Understand thermal fatigue or risk the potential for failure
Repetitive thermal loading on a localised basis can lead to thermal fatigue and extensive surface cracking. Although relatively rare, this phenomenon can have catastrophic consequences because it is generally not well understood.
Fatigue, leading to progressive crack initiation and propagation, is generally caused by cyclic local tensile stress that is often intensified by existing flaws or areas of stress concentration, leading to progressive cyclic crack growth. Failure by fatigue is very common, as cyclic loading is very common, and the crack can readily grow to critical flaw size, leading to catastrophic failure. It is generally regarded by engineers that fatigue cracking does not occur in compression, but this is not the case especially in situations of thermal fatigue which can lead to localised cyclic tensile condition, due to compressive plastic yielding.
In such thermal fatigue situations, often in thick sections a localised heat source (such as that introduced by a clutch acting on a flywheel or by the local impingement of high temperature gas) would cause local expansion in the immediately surrounding material and hence a region of localised compressive stress. If the heat is not dissipated away sufficiently rapidly, the localised expansion can lead to compressive stresses reaching the yield stress of the material in compression at the specific temperature. This material is constrained by the material around it causing the hot compressed material to yield in compression and it therefore occupies less volume than it had previously. On returning to lower (or ambient) temperature this deformed material now needs to fill a slightly larger volume and thus it experiences a net overall tensile residual stress. On repetition of this heating cycle, the tensile stress cycle is repeated - exactly the conditions for conventional fatigue loading. Thus cyclic compressive loading can lead to cyclic tensile cracking, through crack initiation and propagation.
Thermal fatigue on a macroscopic scale is manifested by craze cracking on the surface, for example in high friction (high local temperature) applications such as the flywheel of a clutch, and exhibits a sort of 'cracked mud' -like appearance, often called 'elephant skin' cracking (on a large scale ) or 'snake skin' cracking on a smaller scale. It follows conventional fatigue behavior but the cracking itself is not constrained to a single crack as in normal (tensile) fatigue but is widespread and can cover a large portion of the heated surface.
Thermal fatigue is also known to occur in internal cladding of pressure vessels subject to cyclic high temperature loading as localised expansion is structurally constrained and thus localised compressive yielding occurs, leading to thermal fatigue from the mechanism described above. This generally focuses on high stress concentration regions such as at welds or changes of section, just as conventional tensile fatigue focuses on high stress concentrations, leading to “elephant skin” cracking.
To avoid such thermal fatigue cracking a full understanding of the mechanism and both thermal and mechanical stress fields is required. Such understanding coupled with appropriate design will allow the problem to be overcome.
Although this is not a common failure occurrence, in those situations where these cyclic tensile loading conditions can occur, it does provide a significant challenge.
Published in Technical Tips by Origen Engineering Solutions on 1 January 2016