Bolt Surface Research Articles

Mechanism of bolt breakage in deep mining roadway under dynamic load and advanced strengthening support technology

To reveal the problem of local instability caused by bolt breakage and failure in deep mining roadway under impact dynamic loads, and explore strengthening support technologies that can reduce bolt breakage and improve the stability of roadway support bodies. This article adopts theoretical analysis, numerical simulation, and on-site industrial experiments to conduct research, investigate and obtain the fracture failure characteristics of bolt in deep mining roadway, quantitatively characterize the deformation pressure of roadway surrounding rock and dynamic load of overburden fracture, establish the mechanical criterion of bolt fracture, and analyze the influence law of different anchoring factors on bolt force under static load and dynamic and static load combination. The results show that when the anchor angle is different, the range under dynamic load increases by 169.3 % compared with static load, when the anchor length is different, the range increases by 863.3 % relatively, and the influence is significant; while the anchor support resistance increases by 11.6 % at different times, and the range decreases by 41.5 % at different pre-tightening torque. Then, based on this criterion, the influence law of bolt failure on the stability of surrounding rock is analyzed, and the optimal support scheme is obtained by simulation research, that is, bolt cable +U steel support scheme. After adopting this scheme for support, the deformation of surrounding rock is reduced by about 60 %, and the maximum concentrated stress of surrounding rock of roadway is reduced by 15.0 %. Based on this, the support parameters of the on-site roadway were optimized and a strengthening support scheme was determined, which includes “roof bolt surface support reinforcement + anchor cable combined with U-shaped steel deep reinforcement, and two strong bolts advanced reinforcement”. The on-site strengthening support practice showed that the bolt fracture rate was reduced by 87.3 %, the deformation of the surrounding rock was effectively controlled, and the stability was significantly improved.

Shear Mechanical Properties of Bolt-Grout Interface under Different Bolt Surface Profiles

The shear behavior of the Bolt-Grout interface has a significant effect on the stability of a bolting system. In this paper, a series of shear tests were conducted on Bolt-Grout interfaces, and the effects of rib spacing, rib angles, and normal stress on the shear characteristics and failure modes of the Bolt-Grout interface were investigated. The results showed that the shear strength varied nonlinearly with an increase in rib spacing and angle, and also that it increased linearly with an increase in normal stress. With smaller rib spacings, the effect of rib spacing on peak shear strength was more apparent. The failure modes of the interface can be categorized as shear-slip failure, shear-break failure, and composite failure. The proportion of shear-slip failure and shear-break failure mainly depends on the rib spacing, rib face angle and normal stress.

Failure analysis of frequent fracture accident of torsional shear type high strength bolts for steel structure of a thermoelectric 2×350MW unit project in Northwest China

In this paper, the frequent breakage of high-strength torsional shear bolts used in steel structure of a 2×350MW thermoelectric unit in northwest China is taken as an example. By means of macro and micro fracture analysis, physical and chemical inspection, the causes of frequent breakage of high-strength bolts are analyzed and the improvement measures are put forward. The results show that the failure of the 20MnTiB steel high-strength bolt after assembly is hydrogen embrittlement fracture, and the main cause of the failure is the presence of zinc layer on the bolt surface and a large number of zinc microholes on the shallow surface.

Tensile mechanical properties and damage analysis of layered woven GFRP composite bolts

Composite bolts have become one of the research focuses in screw threads joints due to their lightweight, high-strength, and wave transmission properties. We elucidate the differences in tensile properties, damage processes, and tensile failure forms between two types of layered woven GFRP composite bolts through tests and numerical simulations. It was found that the tensile strength of layered plain woven bolts is 40% higher than layered twill woven bolts. The failure forms of both types of bolts are related to their layered woven structure. The fracture surface of plain bolts is planar, while the fracture surface of twill bolts is apex shaped. Subsequently, a refined thread stretching model was established to study the effect of loading speed on the tensile property of screw threads. It was found that the tensile strength of plain woven screw threads and twill woven screw threads at a loading speed of 1 mm/min was 11% and 6% higher than that at 3 mm/min, respectively. The results show that the bolts and screw threads with layered plain woven have stronger tensile strength than those with layered twill woven, and a decrease in loading speed within a certain range can improve the tensile strength of the screw threads.

