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.
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.
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.
