Creep-Fatigue Terminology

[Guest102023]

Is 'thermomechanical fatigue' just another term for 'thermal fatigue' or 'creep-fatigue'? There appears to be some inconsistency in the literature.
I would like someone to clear this up (so I don't appear unfashionable on my F/A report). Thanks!

 

[metengr]

Thermomechanical fatigue is the combination of mechanical and thermal stress effects on creep behavior.

 

[metengr]

Slight correction - a typo

Thermo mechanical fatigue is the combination of mechanical and thermal stress effects on fatigue behavior.

Creep fatigue is the synergistic effects of creep and fatigue on crack propagation.

 

[Guest102023]

So if we consider a section of pipe thick enough to support a thermal gradient that could cause creep-fatigue after many heating cycles, and we superimpose the stress due to internal pressure, do we have thermo-mechanical fatigue? In this case the hoop stress contribution is minor relative to the thermally induced stresses. I assume there could also be stresses from bending loads due to overall equipment expansion, but I have not considered those (partly because I have no clue what they might be).

 

[metengr]

Think of thermomechanical fatigue as cyclic stresses from thermal gradients + cyclic tensile stresses from internal pressure.

 

[Guest102023]

Moving slightly sideways here: I see the term 'thermal fatigue' used a bit loosely. It might also mean simple fatigue (i.e., cold) in a component that is being stressed by the cyclic thermal expansion of a connected but remote component. Any thoughts on this?

 

[adamuk]

Thermal fatigue seems to be used for 2 mechanisms:

Fatigue due to thermally-induced stresses which produces a defect which looks identical to other fatigue cracks - this is what mechanical engineers seem to understand as thermal fatigue.

or

A mechanism that produces multiple cracks (often seen with corrosion in the cracks which produces the broad defects shown below), but again, due to thermal stresses. This is (also) what materials engineers denote thermal fatigue. The example below was the inside bore of a condensate line into a steam line, the thermal fluctuations due to the varying flow into the steam line produced the cracking shown below (the white colour is the MPI background paint so the defects can be seen easily).

So you are correct!

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk

 

[metengr]

Thermal fatigue is simply fatigue crack propagation that results from the application of cyclic thermal tensile stresses, EOM.

 

[adamtom]

That's correct for thermal fatigue is a type of fatigue fracture, defects initial from the surface, LCF, and creeps may be initial from sub surface.