Monitoring torque in bolts using an embedded fiber Bragg grating sensor

In this study, we embedded a fiber Bragg grating (FBG) sensor into a bolt and experimentally examined whether the wavelength in the FBG changes in response to small elongations of the bolt caused by an acting torque. The FBG sensor was fabricated and embedded in the bolt at different depths and positions to investigate whether these factors directly affect the sensitivity. M20 bolts were used in this study. The greatest sensitivity (0.00298 nm/N-m, R2 = 0.9984) was obtained from the bolt in which the FBG sensor was embedded 7.0 mm from the center of a completely penetrated bolt. The findings suggest that the deeper the embedding depth of the sensor and the closer its position to the bolt surface, the higher the sensitivity. The FBG sensor is suitable for the measurement and long-term monitoring and control of bolt torque.

Insulation Reinforcement for Cable Connector of CRH2 Electric Multiple Unit Under Standard Lightning Impulse Voltage

In order to improve the insulation capacity of cable connector (Cc) in the high voltage equipment box (Hveb), this article proposes an optimization scheme that does not change the size of the Hveb and the structure of Cc fittings. In this article, the change in the impulse voltage withstand value of Cc after installing insulating sheath was verified by the lightning impulse flashover test. Based on the finite-element software and the structure of Cc fittings, a special shaped shielding ring (Sr) was designed to adjust the partial area where the electric field intensity are concentrated. Besides, the optimal parameters of the Sr under different working conditions were determined by the orthogonal design. Finally, utilizing ultraviolet (UV) imaging technology and image processing method, the discharge intensity of Cc fittings were characterized. The results demonstrate that with the combination of insulation measures, the impulse voltage withstand value of Cc can be increased from 155 to 235 kV. The maximum electric field strength on the surface of bolts and metal electrode can be reduced by 57.8% and 72%; UV discharge facular area had an exponential relationship with the applied voltage, the initial discharge voltage of Cc can increase by 243% after installing the Sr.

Analysis on the Shear Stress Propagation Mechanism in the Rock Reinforcement System

Rock reinforcement is significant in maintaining the stability of excavated structures, such as tunnels and underground roadways. However, shear failure in the rock reinforcement system, especially the shear failure at the rock reinforcement bolt surface, induces a threat to the rock reinforcement system. To reveal the shear stress (SS) propagation mechanism in the rock reinforcement system, this article conducted a literature review. First, the investigation approaches that were used by previous researchers to study the SS propagation were summarized. The advantages and disadvantages of experimental tests, analytical simulation, and numerical simulation were compared and analyzed. Then, the SS propagation process in the rock reinforcement system was presented. Two typical SS propagation modes were explained. More attention was given to the SS propagation mode in which the maximum SS propagates from the external end of rock reinforcement bolts to the internal end of rock reinforcement bolts. After that, a discussion section was given. In the discussion section, the significance of the SS propagation was further emphasized. Moreover, the limitations in the analytical simulation and numerical simulation were indicated. It is concluded that when studying the SS propagation mechanism of rock reinforcement bolts, combining experimental tests, analytical simulation, and numerical simulation is a better choice. This study is beneficial for revealing the SS propagation mechanism of the rock reinforcement system.

Microscale damage modeling of bolt-hole contact interface during the bolt installation process of composite structure

During the interference-fit joining process of composite bolted structure, the bolt-hole contact interface would form naturally. Understanding the microscale damage mechanism of bolt-hole contact interface is crucial for guarantying load bearing and sealing performance of the composite bolted structure. In this paper, a microscale finite element model was proposed to model the damage behaviors of the bolt-hole contact interface during the bolt installation process. The micro rough characteristics of composite hole-wall surface, bolt surface and bolt coatings were explicitly considered and modeled. The damage initiation of the composites and bolt (including coatings) were calculated by the modified 3D Hashin criterion and von Mises criterion. The proposed microscale model was validated by comparing the load–displacement curve and damage modes with experiments. The results show that the damages primarily occur on the composite hole-wall and bolt coatings. The damage of coatings is primary caused by the squeezing and friction. The damage ratio of coatings is approximately linear positive related to the interference-fit size. The proposed microscale model not only has the potential application for the prediction of microscopic damage, but also for the formation of leakage channel and the protection of corrosion location.

