Beware of underestimating the potential of bending

NOVEMBER 2014

Inadvertently induced bending stresses often causes failure even at design loads.

Engineering systems are often designed where the perceived nominal stress is principally pure tension.  It is easy to design for this and steels are remarkably robust in resisting such tensile stresses.  However, the stress generated from the same load can lead to very much higher stresses when applied in bending, and if the potential for such bending loads are not fully assessed/understood,  premature fatigue failure and sometimes even catastrophic failure ensues.
 
This is perhaps best illustrated by a case where, in an offshore application, a five ton mass was lifted via a 60mm diameter shank swivel system.  In normal operation the direct tensile stress was less than 18MPa – a minimal stress easily accommodated by the steel shank (and where fatigue cracking would not even initiate).  However, when the five ton, ten meter long component was allowed to settle on the sea bed in a horizontal fashion the five ton lifting load was applied in bending. The bending stress induced by these loads was now over 370MPa, and the component was bound to develop fatigue cracks and subsequently fracture completely after a very short fatigue life and small critical crack length. 

This simple (but costly) example serves to highlight that at the same load very different stresses can be developed; in the one tensile case the resultant stress is very low (less than 20MPa) and there would be no chance of failure (fatigue or fast fracture), whereas the same load, applied in bending, leads to very high stresses where fatigue and ensuing catastrophic failure is inevitable.  The message is to be sure that if systems are designed for pure tension, they do indeed experience only pure tension and not bending (even a small component that could be introduced through some eccentricity or misalignment) as the stresses increase radically. 


Published in Technical Tips by Origen Engineering Solutions on 1 November 2014