THREADED FASTENER FAILURES THREADED FASTENER FAILURES
Introduction
Threaded fasteners are used in almost every kind of electrical or mechanical device, appliance,
machine or structure, from the smallest mechanical watch to the largest ship
There are thousands of other possible shapes, some of which are shown in Figure 2. The shapes may be different but their functions, failure modes and failure mechanisms are similar.
Threaded fasteners are more likely to fracture when they are subject to alternating or fluctuating stresses and when the peak stress levels are high in relation to the endurance limits of the material used.
High tensile steels are used for fasteners in order to reduce the size and eventually the cost of the equipment. These steels are more sensitive to the effects of stress raisers, surface finishes etc.
Increase in stress levels often lead to failures. why they occur and how such failures are to be prevented.
1- Fracture Below the Head in Bolts
Figure 3 shows the location and the appearance of the fracture of a bolt head It was a typical fatigue fracture with two distinct zones in the fractured surface: one was smooth, with tide marks and dull, and the other rough and bright.
This bolt had obviously been under a very high alternating or fluctuating stress.A few bolts with visible cracks under the head were detected. On some bolts, hair line cracks were detected on applying dye penetrant.
There was a sharp corner at the junction between the head and the shank of the bolt. This resulted in
stress concentration . It all depended on the actual radius of curvature and the surface finish at the junction between the head and the shank. These two features varied from bolt to bolt
It must be ensured, while using bolt heads with generous fillets of the type shown in
Figure 4 that the mating component has a chamfer as shown in Figure 5
2- Fracture through Roots of Threads
This type of failure can occur in bolts, screws and studs, which have threaded portions
The fracture starts from the root of the thread, which is sharp. As, there is severe stress concentration, fatigue cracks usually start from such points.
Providing a curved root of thread as shown in Figure 7 can prevent failures of this type
If the threads are produced by rolling instead of machining, the endurance limit of the material gets enhanced and the failure rates reduced.
3- Fracture through Corners at Changes of Section
Sometimes the shank of a bolt is reduced in diameter to that at the roots of the threads in
order to improve its ability to with stand impact loads. In such cases there are two additional
changes of section as shown in Figure 8
This will inevitably create some stress concentration.
However it can be made as low as 1.5 by selecting an appropriate radius for the fillet as shown , and fractures can be prevented.
4- Fracture of Threaded Fasteners due to Excessive Stress
since these are the locations of maximum stress. In that event, it would be necessary to consider the use of material of higher endurance limit. Before doing
so it is advisable to examine the possibility of the stress having become excessive due to some
extraneous factor such as excessive vibration, or inadequate pre-loading of the fastener.
Vibration can become excessive due to a variety of reasons such as:
(a) Excessive clearances in the chain of components involved in supporting the equipment;
(b) Defective or ineffective dampers that may have been provided to minimise vibration or;
(c) Resonance between the natural frequency of the system and the frequency of external impulses.
These possibilities should be examined and appropriate action taken.
Pre-loading of threaded fasteners means tightening the nuts on the fasteners to some specified torque, producing a pre-determined and steady tensile stress in the fastener that is almost equal to the yield point of the material.
Increasing the initial or static tension in the fastener to nearly the point where the material starts developing a permanent deformation can actually prevent fracture due to metal fatigue. It can be proved by mathematical analysis as also by actual tests and measurements that this pre-loading results in a reduction in the alternating component of the stress in the fastener, thereby minimising the development of fatigue cracks.
5- Failures Caused by Relaxation of Tension in Threaded Fasteners
Pre-loading of the fasteners is necessary either to prevent leakage of fluids from pressure vessels or to prevent movement of the components held together as in the case of transformer coils.The force exerted by the fasteners is important for preventing failures of other components.
The fluid inside the pipe or vessel is prevented from leaking by the flanged joint with a gasket between the flanges,Initially, when the joint is installed full tension, nearly equal to the yield point of the bolt, is applied by tightening the nuts with the appropriate torque. In the process, the bolts elongate and the gasket is compressed. There is a balance of tensile and compressive forces.
The material of the gasket, which may be a laminate of some fibrous material with a natural or synthetic binder, continues to shrink under the influence of the pressure on it and possibly the heat if the operating temperature is high. The rate of shrinkage is very small and even the total shrinkage is also small but it is of the same order of magnitude as the elastic elongation of the bolts and the elastic compression of the flanges. The net result is that these elastic deformations get reduced and consequently the tensile forces exerted by the bolts also get reduced significantly. The joint may start leaking or the gasket may get blown out causing catastrophic leakage of fluid. The bolt may not be damaged.
