Don't underestimate the complexities of bolting - beware of the dangers of reduced member stiffness!


Gaskets, dirt, irregular/dished member faces reduce member stiffness and can help promote failure by fatigue mechanisms.

Together with correct installation and pretensioning of threaded fasteners, good joint design is fundamental to prevent failure of the fastener by fatigue mechanisms. 

The proportion of the applied load carried by the bolt is dependent on the ratio of the bolt to member stiffness, Fb=Fi+CP where Fi is the preload, P is the applied load and C=Kb/(Kb+Km) where Kb and Km are the bolt and member stiffnesses respectively.  Increasing the member stiffness and decreasing the stiffness of the bolts both decrease the proportion of the applied load carried by bolts.

What is often overlooked is the effect of distortion and poor 'fit up'  of the members.  Regularly bolts are used to pull the members together - sometimes with only partial success.  In these cases the members are not compressed until the members come into contact. Furthermore, 'member stiffness' is reduced significantly and/or is non linear which, in cyclic load applications, increases both the cyclic load carried by the bolt and the propensity for failure by fatigue mechanisms.

Make sure mating faces are flat and parallel and minimize 'fit up' gaps - where possible make every effort to decrease bolt stiffness and increase member stiffness to prevent premature bolt failure!!

Published in Technical Tips by Origen Engineering Solutions on 1 February 2015