Since 2007
Christophe Berre has been a project leader at TWI where he has worked on fatigue testing of welded joints, resonance testing of pipelines and residual stress measurements. He has written papers on: microstructural modelling of nuclear graphite using X-ray microtomography; finite element modelling of composite materials; finite element modelling of delamination in composite structures and bolted connections and bolting processes on the aerospace assembly line.
Bolted connections are used in many industries to join structural components easily and quickly. These connections can withstand very high loads and offer the possibility of disassembly and re-assembly of the joined parts if necessary. However, as Christophe Berre explains because of their importance in critical parts, a good knowledge of bolting practice is required for the design of structurally reliable connections. This article reviews the applications and usage of structural bolted connections in the industry. It shows a general overview of the state-of-the-art and presents methods to assess and improve the structural integrity of bolted connections.
Structural bolted connections in the industry
Construction and engineering
The integrity of highly complex metallic structures such as bridges, railways, wind turbines, scaffolding, cranes and lattice masts, often relies on bolted connections. In all structural steelwork the connections can be subjected to harsh atmospheric conditions and vibrations which may cause corrosion or bolt loosening.
A very large number of structural steelwork connections are used in metallic frames of buildings. Anchor bolts are also used to maintain high metallic structures, such as wind towers, to the ground, as described in BS EN 61400 Part 1. The American Institute of Steel Construction (AISC) has published useful recommendations concerning the applicable pre-tensions, hole diameters, minimum spacing, minimum edge distance and formulae to calculate the maximum allowable nominal force in bolts. Based on the load and resistance factor design method (LRFD), the Research Council of Structural Connections (RCSC) also provides useful general rules and guidance for the design of joints using structural bolts.
Shipbuilding
In the shipbuilding industry, bolted connections can be used for manholes, lifting appliances and piping systems. The connections are then subjected to machinery-induced vibration and seawater corrosion. Materials used must then be corrosion-resistant such as stainless steel or bronze.
Automotive industry
In the automotive industry, bolts are used to assemble the chassis, to attach wheels, suspension and body panels. Resistance to vibration is a major factor for the integrity of these connections, which can be subjected to high levels of impact.
Aerospace industry
Commercial aircraft contain between one-and-a-half and three million mechanical fasteners and can be used to connect dissimilar materials such as metal-to-composite connections. Bolts are used in critical parts of the aircraft such as the landing gear, wings or spoilers. Materials and connections are therefore vital parts of the structure and must withstand high dynamic loads over long periods of time, in a particularly harsh and corrosive environment, over a large range of temperatures. Light-weight materials such as titanium or Inconel are preferred for their good performance across the required temperature range.
Oil and gas industry
In the oil and gas industry, bolted joints are often used to connect pipework, heat exchangers and pressurised equipment. Bolts such as those specified in BS 8010 Part III are used in flanged connections and are required to withstand high temperatures and high pressures in for example pressure vessels, gas turbines, drilling systems, valves, flange pipe joints, risers and tethers. In such environments, the prevention of leakage requires the use of gasketed flange connections in which the interactions between the different parts are particularly complex. Bolted flanges may rotate under torsional forces which may result in leakage. Operating temperatures can be up to 750°C and materials must be able to withstand forces due to cyclic loading, corrosion, and vibration. Guidance on the design of bolted joints in pressure vessels, steam turbines and flange connections can be found in PD 5500 and BS EN 13445-3. Active research is currently ongoing, particularly on the sealing performance of gaskets.
Bolted connections and tightening methods
Structural bolts
Bolted joints can be subjected to shear, tension or both. The geometry of the connection can be a lap joint, flanges, made of a row of several fasteners or with different geometrical patterns. There are three main types of bolting materials: carbon-steel, high-strength steel or quenched and tempered alloy steel. Mechanical and physical properties are provided by BS EN ISO 898-1. Some characteristics can be defined as follows and are illustrated in Figure 1. Markings are shown on the bolt head to ensure the characteristics of a fastener.
