Fatigue Failure in Structural Elements: Axial, Shear & Torsional Loads

[Junior19]

TL;DR

Shear Stresses

"The stresses acting on a structural element subjected to axial, shear or torsional loads are constant and do not change sign throughout the loading cycle. Therefore, alternating stresses, which cause fatigue, do not occur in these cases. On the other hand, in a beam subjected to bending load, the stresses vary in magnitude and direction during a loading cycle, thus producing alternating stresses that can cause fatigue failure."

Does anyone know of any academic paragraph that supports this idea? specifically the part related to shear stresses. Thanks

 

[Baluncore]

Welcome to PF.

Maybe we need some context here.
Where is that quote from, a link or a reference would help.

 

[Junior19]

Hello and thank you for replying. I am doing a research project related to power transmission shafts and unlike others, I don't want to ignore the shear stresses generated by the forces acting on the shaft. I have already done the analysis in terms of static stresses and deformations, however, in the dynamic stresses (fatigue), I do not have much information related to the shear stresses I mentioned. In my opinion, axial, torsional and shear stresses have a constant behavior, in terms of direction, so their alternating stress is equal to 0, however, this is not the case of bending forces, which do present alternating stresses. In the books, all this is mentioned, except the part related to shear stresses, of which nothing is mentioned, so I am not sure if what I think is correct. That is why I consulted with Chat GPT, I gave him all the context and what I think, and he confirmed it with the paragraph I put at the top (it is as he wrote it), however, I think it is something generated from his knowledge, since he could not give me a specific reference of the book or article from where he extracted it. I think it is not appropriate to cite GPT chat in research projects, because it is something new, and for all this I am looking for an academic reference to support this idea that shear stresses do not have alternating stresses, although its magnitude does vary, but not its direction. That is all.

 

[jrmichler]

The ChatGPT answer is technobabble. And a perfect example of why ChatGPT is not a reliable source for engineering and physics questions.

You need to learn for yourself how to calculate loads, calculate stresses, and what loading conditions cause fatigue. After that, you learn how to calculate fatigue stresses. This is all basic learning that is found in textbooks on strength of materials and machine design.

This is an area where you need to learn the basics. Attempting to cheat using ChatGPT will just get you into trouble.

 

[Lnewqban]

Welcome, @Junior19 !

Take for example the fatigue that the fuselage of commercial airplanes suffers, only by cycles of pressurization.
That refutes the first part of the idea's statement.
The tubular fuselage is a structural element subjected to pressure load that does not change sign throughout the loading cycle.

Regarding a high rpm's shaft transferring constant torque, but suffering bending deformation and shear loads due to non-balanced masses, misalignment of bearings, couplings, etc., is a rotating beam subjected to constant bending and shearing loads.

As the cross section of that beam rotates about its neutral axis, those stresses vary in magnitude and direction in a rapid cycling manner.
That confirms the second part of the idea's statement.

I am sure that Internet searches could bring back to you a lot of research work on this problem.
The only reference that I know and can share is both chapters IX of Timoshenko's books (parts 1 and 2) on Strength of Materials.

Please, see:
https://docs.google.com/file/d/0Bw8...Zjg/edit?resourcekey=0-GEkdRBIXq66OFnCBnteVSw

https://docs.google.com/file/d/0Bw8...Y28/view?resourcekey=0-NDWpBrvRWodj29L7tZAt2Q

 

[A.T.]

I don't want to ignore the shear stresses generated by the forces acting on the shaft.

If you have specific forces in mind, then it should be obvious if they are rotating with the shaft or not. If the shear forces are constant in the rotating rest frame of the shaft (magnitude & direction), then the shear stresses are constant too. Otherwise they are alternating.