Replacing the gasket and re-tightening the bolts can easily effect repairs. However, the gasket failure
may be said to be due to failure of the bolt to provide the required force as a result of the shrinkage.
The remedy, for preventing such failures, is to tighten the nuts periodically in the initial months of service until the gaskets stabilise and the nuts do not turn on application of the specified torque.
6- Failures of Fasteners due to Relaxation Caused by Metal Creep
If in the case of the flanged joint shown in Figure 9, the operating temperature is higher than 300 °C, we have to consider the phenomenon of metal creep, shown in Figure 10
Metal creep is a property of metals. Steel bolts elongate continuously when subjected simultaneously to a tensile stress and a temperature higher than their Creep Temperature Limit (CTL). The time rate of elongation depends on (a) the tensile stress and (b) the excess of the operating temperature over the creep temperature limit of the metal. If allowed to continue in service without
periodical re-tightening, eventual failure of such components, is certain. Proper design can help to minimise the creep and timely tightening and replacement of such components can prevent failures in service.
Metal creep can lead to indirect effects in flanged joints. The tension in the bolts will reduce as a result and this will cause leakage and/or failures of gaskets. How frequently the nuts should be re-tightened and the bolts replaced depends on design details, operating temperatures etc.
7- Locking Arrangements for Nuts in Threaded Fasteners
Spring washers, serrated washers, castle nuts with split pins, nuts with nylon ring inserts, split nuts, etc. and many other devices have been used to prevent nuts from becoming loose.
Whenever the nuts have to be tightened sufficiently to produce a stress in the bolt close to the yield point, such locking devices are redundant. The friction between the contact surfaces of the nut is adequate to hold it in position. If there is severe vibration, it is advisable to use locking devices.
8- General Design of the Fastener
the basic design of the fastener may be considered to be possibly defective. It may be necessary to use larger fasteners or material of higher mechanical strength may have to be selected. Detailed calculations and analysis will be necessary at this stage. The designers and manufacturers of the equipment will have to be consulted
9- Failures of Screws due to Errors in Lengths
There are some cases of fastener failures due over sight. Failures of screws due to bottoming
in blind tapped holes is one of them.
The overall length of the screw may be more than the required length, or the depth of the tapped hole may be less than the required depth. It is also possible that some foreign matter such as machining swarf has collected in the blind hole
the effect is that even when the screw S seems to be and actually is tightened fully, the component B which the screw is supposed to hold down remains loose.
The effect could be fracture of either the component B or of the screw S if the assembly is subject to vibration as in rolling stock. If the application involves electrical contact, overheating and burning may be the result.
The same results will follow if the unthreaded length of a bolt is incorrect as
illustrated in Figure 12
The use of fasteners of the correct lengths is an obvious technical remedy. On the managerial aspects the remedies are not so distinct. In large organisations to prevent such errors and to meet other requirements modular training systems should be organised for the workers.
10- Failures of Threaded Fasteners due to Stripping of Threads
Sometimes, in equipment that has been in service for a long time, threaded fasteners may fail by stripping of threads. The cause is usually corrosion. Either one or both of the internal and external threads may have worn or corroded to such an extent as to reduce the extent of overlap between the two. The remedies include checks on this aspect during periodical overhauls, replacement of fasteners with any visible signs of corrosion, measures to prevent corrosion.
Even new fasteners have failed in this mode due to machining errors, in organisations which did not inculcate basic engineering discipline with regard to dimensions, tolerances, etc.
SUMMARY
The do's and don'ts for preventing fastener failures are:
--Special care has to be taken when fasteners are subject to vibration, alternating or fluctuating stresses and corrosion.
--If there are changes in section of the fastener, and at corners under the heads, the design should include fillets of ample radius. Routine inspections should include checking for cracks at vulnerable location
--In highly stressed fasteners, the roots of the threads should be rounded.
--If fastener design seems satisfactory, failure investigation is directed towards possibilities of excessive stress due to vibration, overloading, unequal load sharing, etc.
--If design calls for control on tightening torque, torque wrenches should be used for tightening the fasteners.
--Look for and eliminate relaxation of fastener tension during service due to effects of metal creep, shrinkage of non-metals, vibration, failures of locking devices, etc.