Online detection of external thread surface defects based on an improved template matching algorithm

Surface defects in the bolts can affect the assembly speed of the assembly line, which in turn affects the work of the entire assembly line. At the same time, defects in bolts may also affect the quality of the product. At present, it is difficult for bolt surface defect detection to meet the requirements of fast detection, high recognition rate, and online detection at the same time. This paper studies a defect detection algorithm designed based on an improved template matching algorithm. It solves the problem of difficult template matching for bolted images during online inspection, and has a significant speedup compared to the original template matching algorithm. The final experimental results show that the method can meet the requirements of online, high speed and high accuracy to some extent.

Analysis and research on fracture cause of connecting bolt of wind turbine spindle

On a certain day in 2018, the main shaft connecting bolt of a wind turbine in a wind farm broke and failed during operation. In order to find out the cause of fracture, the fractured spindle connecting bolt was comprehensively detected and analyzed by means of appearance state analysis, chemical element composition analysis, mechanical property detection analysis, microstructure detection and fracture micro area analysis. The results show that the main reason for the fracture of the main shaft connecting bolt is that the hydrogen penetrates into the bolt surface due to improper process control during the production process, resulting in hydrogen embrittlement in the local area of the thread root. Thus, during the operation of the fan, under the combined action of bending, shear and tensile stress, the connecting bolt of the main shaft cracks at the bottom of the first thread on the side of the self rigid rod with the most concentrated stress, and expands in the form of “herringbone” brittle fracture until the overall fracture failure. It is recommended to check the bolts in the same batch to prevent hydrogen embrittlement in the whole batch.

Numerical simulation of fully encapsulated rock bolts with a tri-linear constitutive relation

This paper modified the original structural elements of pile in FLAC3D to analyse the performance of rock bolts. For the contact surface between the bar and the binding grout, bonding behaviour was modelled using the tri-linear constitutive relation. Pull tests were used to testify the effectiveness of the modified pile elements. A satisfactory correlation was acquired between the modelling and physical results. This testified that the improved pile elements can successfully analyse the performance of bolts. Then, a numerical coal roadway was established and reinforced with the modified pile elements. After equilibrium, a non-uniform force distribution occurred along bolts. The axial force around the middle was maximal and the tensioning force at two ends of the bolt was minimal. A parametric analysis was performed to analyse the effect of relevant factors on performance of bolts. The results showed that with the bonding strength of the bolt surface raising, the neutral point shifted towards the excavation direction. Moreover, when the applied vertical stress was beyond a limit, the axial force in the bolts installed in the roadway side reduced dramatically. However, for the bolts installed in the roof, the axial force in the bolts was still raising.

Experimental study of slip‐resistant connections under cyclic load

AbstractThe slip‐resistant shear connections with preload bolts have traditionally use in the steel structures exposed to cyclic / fatigue loading or in the cases when the bolt slipping or connection deformations should be restricted. Hence, the shear load is transferred through the friction surfaces rather than the contact between bolt surface and hole. The main influencing parameters determine the connection slip resistance are the friction between the contact surfaces and the level of preload in the bolts. This paper presents an experimental study of preloaded bolted connections under cyclic load. Experiment was performed on double lap shear connections using high strength bolts and lock bolts, both M20 of grade 10.9. The steel plates used in experiment are grade S355JRG2. Sandblasting was performed using quartz sand as the abrasive after which the zinc‐silicate coating was applied on plate surfaces. To address the required pretension force in the bolts, the strain gauge method was employed. All strain gauges were calibrated. Six specimens were exposed to cyclic loading after monitoring initial and short‐term losses of pretension forces in the bolts. The main objective of the presented study is to investigate the influence of the cyclic load on the losses of pretension force in high strength bolts and lock bolts. The results of comparative experimental analysis are presented and discussed.

