Bolt Material Research Articles

An empirical component-based model for high-strength bolts at elevated temperatures

High-strength structural bolts are used in nearly every steel beam-to-column connection in typical steel building construction practice. Thus, accurately modeling the behavior of high-strength bolts at elevated temperatures is crucial for properly evaluating the connection capacity, and is also important in evaluating the strength and stability of steel buildings subjected to fires. This paper uses a component-based modeling approach to empirically derive the ultimate tensile strength and modulus of elasticity for grade A325 and A490 bolt materials based on data from double-shear testing of high-strength 25 mm (1 in.) diameter bolts at elevated temperatures. Using these derived mechanical properties, the component-based model is then shown to accurately account for the temperature-dependent degradation of shear strength and stiffness for bolts of other diameters, while also providing the capability to model load reversal.

Fatigue failure of the bolts connecting a Francis turbine with the shaft

Abstract This paper describes the failure analysis of bolts used to connect a turbine to the shaft of a hydroelectric power generator. Three of ten bolts were found broken during a disassembly process to replace the turbine. The methodology included visual inspection of the thread to detect eventual stress concentrations. Using loads measured on the turbine shaft for different power levels and the actual power generation history for a period of one year, a stress history on the bolts was obtained. With this stress history, damage was calculated for three preload values. The results showed some scratches on the root of the thread produced during the machining process and by subsequent corrosion during operation, which likely significantly affected the fatigue life. A preload that was smaller than the recommended one was found in the maintenance report, which put the bolts at risk of loosening. A fatigue life of only 16.4 years was estimated if loosening occurs on the bolts. Better preload control during the assembly process and a change in the bolt material to stainless steel are the primary recommendations from this analysis.

Development of high-strength bolt material of Al–Mg–Si alloy by ECAP and various aging treatments

At present, the strength of most of Al–Mg–Si alloy bolts is not enough, and thus highly expected to be increased, for example, to >500 MPa in ultimate tensile strength (UTS). In this study, mechanical strength of A6056 Al–Mg–Si alloy was aimed at improving, without diminishing the elongation, by combined application of equal-channel angular pressing (ECAP) and various aging treatments. Multi-pass ECAP and pre-aging (PA) treatment at 373 K for 1.2 ks was found to be effective in strengthening the bolt material subject to room-temperature storage followed by artificial aging. Especially, PA plus ECAP 2pass treatment exerted the biggest impact on the strength (i.e., 514 MPa in UTS) with a reasonable elongation to fracture (i.e., 16%). Such a high strength and good ductility exceeds minimum requirement for aluminum-made bolts registered in JIS B1057, and thus the developed PA-ECAP 2pass specimen can be utilized potentially as a high-strength bolt material.

Mechanical behaviour of fully grouted GFRP rock bolts under the joint action of pre-tension load and blast dynamic load

In mining operations, the stability of excavations is critical to ensure the safety of production. Rock bolt support is the most effective and economical method commonly used to improve ground stability. The glass-fiber-reinforced plastic (GFRP) bolt has been developed and is becoming widely used because of its corrosion resistant character and its high tensile strength. However GFRP bolts are more susceptible to damage by blast loads and therefore it is important to understand the mechanical behaviour of GFRP bolts under dynamic loads so the support system can be better designed. In this paper we propose a new approach to describe the mechanical behaviour of fully grouted GFRP rock bolts under the combination of pre-stressed static load and blast dynamic load using the Kelvin’s solution. The method was applied to a case study and it was found that large axial and shear stresses of fully grouted GFRP rock bolts are mainly distributed in a small region near the bolt collar. It was further demonstrated that the strength of blasting, the distance between the rock bolt and the blasting point and the Young’s modulus of the rock mass and the bolt material are the most important factors having significant influences on the mechanical behaviour of GFRP bolts. For the cases examined where the bolt is within the range of 15 m from the blasting point, the influence of blast load on the axial and shear stresses of the rock bolt is significant and the dynamic load is the main factor causing the support damage. The proposed model provides a method to quantitatively evaluate the effect of blasting on the mechanical behavior of GFRP rock bolts so that a better ground support system can be designed in practical mining applications.