--Prevent bottoming of screws in blind tapped holes and inadequacies in the lengths of threaded portions in bolts.
--Locate signs of corrosion in both internal and external threads. Replace corroded fasteners.
--If none of the above reveal any defect, check the composition and mechanical properties of the material, while investigating fastener failures.
THREADED FASTENER FAILURES
Introduction
Threaded fasteners are used in almost every kind of electrical or mechanical device, appliance,
machine or structure, from the smallest mechanical watch to the largest ship
 |
| FIG 1 |
There are thousands of other possible shapes, some of which are shown in Figure 2. The shapes may be different but their functions, failure modes and failure mechanisms are similar.
 |
| FIG 2 |
High tensile steels are used for fasteners in order to reduce the size and eventually the cost of the equipment. These steels are more sensitive to the effects of stress raisers, surface finishes etc.
Increase in stress levels often lead to failures. why they occur and how such failures are to be prevented.
1- Fracture Below the Head in Bolts
Figure 3 shows the location and the appearance of the fracture of a bolt head It was a typical fatigue fracture with two distinct zones in the fractured surface: one was smooth, with tide marks and dull, and the other rough and bright.
This bolt had obviously been under a very high alternating or fluctuating stress.A few bolts with visible cracks under the head were detected. On some bolts, hair line cracks were detected on applying dye penetrant.
 |
| FIG 3 |
There was a sharp corner at the junction between the head and the shank of the bolt. This resulted in
stress concentration . It all depended on the actual radius of curvature and the surface finish at the junction between the head and the shank. These two features varied from bolt to bolt
 |
| FIG 4 |
Figure 4 that the mating component has a chamfer as shown in Figure 5
 |
| FIG 5 |
This type of failure can occur in bolts, screws and studs, which have threaded portions
The fracture starts from the root of the thread, which is sharp. As, there is severe stress concentration, fatigue cracks usually start from such points.
 |
| FIG 6 |
 |
| FIG 7 |
If the threads are produced by rolling instead of machining, the endurance limit of the material gets enhanced and the failure rates reduced.
Sometimes the shank of a bolt is reduced in diameter to that at the roots of the threads in
order to improve its ability to with stand impact loads. In such cases there are two additional
changes of section as shown in Figure 8
 |
| FIG 8 |
However it can be made as low as 1.5 by selecting an appropriate radius for the fillet as shown , and fractures can be prevented.
4- Fracture of Threaded Fasteners due to Excessive Stress
since these are the locations of maximum stress. In that event, it would be necessary to consider the use of material of higher endurance limit. Before doing
so it is advisable to examine the possibility of the stress having become excessive due to some
extraneous factor such as excessive vibration, or inadequate pre-loading of the fastener.
Vibration can become excessive due to a variety of reasons such as:
(a) Excessive clearances in the chain of components involved in supporting the equipment;
(b) Defective or ineffective dampers that may have been provided to minimise vibration or;
(c) Resonance between the natural frequency of the system and the frequency of external impulses.
These possibilities should be examined and appropriate action taken.
Pre-loading of threaded fasteners means tightening the nuts on the fasteners to some specified torque, producing a pre-determined and steady tensile stress in the fastener that is almost equal to the yield point of the material.
Increasing the initial or static tension in the fastener to nearly the point where the material starts developing a permanent deformation can actually prevent fracture due to metal fatigue. It can be proved by mathematical analysis as also by actual tests and measurements that this pre-loading results in a reduction in the alternating component of the stress in the fastener, thereby minimising the development of fatigue cracks.
5- Failures Caused by Relaxation of Tension in Threaded Fasteners
Pre-loading of the fasteners is necessary either to prevent leakage of fluids from pressure vessels or to prevent movement of the components held together as in the case of transformer coils.The force exerted by the fasteners is important for preventing failures of other components.
The fluid inside the pipe or vessel is prevented from leaking by the flanged joint with a gasket between the flanges,Initially, when the joint is installed full tension, nearly equal to the yield point of the bolt, is applied by tightening the nuts with the appropriate torque. In the process, the bolts elongate and the gasket is compressed. There is a balance of tensile and compressive forces.
 |
| FIG 9 |
The material of the gasket, which may be a laminate of some fibrous material with a natural or synthetic binder, continues to shrink under the influence of the pressure on it and possibly the heat if the operating temperature is high. The rate of shrinkage is very small and even the total shrinkage is also small but it is of the same order of magnitude as the elastic elongation of the bolts and the elastic compression of the flanges. The net result is that these elastic deformations get reduced and consequently the tensile forces exerted by the bolts also get reduced significantly. The joint may start leaking or the gasket may get blown out causing catastrophic leakage of fluid. The bolt may not be damaged.