Influence of Configuration Error in Bolted Joints on Detection Error of Clamp Force Detection Method

Clamp force errors in bolted joints often cause accidents in various mechanical structures. Therefore, the clamp force must be controlled accurately and maintained for securing the reliability of mechanical structures such as vehicles. However, the clamp force cannot be controlled easily during tightening. Moreover, it is difficult to detect the clamp force after tightening. We previously proposed a method to easily detect the clamp force of a bolted joint that has been tightened. In that method, the bolt thread protruding from the nut is pulled while the nut’s upper surface is supported. The relationship between tensile force and displacement at the pulling point where the tensile force is applied differs before and after the tensile force reaches the clamp force. The method detects the tensile force at the point, where the relationship changes, as the clamp force. In this study, we investigate the influence of squareness error on the bearing surface of the clamped part in a bolted joint on the detection error of the method using experiments and finite element (FE) analysis. The experimental results show that the squareness error has an influence on the detection accuracy. The average detection error in the experiments increases by approximately 10% with an increase in the squareness error. To understand the cause of this phenomenon, we investigate the effects of backlash between mating thread surfaces of bolts and nuts on the detection error. The results show that the error decreased because of the backlash. Consequently, it is assumed that the error is caused by the non-separation of the mating thread surfaces when the tensile force reached the clamp force. Furthermore, the FE analysis results show that the squareness error on the bearing surface of the clamped part has an influence of the squareness error on the detection accuracy. The results indicate that we should control the tolerance of squareness errors on the bearing surface of the clamped part when the clamp force detection method is applied to bolted joints.

Eksperimentalno istraživanje gubitaka prednapinjanja u visokovrijednim vijcima pod cikličkim opterećenjem

Slip-resistant shear connections with preloaded bolts have traditionally been used in steel structures exposed to cyclic / fatigue loading or in cases when bolt slipping or connection deformations should be restricted. Hence, shear load is transferred through friction surfaces rather than through contact between the bolt surface and the hole. The main influencing parameters determining slip resistance of the connection are the friction between contact surfaces and the bolt preload level. This paper presents an experimental study of preloaded bolted connections under cyclic load, the main objective being to investigate the influence of cyclic load on the losses of preload force in high strength bolts and lock bolts.

Microbiologically influenced corrosion of cable bolts in underground coal mines: The effect of Acidithiobacillus ferrooxidans

Reports on corrosion failure of cable bolts, used in mining and civil industries, have been increasing in the past two decades. The previous studies found that pitting corrosion on the surface of a cable bolt can initiate premature failure of the bolt. In this study, the role of Acidithiobacillus ferrooxidans (A. ferrooxidans) bacterium in the occurrence of pitting corrosion in cable bolts was studied. Stressed coupons, made from the wires of cable bolts, were immersed in testing bottles containing groundwater collected from an underground coal mine and a mixture of A. ferrooxidans and geomaterials. It was observed that A. ferrooxidans caused pitting corrosion on the surface of cable bolts in the near-neutral environment. The presence of geomaterials slightly affected the pH of the environment; however, it did not have any significant influence on the corrosion activity of A. ferrooxidans. This study suggests that the common bacterium A. ferrooxidans found in many underground environments can be a threat to cable bolts’ integrity by creating initiation points for other catastrophic failures such as stress corrosion cracking.

Experimental Study on the Deformation and Failure Characteristics of Anchor under Graded Loading and Corrosion

During the entire life cycle of rock and soil anchors, owing to the influence of adverse factors such as the working environment, load change, and anchor performance degradation, the working load of the anchor will continue to increase and the mechanical properties will continue to deteriorate, which significantly affects the safety and stability of rock and soil anchors. Therefore, this study focuses on the deformation and failure characteristics of anchors under loading and corrosion conditions by means of indoor simulation tests under laboratory conditions. The results indicate the following. (1) There are obvious cracks on the surfaces of specimens 2# and 10#. In the two groups of specimens, the corroded bolt surface exhibited a corrosion phenomenon. This indicates that the corrosion environment conditions cause a certain degree of damage to the anchored rock mass. (2) Under the same gradient load condition, three observable cracks were found in the 10# anchorage specimens and one observable crack was found in the 2# anchorage specimens. Therefore, it is clear that the damage degree of the anchor increases with an increase in the corrosion time. (3) Under the condition of corrosion environment, the strain in the lower part of the specimen is generally greater than that in the upper part of the specimen, and the failure of this group of specimens in the loading process is mostly splitting failure, which is basically generated along the trend of the strain nephogram, and shear failure occurs with the extension and diffusion of cracks.

