why do we have yeild instead of just using proof 1

[jjr1111]

Dumb question, but curious on thoughts. I know with fasteners proof is used, as the max of the elastic limit, but then we have yield which is a .2% plastic deformation, so more of a measure of when failure begins. My question is I see yield used much more commonly, but wouldn't it be better to use proof for the majority of the time, or teach it earlier?

I don't think I was even introduced to the idea of proof until later in strength of material when we studies bolts, and kind of thought that if I just designed to yield that I'd avoid failure, but that's not the case.

 

[BrianE22]

Yielding is considered failure in many applications.

 

[SWComposites]

"proof"?? do you mean ultimate (maximum) load? I've never heard "proof" used with fasteners. Its typically used in the context of things like pressure vessels, where "proof load" is used as a test load to "prove" the vessel has sufficient capability above operating pressures.

 

[Tmoose]

https://www.bossard.com/global-en/a...operty-class-46-to-129/proof-loads-of-screws/

 

[jjr1111]

Yeah, I wasn't really taught it when I took my materials class for my ME degree, and don't think I even encountered it until a strength of materials class. I guess I just saw it as a very useful concept that seems to only get used for bolt connections, and felt it would have been better to introduce sooner. I'll get off my soap box now

 

[EdStainless]

Yield strength is an engineering construct and not a true material property.
Since the materials that we use do not have a true straight section of the tensile curve.

We used to have to measure modulus on high strength steel wire (~300ksi UTS).
We would load to 150ksi, unload to about 35ksi, and then reload.
We would take the slope of the reload curve and we got reasonable results.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MintJulep]

"Proof Load" exists for fasteners because the geometry is pre-defined.

For an arbitrary geometry using stress normalizes the evaluation, making it more straightforward to design or modify geometry to best match the application and constraints.

 

[desertfox]

Hi jj1111

My understanding is proof load is used when the yield point of a material isn’t clearly defined.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein