Which types of bolt failure are most common?

This article is from the March 2024 Bolt Science Newsletter. Long ago when I worked as part of a large team of design engineers, there were people who stated that fatigue was the commonest cause of bolt failures, others said that it was insufficient preload, and fatigue was an indirect cause. I was reminded of this when I read a recent technical paper (Review on recent advances in structural health monitoring paradigm for looseness detection in bolted assemblies, in Structural Health Monitoring, March 2023). This included an interesting pie chart that I've reproduced below. This shows the distribution of articles based on bolt failures published from 2016 to 2021 obtained by keywords bolt failure, bolted joints, and machine learning.

Pie chart showing percentage failure rates of bolts

I thought about how close the chart tallies with percentages of actual failures rather than papers written about failures. It is difficult to come to any definitive conclusion as to the failure distribution across all industries, but the chart roughly tallies with my own experience of failures in the mechanical engineering sector. The largest proportion of failures that I have come across have been due to bolts coming loose, often as a result of insufficient preload for one reason or another. Bolt fatigue is usually, but not always, the result of insufficient preload. This can either be due to joint separation (a gap occurring in the joint when axial forces are applied) resulting in the bolt sustaining a high alternating stress. Alternatively, in shear loaded friction grip applications, joint slip occurs causing bolt bending and subsequently, a bending fatigue failure.

M24 Bolt Failure If joint slip occurs in a friction grip joint, and the bolt is prevented from self-loosening by a locking device being present, after several thousand cycles, bolt fatigue can occur since as the joint moves, the bolt bends. This can be readily demonstrated on a Junker transverse vibration test in which a locked bolt fails by fatigue after about 3000 cycles. A locking device is useful in preventing self-loosening but not in preventing fatigue if the loading that can cause joint slip is frequently repeated. On friction grip shear loaded bolted joints, loosening and fatigue can be due to the same underlying mechanism. That is, joint slip due to insufficient preload.

Although the paper doesn't define what they mean by a shear failure, it is likely to be thread shear (thread stripping) together with failures associated with the direct shearing of bolts. Direct shearing, in which the bolt is sheared in half is unusual in mechanical engineering. Most joints are friction grip and so rely upon the friction between the joint faces as a result of the bolt preload; the bolt itself does not directly sustain the shear. Thread stripping in comparison is not that unusual but typically occurs when a bolt is used in a tapped hole and the length of thread engagement is insufficient.

Other less common failures include direct overload in which the bolt fails by tensile fracture. In mechanical engineering this is unusual since joint separation occurs before tensile fracture and, if repeated, results in a fatigue or a loosening failure. Overload failures are usually due to one-off very high forces being sustained by the joint under extreme conditions.

Another less common failure is hydrogen embrittlement. Most engineers will not encounter such a failure and it is due to the presence of hydrogen causing cracks to occur in the microstructure of high strength bolts (usually property class 12.9 but occasionally 10.9). The hydrogen being introduced during the bolts manufacture (from a process such as electro-plating) or from the environment (such as corrosion).

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You may also be interested in an article on this failure:

 

Failure of an M24 Engine Mounting Bolt

Published in the Fastener Technology International magazine in August 2006, this article looks into the causes of the failure of M24 engine mounting bolts. An M24 property class 8.8 bolt was used to secure one of four engine mounts to the chassis of a bus. Following the introduction of the bus into service and some operational experience, reports started to be received that bolts were occasionally found loose and, on a number of occasions, the bolts were failing. To prevent what was perceived to be a loosening problem, a split pin was introduced that passed through the bolt thread immediately below the nylon insert nut to prevent the possibility of the nut backing off. This fix proved to be only partially successful and instances were still being reported that the nuts continued to back off, leading to the split pin being completely sheared off in some instances. Fatigue failures continued to be experienced. This failure illustrates two problems that inadequate preload can manifest itself as. Fatigue failure is a common by product of inadequate preload; joint movement, because the friction grip was inadequate, results in stresses being induced into the bolt that it was never designed to sustain. This same movement, when the fatigue strength of the bolt is able to sustain the induced stresses, will result in the tendency for the fastener to self loosen.

The importance of achieving and maintaining an adequate preload is often the crucial factor in ensuring that the structural integrity of the joint is assured.