Experimental Study on Determining Design Parameters of Non-Prestressed BFRP Anchor for Supporting Soil Slope

Basalt fiber-reinforced polymer (BFRP) is a high-strength, lightweight material with an anti-corrosion property and a thermal expansion coefficient that is close to that of concrete. When used as an anchor bolt material, the corrosion problem of steel anchors can be solved fundamentally. In recent years, the BFRP bar has been applied in geotechnical anchorage engineering. In this study, the mechanical properties of the BFRP bar were studied by tensile, shear, corrosion resistance, and bonding strength tests with cement-based materials. In accordance with the design specifications of the steel anchor and existing research results, the design parameters of the non-prestressed BFRP anchor in supporting soil slope were proposed. To examine the reinforcing effect of the BFRP anchor, this study adopted the recommended parameters to design a BFRP anchor in an actual soil slope. A field contrast test of the steel anchor was also carried out. Experimental studies yielded the following conclusions. First, the tensile-strength safety factor of the non-prestressed BFRP bar should not be less than 1.6 (permanent) and 1.4 (temporary). The tensile-strength standard value was 80% of the ultimate tensile strength, and the value of the BFRP bar (diameter ≥ 12 mm) commonly used as an anchor was 710 MPa. The bonding-strength standard value between the BFRP bar and cement was equal to the average value divided by 2.1 of the pull-out test, and the common value was 2.8 MPa. The monitoring field test data proved that the designed BFRP anchor demonstrates better control displacement and can effectively retain soil slope. Moreover, the reinforcing effect of the BFRP anchor was equivalent to that of the steel anchor. Thus, the design of the BFRP anchor is reasonable, and the equal strength substitution method can be used to design the BFRP anchor.

Use of Polyamide-6 Type Engineering Polymer as Grouted Rock Bolt Material

In this study, usability of Polyamide-6 type engineering polymer as a new rock bolt material was investigated carrying out a series of laboratory and field scale experiments. The Polyamide-6 type polymer tested as a grouted rock bolt material was assessed to have a good body strength, large elastic deformation limit and high energy absorbing capacity. The load bearing capacity of Polyamide-6 rock bolts was found to dominantly depend on the mechanical anchorage in front of the shank as a result of having low adhesion to the cement grout values. In this study, polyamide rock bolts have been designed with different mechanical anchors. It was suggested to start the systematical use of polyamide rock bolts having ideal support reactions under both static and dynamic load conditions in rock engineering. Especially, Polyamide-6 rock bolts were assessed to be economically usable in rock masses with squeezing, swelling or bursting problem, which need for non-decreasing support pressure while being higly deformed and good energy absorption capacity.

Causes of Failure of High-Tensile Stud Bolts Used for Joining Metal Parts of Tower Crane

The causes of the failure of a high-tensile stud 2M48-6gx500 10.9 made from steel grade 30HGSA which led to a temporary inoperability of a tower crane were investigated. The bolts were used to assemble the tower sections and collapsed after 45 days from the moment the crane was commissioned. The cracks in the fracture are identified as fatigue with the characteristic sites of nucleation, sustainable development and static dolomite. To determine the possible causes of stud bolts destruction, metallographic, durometric and mechanical tests were carried out from which it follows that the stud bolt material in its original state corresponded to the delivery conditions. The destruction of the stud bolt appears to have resulted from a combination of several unfavorable factors: uncertainty about the actual tension of the stud bolt due to the lack of information about the magnitude of the twist factor; partial displacement of the centers of the brackets holes and rotation of the stud bolt axis during the sections’ assembly; no tight contact on the support surfaces of the section brackets. All this led to a discrepancy between the actual design of the stud bolt, the appearance of additional forces and the destruction of the stud bolt.

Fastener investigation in JET

JET is an experimental fusion reactor consisting of magnetically confined, high temperature plasma inside a large ultra-high vacuum chamber. The inside of the chamber is protected from the hot plasma with tiles made from beryllium, tungsten, carbon composites and other materials bolted to the vessel wall. The study was carried out in response to a JET fastener seizing inside the vacuum vessel.The following study looks at characterising the magnitude of the individual factors affecting the fastener break away torque. This was carried out using a statistical approach, the Taguchi method: isolating the net effect of individual factors present in a series of tests [1](Grove and Davis, 1992).Given the severe environment within the JET vessel due to the combination of heat, ultra-high vacuum and the high contact pressure in bolt threads, the contributions of localised diffusion bonding is assessed in conjunction with various combinations of bolt and insert material.

Microstructural evolution and mechanical properties of a nickel-based superalloy through long-term service

Microstructures and mechanical properties of nickel-based superalloy (GH783 alloy) bolts used in a power plant 660MW Ultra supercritical unit were investigated. The results showed that the material's strength increased monotonically but the elongation and impact toughness both decrease as an exponential decay trend with the service time. Microstructural examinations revealed that γ′ phase within the alloy matrix coarsened significantly leading to an enhancement in strength. Meanwhile, additional γ′ precipitated gradually within the grain-boundary β phase, causing the weakening of the grain boundaries, thus a reduction in ductility. The evolution tendency here obtained could be used to approximately predict the variation tendency of mechanical properties with the service time for GH783 alloy used as bolt material.

