Dear Ng Lin Feng

High-cycle fatigue focused on situations that require more than 10*4 cycles to failure where stress is low and deformation is primarily elastic. In high-cycle fatigue situations, materials performance is commonly characterized by an S-N curve, also known as a Wöhler curve. In this sense the yield stress represents an upper Limit.

While in low-cycle fatigue the stress is high enough for plastic deformation to occur, the accounting of the loading in terms of stress is less useful and the strain in the material offers a simpler and more accurate description. Low-cycle fatigue is usually characterised by the strain relation according to the Coffin-Manson relation. However since only relatively small strains respectively crack growth only can be accepted, the ultimate tensile strength would too large to be useful.

High cycle fatigue test are conducted in the elastic regime(below the yield strength of the material). Some metals and alloys (ferrous alloys, titanium alloys) have distinct fatigue limits, which means below that load/stress(mean, max or otherwise) there will not be fatigue failure; however if the material is not showing a limit, a certain amount of cycles is considered(106 or 108) which does not cause failure. There is not a min. value that can be generalized but it can be as low as 0.45UTS. The limit is usually normalized wrt ultimate tensile strength/yield strength in order to compare different materials(different grades of the alloy/ different heat treatments,grain size.....).

Thank you Dr Rüdiger Rentsch for the explanation. I realize some people are required to reach an endurance limit for their fatigue test. They set endurance limit at 1 million cycle. Is it compulsory to reach that level since we just need to know the life cycle at certain level?

For damage tolerant design parts will periodically be inspected for cracks and to replace the part once a crack exceeds a critical length. This approach usually uses the technologies of nondestructive testing and requires an accurate prediction of the rate of crack-growth between inspections. For aircrafts the designer sets some maintenance checks schedule frequent enough that parts are replaced while the crack is still in the "slow growth" phase. Hense it is not comulsory, but depends on the application maintenance measures as well. If you like the answers you can Show it by voting up or down. Best regards, Ruediger

Thank you Dr Rüdiger Rentsch & Erkan Asik.

Then why does endurance limit normally set at 1 million cycle? Is it most of the material achieve a straight horizontal line in S-N curve, starting at 1 million cycle?

https://www.researchgate.net/post/Why_Steels_and_Ti_Alloys_have_Fatigue_Limit?_tpcectx=qa_overview_following&_trid=GVmFDOB7cJVbkj0agHrxHfrk_

Please see the discussion in the above link about endurance limit.

Is fatigue crack propagation testing performed on maximum load Pmax under the yield strength or under the UTS? Thank you