Replacing the gasket and re-tightening the bolts can easily effect repairs. However, the gasket failure
may be said to be due to failure of the bolt to provide the required force as a result of the shrinkage.
The remedy, for preventing such failures, is to tighten the nuts periodically in the initial months of service until the gaskets stabilise and the nuts do not turn on application of the specified torque.
6- Failures of Fasteners due to Relaxation Caused by Metal Creep
If in the case of the flanged joint shown in Figure 9, the operating temperature is higher than 300 °C, we have to consider the phenomenon of metal creep, shown in Figure 10
 |
| FIG 10 |
periodical re-tightening, eventual failure of such components, is certain. Proper design can help to minimise the creep and timely tightening and replacement of such components can prevent failures in service.
Metal creep can lead to indirect effects in flanged joints. The tension in the bolts will reduce as a result and this will cause leakage and/or failures of gaskets. How frequently the nuts should be re-tightened and the bolts replaced depends on design details, operating temperatures etc.
7- Locking Arrangements for Nuts in Threaded Fasteners
Spring washers, serrated washers, castle nuts with split pins, nuts with nylon ring inserts, split nuts, etc. and many other devices have been used to prevent nuts from becoming loose.
Whenever the nuts have to be tightened sufficiently to produce a stress in the bolt close to the yield point, such locking devices are redundant. The friction between the contact surfaces of the nut is adequate to hold it in position. If there is severe vibration, it is advisable to use locking devices.
8- General Design of the Fastener
the basic design of the fastener may be considered to be possibly defective. It may be necessary to use larger fasteners or material of higher mechanical strength may have to be selected. Detailed calculations and analysis will be necessary at this stage. The designers and manufacturers of the equipment will have to be consulted
9- Failures of Screws due to Errors in Lengths
There are some cases of fastener failures due over sight. Failures of screws due to bottoming
in blind tapped holes is one of them.
The overall length of the screw may be more than the required length, or the depth of the tapped hole may be less than the required depth. It is also possible that some foreign matter such as machining swarf has collected in the blind hole
 |
| FIG 11 |
the effect is that even when the screw S seems to be and actually is tightened fully, the component B which the screw is supposed to hold down remains loose.
The effect could be fracture of either the component B or of the screw S if the assembly is subject to vibration as in rolling stock. If the application involves electrical contact, overheating and burning may be the result.
The same results will follow if the unthreaded length of a bolt is incorrect as
illustrated in Figure 12
 |
| FIG 12 |
10- Failures of Threaded Fasteners due to Stripping of Threads
Sometimes, in equipment that has been in service for a long time, threaded fasteners may fail by stripping of threads. The cause is usually corrosion. Either one or both of the internal and external threads may have worn or corroded to such an extent as to reduce the extent of overlap between the two. The remedies include checks on this aspect during periodical overhauls, replacement of fasteners with any visible signs of corrosion, measures to prevent corrosion.
Even new fasteners have failed in this mode due to machining errors, in organisations which did not inculcate basic engineering discipline with regard to dimensions, tolerances, etc.
SUMMARY
The do's and don'ts for preventing fastener failures are:
--Special care has to be taken when fasteners are subject to vibration, alternating or fluctuating stresses and corrosion.
--If there are changes in section of the fastener, and at corners under the heads, the design should include fillets of ample radius. Routine inspections should include checking for cracks at vulnerable location
--In highly stressed fasteners, the roots of the threads should be rounded.
--If fastener design seems satisfactory, failure investigation is directed towards possibilities of excessive stress due to vibration, overloading, unequal load sharing, etc.
--If design calls for control on tightening torque, torque wrenches should be used for tightening the fasteners.
--Look for and eliminate relaxation of fastener tension during service due to effects of metal creep, shrinkage of non-metals, vibration, failures of locking devices, etc.
--Prevent bottoming of screws in blind tapped holes and inadequacies in the lengths of threaded portions in bolts.
--Locate signs of corrosion in both internal and external threads. Replace corroded fasteners.
--If none of the above reveal any defect, check the composition and mechanical properties of the material, while investigating fastener failures.
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