Beware of the insidious effects of creep
The combined effects of high stress and temperature and resulting failure by creep mechanisms is well known - but is often overlooked in relatively low temperature environments.
Creep is the slow and irreversible plastic deformation of a material operating under constant load that occurs over an extended period of time. Theoretically it takes place at any temperature above absolute zero, but only becomes significant at higher temperatures of over 40% of the melting temperature of the material, with the rate of deformation increasing strongly with temperature.
This type of material deformation occurs when the applied load is below the yield point of material yet permanent deformation still occurs. The continuous flow of material during creep can eventually result in very large plastic deformation and a significant modification to the initial microstructure. This would eventually lead to failure of the material due to creep rupture, with time to rupture ranging from hours (at high stress and temperature), to years (at low stress and temperatures).
This plastic deformation is caused by the movement of defects within the microstructure of the material, such as dislocations, vacancies, or interstitial atoms. These defects move, interact and join up to form micro voids, increasing the internal stress of the part. These voids then join up to form creep cracks and result in the ultimate failure of the material. Creep also occurs in compression.
Although creep is generally viewed as a high temperature failure mechanism, it can often occur in relatively low temperature environments where materials with low melting temperatures, such as polymers, are used. This is particularly important in situations where high stresses are also experienced such as epoxy grouted anchors, gaskets and when paint is inadvertently applied between mating surfaces of bolted joints. In these cases viscoelastic creep of the polymer results in the loss of preload and subsequent leaking of the joint or failure of the fasteners by fatigue mechanisms.
Published in Technical Tips by Origen Engineering Solutions on 1 July 2016