Ambient temperature creep
This Tech Tip concerns failures that can arise from creep, but at ‘normal’ (ambient) temperature conditions. The well-known condition for any manifestation of creep deformation leading to premature cracking, or ultimate failure, is that temperatures need to exceed about 40% of the material yield strength, for a sustained length of time. These conditions are seldom encountered in normal applications. However, when the material is polymeric, there is a very definite and common case of seemingly low temperature creep.
Epoxies are polymers that when activated form cross-linked chains with themselves of co reactants to set hard, with generally very useful mechanical chemical and thermal properties. However they are susceptible to degradation if exposed to ‘high’ temperature (relative to their melting point) and when exposed to UV radiation. The ‘high temperature’ referred to here is not high in general terms but rather simply in relation to the epoxy glass transition temperature. In epoxies the temperature at which epoxies soften is in the vicinity of 60-110⁰C, and creep initiates at even lower temperatures. Structures incorporating epoxy elements, and under sustained load, can thus readily creep, often with most unexpected consequences. The two cases referred to in this article, illustrate the effect of creep at ambient conditions.
In a local steel production facility, scale produced during quenching process was removed from the cooling water in a concrete settling/filter tank. A key structural element of this filtering and separation process was a spoked ring steel structure, which was attached to the tank wall by means of epoxy grouted bolts. After a number of years in operation, creep of the epoxy caused a loss of preload in the pretensioned bolts securing the brackets to the concrete wall. This loss of pretension resulted in the bolts being loaded in cyclic shear which caused the bolts in incompletely filled grouting holes to fail by unidirectional bending fatigue. Although not catastrophic in itself, remedial strengthening of the support brackets and the addition of bolts allowed the structure itself to become overloaded and fail during a subsequent operational event. This in turn resulted in substantial damage and the deaths of a number of people in the subsequent clean up and rehabilitation process. Although it can be argued that the final failure was not due to creep of the epoxy grouting but rather due to overloading, the initial fatigue failure of the bolts which preceded the incident was a direct result of poor grouting practice and the loss of pretension in the bolts due to creep.
Unfortunately this is not an isolated event. A similar case of ambient temperature failure due to creep of epoxy grouted bolts, involved a failure of an underground road tunnel in Boston, USA, in 2006. In this case a section of the tunnel roof, some 6m by 12m, collapsed, pinning a car and resulting in the death of a passenger. Subsequent testimony highlighted that the bolts supporting the roof structure were fixed with epoxy, the long term creep resistance of which was not satisfactory. The load was clearly sustained tensile loading, and the epoxy reportedly used was the wrong type (with short term creep resistance) not suited to long term load bearing applications. The roof bolting system apparently cost only $1250 but the consequent damage and case was in the region of $54m.
The message, is that although we normally think of creep as a high temperature phenomenon, it can occur at ambient temperatures in polymeric materials. So take care in what one specifies!
This is the last Tech Tip for the year - we trust you have found the tips informative and they have helped improve integrity/prevent failure.
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The Origen team wishes you and yours a failure free festive season and a cracking New Year
Published in Technical Tips by Origen Engineering Solutions on 1 December 2017