Understanding Fatigue Failure and S-N Curves

Fatigue failure is a failure mechanism which results from the formation and growth of cracks under repeated cyclic stress loading, leading to fracture. It can result in failure at stress levels well below the material yield or ultimate strengths.
In this video I discuss the mechanisms behind fracture, and how S-N curves can be used to predict the fatigue life of a component.
I also discuss the effect of having a tensile mean stress, and how we can use techniques like the Rainflow counting method and Miner's rule to consider complex cyclic stresses.
The Efficient Engineer is a channel aimed at mechanical and civil engineers. The mission is to simplify engineering concepts, one video at a time!
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[@petercalow1028]

HELLO, I have an exam in 2 and a half hours. Can you tell me why the initial fatigue results are scattered? Cant find the answer anywhere. Great video otherwise!! Thankyou!!

[@TheEfficientEngineer]

Hi Peter! Most of the scatter will typically arise in the crack initiation phase, where there is more variability, than in the crack growth phase. This initial phase will be sensitive to things like the surface conditions of the test piece. Of course there will also be some variability associated with the test, e.g. the equipment that is used. Hope that helps - good luck with the exam!

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@@TheEfficientEngineer Ahhh yes, thank you so much for replying so quickly. Will need the luck!

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Great video introducing fatigue! Could mention that damage summation by Miner is only approximation as for variable loading actual loading history can make huge difference. Fun fact initial high loading (even partial yielding) for steels can improve high cycle fatigue.

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[@thesuperkat943]

On the stress amplitude graphs at 4:15, you should’ve plotted some of the other theories, especially since the Goodman line and Gerber Line do not guarantee infinite life, as they can fail from going past the yield point of a material, giving them finite life. This is true for most theories, but especially true for these

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[@BoZhaoengineering]

you explain it very clear! thank you very much! just more about rainflow counting method, in matlab there is built in function that give you the cycle number, range and mean of a complex fatigue record in time domain.

[@TheEfficientEngineer]

Great tip, thanks! There are also Python modules out there that can do the same, e.g. pypi.org/project/rainflow/

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[@rogeriotavares1751]

Great video! Very objective and well explained! Only one doubt: the vertical axis of the S-N curve is not normally the stress amplitude instead of the stress range? Thank you in advance for your answer!

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Hey hi. You have made great videos. Could you also do this type of videos on vibration topics. That will be very nice to understand.

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