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.

Tensile Behaviour of Galvanised Grade 8.8 Bolt Assemblies in Fire

In structural fire engineering, the importance of bolt assemblies is often overlooked. Connection design uses the temperature-dependent bolt strength-reduction factors prescribed in Eurocode 3, despite the existence of two distinct failure modes under tension; necking of the bolt shank, and thread-stripping. While literature exists to predict failure modes at ambient temperature, there is no method for failure mode prediction for elevated temperatures where ductility is critical to avoid collapse. Galvanised M20 structural bolt assemblies and bolt material from a single batch have been tested under tension at a range of temperatures and strain-rates typical of those experienced in fire. Turned-down bolt test data produced stress-strain curves characteristic of different microstructures at ambient temperature, despite a tempered-martensitic microstructure being specified in the standards. The failure modes of bolt assemblies were found to be dependent on the as-received microstructure at ambient temperature. At elevated temperatures, however, only thread-stripping was observed.

Key technologies and equipment for a fully mechanized top-coal caving operation with a large mining height at ultra-thick coal seams

Key technologies and equipment for a fully mechanized top-coal caving operation with a large mining height at ultra-thick coal seams

A Study on the Fracture Control of Rock Bolts in High Ground Pressure Roadways of Deep Mines

According to the frequent fractures of rock bolts in high ground pressure roadways of deep mines, this paper analyzes the mechanism of fractures and concludes that high ground pressure and material de- fects are main reasons for the fracture of rock bolts. The basic idea of fracture control of rock bolts in high ground pressure roadways of deep mines is to increase the yield load and the limit load of rock bolt materials and reduce the actual load of rock bolts. There are four ways of controlling rock bolt fracture: increasing the rock bolt diameter, strengthening bolt materials, weakening support rigidity and the implementation of double supporting. With the roadway support of the 2302 working face of a coal mine as the project background, this paper carries out a study on the effect of two schemes, increasing the rock bolt diameter and the double supporting technique through methods of theoretical analysis, numerical simulation and so on. It determines the most reasonable diam- eter of rock bolts and the best delay distance of secondary support. Practices indicate that rock bolt fracture can be effectively controlled through the double supporting technique, which strengthens the roof and two sides through the first supporting technique and strengthens side angles through the secondary supporting technique.

Force-Deformation Relationship of Bearing-Type Bolted Connections Governed by Bolt Shear Rupture

Well-designed bolted connections can exhibit excellent ductile behavior through bearing mechanism until the occurrence of bolt shear rupture. The ultimate strength analysis of eccentric bolted connections is an economical and mechanistic approach which uses such ductility. However, the bolt load-deformation relationship, which forms basis of the current practice, is based on very limited combinations of bolt and steel materials. The primary objective of this study was to establish the general bolt force-deformation relationship based on systematic single-bolt bearing connection tests. The test results showed that the projected area of the bolt hole and the strength and thickness of the plate to be connected are the main factors affecting the force-deformation relationship. The results of this study can be used for the instantaneous center of rotation method (ICRM) to achieve more accurate analysis and economical design of a variety of group-bolted connections subjected to eccentric shear.

Optimizing Piezoelectric Stack Preload Bolts in Ultrasonic Transducers

The selection of the preload bolt is often an afterthought in the design of Langevin type “sandwich” transducers, but quite often it is the root cause of failure for power ultrasonic applications. The main role of the preload bolt is to provide a “prestress” in the piezo stack to prevent interface “gapping” or tension in glued joints which can result in delamination. But as an integral part of a highly tuned dynamic system, the resulting parasitic resonances in these preload bolts, such as bending or longitudinal modes, are often difficult to predict and control. This research investigates many aspects of preload bolt design for achieving optimal transducer performance, including basic size and strength determination based on drive amplitude, as well as ensuring adequate thread engagement to the mating horn. Other aspects such as rule-of-thumb configuration and length guidelines to reduce parasitic resonances are also investigated. Optimizing the uniformity of stress in the piezoceramics is also considered, which is affected by end mass length, counterbores and proximity to threading. The selection of the bolt material based on stiffness is also investigated as related to electromechanical coupling. The investigation focuses solely on Langevin type transducers used for semiconductor wire bonding, and which are comprised of the common Navy Types I and III (PZT4 and PZT8) piezoelectric materials. Several metrics are investigated such as impedance, displacement gain, and electromechanical coupling factor. The experimental and theoretical research methods include Bode plots, scanning laser vibrometry and finite element analysis.

