Technical Article Published in the August 2015 Edition of Fastener Technology International

Bill Eccles of Bolt Science had an article published in the August 2015 edition of the Fastener Technology International magazine. The article titled 'Bolt Fatigue Failure due to Insufficient Tightening' is a case study discussing the consequences of not adequately tightening bolts.

Cross Section of a Fatigue Failure of a Bolt

Not all fastener failures are due to a manufacturing or material defect, or even a design flaw. With bolts, and threaded fasteners in general, failure to tighten them sufficiently is a common cause of failure. It is by no means obvious why fasteners should be tightened to the extent that they are, especially those of larger diameter. Hence, on occasion, without the right controls in place, they can easily be under tightened.

The plates comprising a joint are clamped together by tightening the bolts. In doing so any subsequent loading that is applied to the joint unclamps it rather than additionally loading the bolt to any significant amount. If the preload is insufficient the joint can be completely unloaded, that is, no clamp force present between the plates. The terms frequently used for this condition are joint decompression or joint separation. Any subsequent additional loading, once the joint decompression point is reached, is completely sustained by the bolt. If the load that is applied to the joint fluctuates, it results in the bolt sustaining a high alternating stress. Fatigue failure, normally in the threaded part of the bolt, is often the consequence. If joint decompression can be prevented by adequate tightening, in most, but not all, instances, fatigue failure will not occur.

Cross Section of a Fatigue Failure of a Bolt

Fatigue is a common, possibly the commonest, cause of failure for fasteners in service. The root cause in many such situations is however a lack of bolt preload. A case of fatigue failures of bolts used in a specialist machine illustrates this point and is reported on in this article.

The conclusion, fatigue failure of bolts as a result of inadequate tightening is relatively common. Ensuring that fasteners are tightened sufficiently is one of the key factors to ensuring product reliability with bolted joints.

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