Bolt sizes range from M1, M6 to M100 where M specifies ISO metric threads and the number, ie 1, 6 to 100, specifies the nominal thread diameter in millimetres. The bolt class is defined between 3.6 and 12.9 in which the first figure multiplied by 100 gives the nominal tensile strength in N/mm2 and the second figure indicates 10 times the ratio between the lower yield stress and nominal tensile strength (BS EN ISO 898-1). The minimum tensile strength of ISO bolts is therefore between 300MPa and 1200MPa and the lower yield strength is typically 60, 80 or 90% of the minimum tensile strength. The grade depends on the material used and on the manufacturing process.
General 'ordinary' bolts Grade 4.6 (BS EN 20898-1) are used for engineering applications and high-strength friction grip bolts (HSFG) can be used for connections requiring higher clamping loads. Grades 8.8 and 10.9 are the most common HSFG bolts used. Their extra strength allows for loads to be transferred through the joint by friction between the faying faces rather than by shearing of the bolt. However, high strength materials of class 10.9 and above are more susceptible to stress corrosion and hydrogen cracking.
Washers
Washers are commonly used to increase the fatigue strength of the connection. There are various benefits to the use of washers such as listed below.
- Better torque control during installation.
- Protection of the outer surface of the connected material.
- Enlargement of the load bearing area and reduction of contact pressures in pre-tensioned bolts.
- Maintenance of a high clamping force.
- Minimisation of bending in the bolt in the case of taper washers.
Gaskets
To prevent leakage and for additional sealing of pressure joints, gaskets of low stiffness can be added to the connection; see Figure 2. Common gasket materials are polytetrafluoroethylene (PTFE), exfoliated graphite sheet and rubber. Stresses in gasketed connections are more evenly distributed but the highly non-linear behaviour of the joint is difficult to model numerically.
Tightening loads
A correct tightening load must be applied to the bolt to ensure that the connection will not loosen during service due to insufficient clamping force but also that the connection will not prematurely fail under fatigue because of too high a clamping force.
The clamping load must be between 70 and 80% of the proof or yield load of bolts having a friction coefficient between 0.15 and 0.2. The amount of clamping load is increased for lower friction coefficients. Where protective coating is used, care must be taken not to induce any thread stripping failure during installation which could remove the protection.
Torque wrenches
Torque wrenches are pre-set to a defined torque and are useful to fasten a large amount of bolts quickly. However, their accuracy is limited, typically around +/- 20%, and their utilisation requires consistent conditions which may be with or without lubrication. Hydraulic jacks and impact wrenches use mechanical elongation of the bolt to apply very high pre-loads.
Direct tension indicators
Direct tension indicators (DTI) are compressible washers fitted with bumps and allow for a quick and easy visual inspection. They can be placed either below the head of the bolt or below the nut. The DTI mechanism may sometimes be quite delicate for industrial applications subjected to harsh conditions and/or high loads. General requirements and recommendations for washer-type DTIs are given in BS 7644.
Turn-of-the-nut method
The turn-of-nut method is based on the angle of rotation of the nut and is expected to stretch the fastener to a minimum of 75% of its ultimate tensile strength. This method is claimed to be quite efficient with accuracies in pretension around 5% reported for a uniform batch of bolts.
Heating devices
Heat elongation fastening is more accurate but the heating of the bolt requires a long time. In this procedure, the bolt is heated to the required temperature then inserted into the hole and the nut is simply nipped up. Pre-tension is induced in the bolt by contraction on cooling.
Structural integrity and improvement by design
Causes for failure
TWI has often dealt with failure investigations in bolted connections. In some of the cases, failure resulted from accidental overload during tightening, corrosion, fatigue fretting, design faults, the effect of prying action or misalignments.
TWI has also conducted extensive fatigue testing of untightened bolts of various grades, diameter and manufacturing route, subjected to axial loading. The results were published in a useful compilation document produced by the UK Health and Safety Executive in 1998.
