TF
Thermal Fatigue
TF – Thermal Fatigue cracking, often referred to as thermal fatigue, is a damage mechanism caused by cyclic loading produced by temperature fluctuations. Thermal stresses are produced in the material, which can lead to cracking. There are two types of thermal stresses: uniform thermal expansion/contraction stresses, which cycle as a piece of plant is heated up and then cooled, for example during start up and shutdown; and thermal shock, where a cold fluid impinges onto the hot surface of a component. Thermal shock occurs during the quenching of castings and, if the change in temperature is too great, cracking can occur. In operating plant, thermal shock can be caused by the turbulent mixing of fluids of different temperatures around a T-joint in a piping system.
TF can exhibit different morphologies, which provide NDT with a challenge. In PANI 2, one of the test samples was a thermal fatigue crack removed from a coal-fired power station. A similar crack found on the plant had such smooth crack faces that ultrasonics was only able to detect the corner response and then a small tip echo at some depth in the wall. There were no responses from the crack face between these two limits. As the crack had propagated around the full circumference of the pipe, the corner response could easily be mistaken for part of the weld root or counter bore.
TF produced by thermal shock can produce lots of small surface cracks, which can be randomly oriented, producing a crazed cracking or ‘elephant skin’ appearance. The challenge is to then detect any larger crack in the presence of this crazed cracking.
For more information on TF cracks see:
J Wåle, ‘Crack characterisation for in-service inspection planning: an update’, available at: http://www.stralsakerhetsmyndigheten.se/Global/Publikationer/Rapport/Sakerhet-vid-karnkraftverken/2006/SKI-Rapport-2006_24.pdf
What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org
TF can exhibit different morphologies, which provide NDT with a challenge. In PANI 2, one of the test samples was a thermal fatigue crack removed from a coal-fired power station. A similar crack found on the plant had such smooth crack faces that ultrasonics was only able to detect the corner response and then a small tip echo at some depth in the wall. There were no responses from the crack face between these two limits. As the crack had propagated around the full circumference of the pipe, the corner response could easily be mistaken for part of the weld root or counter bore.
TF produced by thermal shock can produce lots of small surface cracks, which can be randomly oriented, producing a crazed cracking or ‘elephant skin’ appearance. The challenge is to then detect any larger crack in the presence of this crazed cracking.
For more information on TF cracks see:
J Wåle, ‘Crack characterisation for in-service inspection planning: an update’, available at: http://www.stralsakerhetsmyndigheten.se/Global/Publikationer/Rapport/Sakerhet-vid-karnkraftverken/2006/SKI-Rapport-2006_24.pdf
What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org
