 10.9 M20 black finished high strength structural steel bolts in a single row. Shims using brass, aluminium, high strength steel, semi-metallic brake pads and copper-iron alloy were tested. Results show that all five shim materials tested can provide a stable hysteretic behaviour under a cumulated sliding distance of more than 4000 mm. The effect of installed bolt tension, loading rates and bolt-bearing mode are also discussed. Given the stable hysteretic response and the low strength degradation observed, the proposed slip friction device can be considered as a low damage energy dissipater.