Loosening behaviour of threaded fasteners under cyclic shear displacement

This paper presents an experimental study on the loosening behaviour of threaded fasteners under cyclic shear displacement. The effects of the initial preload and the amplitude of the shear displacement on the loosening behaviour of the common zinc-coated bolts are investigated. Diamond-like carbon (DLC) coating is utilised to treat the surface of bolts for improving the wear resistance of threads. The thread damage is analysed after the tests. Results show that fatigue wear is the major wear mechanism on the first thread surface. The looseness of threaded fasteners is small under high initial preload and low amplitude of shear displacement. For DLC-coated bolts, the thread damage is slight, and DLC-coated bolts have a better anti-loosening property comparing with zinc-coated bolts.

Modeling and Monitoring of Wood Moisture Content Using Time-Domain Reflectometry

Time-domain reflectometry (TDR) can monitor the moisture content (MC) of water saturated logs stored in wet-decks where the MC exceeds the range that can be measured using traditional moisture meters (>50%). For this application to become routine, it is required that TDR monitoring of wet-decks occurs after establishment, and tools are needed that automate data collection and analysis. We developed models that predict wood MC using three-rod epoxy encased TDR probes inserted into the transverse surface of bolts (prior wet-deck studies were installed on the tangential surface). Models were developed for southern pine, sweetgum, yellow poplar, hickory, red oak, and white oak using a Campbell Scientific TDR100. For each species, at least 37 bolts were soaked for a minimum of three months and then air dried with TDR waveforms, and MC was periodically recorded. Calibrations were developed between MC and the TDR signal using nonlinear mixed effects models. Fixed effects ranged from excellent (southern pine R2 = 0.93) to poor (red oak R2 = 0.36, hickory R2 = 0.38). Independent of wood species, random effects all had a R2 greater than 0.80, which indicates that TDR detects changes in MC at the individual sample level. Use of TDR combined with a datalogger was demonstrated in an operational wet-deck that monitored changes in MC over 12 months, and in a laboratory trial where bolts were exposed to successive wet-dry cycles over 400 days. Both applications demonstrated the utility of TDR to monitor changes in wood MC in high MC environments where periodic measurement is not feasible due to operational safety concerns. Because a saturated TDR reading indicates a saturated MC, and because of the relatively accurate random effects found here, developing individual species models is not necessary for monitoring purposes. Therefore, application of TDR monitoring can be broadly applied for wet-decks, regardless of the species stored.

Characterization of 3D printed bolts based on digital image correlation and infrared thermography

This study selects 3D printing to innovate traditional research approaches regarding bolting. Three bolts composed of different manufacturing materials—aluminum alloy (AL), die steel (DS) and stainless steel (SS)—are successfully 3D printed, and the desired geometric profile is achieved with high precision. Prior to printing, the digital file of the prototype (PT) bolt is obtained by a 3D laser scanning system. Afterwards, mechanical tensile tests, infrared thermography (IRT) tests and digital image correlation (DIC) tests are carried out. The mechanical testing results indicate that the peak strength of the DS bolt is approximately twice that of the PT bolt. The IRT results show that the maximum temperature on the surface of all bolts except the AL bolt increases as the load increases, and the minimum temperature on the bolt surface is barely impacted by loading. DIC indicates a feasible relationship between the load and strain for the PT bolt, AL bolt and DS bolt. Overall, the AL bolt is not suitable for replicating the PT bolt, whereas the DS bolt and SS bolt are suitable choices. The conclusions established in this study provide new measures for future research regarding laboratory bolting tests and even engineering bolting trials.

Failure Analysis of a Mobile Crane: A Case Study

This paper presents metallurgical failure analysis method to find the root cause of a mobile crane turret bolts failure. Focus of the metallurgical analysis was to identify the root causes of the failure of turret bolts which led to the collapse of the mobile crane. The turret bolts were made of high strength steel. Comprehensive engineering analysis suggested that the turret bolts failure was initiated with fatigue. The fatigue was most likely initiated by the corrosion at the bolt surface and accelerated by the loosening of bolts due to long servicing life and/or fluctuating tensile load. Based on the investigations, recommendations are provided to guide the operation of the mobile crane to prevent the premature failure.