Recent Advances in PVP Bolted Joint Technology

This paper outlines several areas of advancement in pressure vessel and piping bolted joints that have been researched in recent years. The topics include:•The creation of a quantitative risk assessment for determining the likelihood of joint leakage, which allows the assessment of the most at-risk joints in any given plant or project. This enables resources and urgency to be assigned appropriately to only those joints that require it.•Further examination of the creep/relaxation characteristics of bolt materials will be presented. This information is based on recent laboratory testing performed on a range of bolt materials and will eventually enable the prediction of the influence of metallic component relaxation on joint integrity.•Assembler Qualification will also be examined, based on the system of training and demonstration that has been developed for the IES “Train-the-Trainer” program that fully complies with ASME PCC-1 Appendix A.

Failure analysis of connecting bolts used for anchoring streetlights of a mountain highway

This paper presents a failure analysis of connecting bolts used for holding streetlights inside the tunnels of a mountain highway. The fracture occurred in correspondence to crevices formed in bolted joints. Fractographic and metallurgical studies indicate that the bolt failure is due to a deep crevice corrosion process that is further aggravated by the presence of chlorides coming from the spray formed inside tunnels during the winter when sodium chloride is spread for de-icing roads.Metallographic studies highlighted also that corrosion affects only bolts, while all the other joints’ components are intact. EDS analyses revealed that the bolt material was a Cu-bearing stainless steel that, in chloride media, has a bad corrosion resistance because Cu destabilises the alloy passive film. Therefore, it is recommended that the bolt material should be changed to prevent similar failures and that in such environment crevices should be avoided in design.

Fatigue Characteristics of Nonferrous Bolts at Elevated Temperature

In order to investigate fatigue characteristics of nonferrous bolts at elevated temperature, fatigue tests of bolted joints which were tightened with three kinds of nonferrous bolts were been conducted at 100°C atmosphere. The test bolts were made of A5056 aluminum alloy and AZ31 and AZX912 magnesium alloy. Creep tests of the bolts at 100°C atmosphere were also conducted. The results showed that the fatigue limit of A5056 bolt was the highest of all regardless of the ambient temperature. The fatigue limits of AZ31 bolt and AZX912 bolt also were a half of the fatigue limit of A5056 bolt at both ambient temperature. Bolt clamping force losses due to creep deformation were observed for all bolts during fatigue tests at elevated temperature. Hence as additional tests, the creep tests which was controlled either the tensile force or the displacements respectively were conducted. As the results it was seen that the clamping force losses for all bolts were remarkably large although the each creep deformation was different for each bolt material. Therefore the results indicates that we have to pay attention to the clamping force reduction due to creep deformation if we use the nonferrous bolt in high temperature.

Seizing of Gas Turbine Vane Fixing Bolts Made of X22CrMoV12–1

Abstract It is known from field experience that gas turbine vane fixing bolts, made from high-alloy chromium steel, got stuck in their mating bores and often caused problems upon disassembly. Many of them had to be drilled out rather than removed normally. Several metallurgical in­vestigations determined excessive surface oxidation to be the physical cause for the seizing of the bolts. It was recom­mended to change the bolt material from high-alloy steel to a nickel base superalloy with significantly higher oxidation resistance than steel in order to prevent surface scaling and seizure.

Experiments on Effects to Rock Bolt Pretension by Thread Rolling Accuracy

Pretension is a key factor affecting the performance of rock bolt. It is usually applied on the bolt by tightening the nut. The efficiency of the torque-to-pretension conversion is significantly influenced by the thread rolling accuracy. In this study, the influence of the thread rolling accuracy upon the pretension is investigated. Factors influencing the thread rolling efficiency, i.e., rock bolt material quality, rolling technic and equipments, are examined. To study the relationship between torque and pretension, a laboratory table was specially designed and a series of tests were carried out on the rock bolts with different thread accuracy. The result indicates that the higher the thread rolling accuracy, the higher the coefficient of torque-to-pretension conversion. And the coefficient can be improved by daubing lubrication oil on the thread. In China, the rock bolt thread encounters the issue of low rolling accuracy which results in coarse thread surface. As a result, the efficiency of the torque-to-pretension conversion is greatly reduced. In addition, the rock bolt is susceptible to break at the thread end. Essential approach to resolve these issues is to significantly improve the thread rolling accuracy by adopting advanced equipment.