Fatigue assessment
BS 7608 provides the method for the fatigue assessment of steel structures. In this Standard, bolts subjected to tension are defined as Class X and bolts subjected to shear can either be defined as Class C, D, E or G, depending on their type and material. For Class X, S-N curves for axially loaded bolts are defined in terms of the ratio of the stress range applied on the tensile area of the bolt and the minimum tensile strength of the bolt material. Calculation of the applied stress range in the bolt must include the effects of axial and bending loads, the pre-load remaining in the bolt and any prying action. However, the stress concentrations due to the threads and head radius are already included in the S-N curves and do not need to be estimated. The fatigue limit of bolts is approximately 6% of the stress range divided by the minimum tensile strength of the material. In seawater installations, the fatigue life is reduced by a factor of two if the joints are unprotected.
Pre-tension monitoring and stress relaxation
The loss of clamping force in a bolted joint is a major concern in many structural applications subjected to vibration due to external sources such as wind, traffic and machinery. Bolt loosening is generally due to successive bending and shearing. Suitable pre-tension must be applied to ensure that the in-service loads are distributed on the faying surfaces rather than through the bolt. Stress-relaxation monitoring can be carried out during service by direct length measurements using a micrometer, or using more complex techniques such as ultrasound, laser or piezoelectric sensors. Micrometer measurements such as C- or depth micrometers are very often used, but distortion of the bolt may induce some errors in the measurements.
The stress relaxation in joint models can be experimentally tested following the methodology defined in BS EN 10319 or by the so-called Junker test. The Junker test is defined in DIN 65151 and uses a vibrating table to test the loosening resistance of a bolted joint. The bolt pre-tension is measured during the test by a load cell and can then be plotted against the number of shear cycles.
Design principles
Fatigue can be reduced by selecting appropriate materials to prevent crack initiation, by reducing load excursions and reducing stress in parts. There are also optimum dimensions for each specific bolting application and care must be taken in deciding on parameters such as the diameter, pitch and diametric fit with the nut. BS 3580 provides useful recommendations for the design and the improvement of fatigue performance of bolts.
Stresses in bolts are not evenly distributed around the head, the body and the end of the bolt. The stress distribution along the thread depends on the bending of the bolt, the tensile stress in the core of the bolt, and the concentration effect of the thread root. The maximum load along the thread helix is always located at the point where the nut engages the thread. Additional torsional stresses are due to the friction of the thread and must be taken into account in the calculation of the overall strength of the bolt.
Maximum loads shall not fall on stress concentration areas of the fastener and thread run-out points should be gradual. To obtain a well distributed stress at the contact area between the nut and the bolt, the nut material should be similar to or softer than the bolt. The most favourable mode of failure of a bolt is across the core of the bolt rather than by thread stripping because it is more gradual and easier to detect than failure in the threads. Manufacture of the thread by cold rolling or the application of a cold rolling process to the root also improves the fatigue resistance of the thread.
Prying action
Prying action may also occur in a bolt assembly loaded in axial tension and is caused by the eccentricity of the bolt to the main axial direction of the tension force. The moment resulting from the lever action increases the total axial tension felt by the bolt and adds a bending moment to the total forces of the bolt. Prying action depends on the distance of the bolts from the loading axis and from the stiffness of the connected plates and must be taken into account in the design (Figure 3).
Summary
This review has highlighted the broad range of application of bolted connections covering many industry sectors and types of structure. Research conducted over several decades has provided a good understanding of the engineering performance of the bolts themselves and much of this information is available in design codes.
In TWI's experience, misalignment and poor fit-up are the primary causes of problems in bolted connections. Inevitably these deficiencies lead to the application of increased torque in attempting to bring the mating faces together. If fit-up gaps remain, prying action and uneven load sharing between bolts result in additional bending stresses, which may lead to premature failure. These shortcomings are not recognised or allowed for in the treatment in most codes; good fit-up and even load distribution is an implicit assumption of the treatment adopted in standards.
Acknowledgement: Martin Ogle and Peter Tubby are gratefully acknowledged for their contribution to this article.
