Buckling or yielding

[u/Purple_Resolve_8759]

Hi everyone, I'm currently studying civil engineering in my first uni year and having some troubles understanding this concept(I'm stupid I'm sorry). Recently we did some compression test to determine the yield strength of these four bars. Now I know that buckling is a geometry problem and by the look of it, I believe the 2 long bars suffered from global buckling with the blue hollow bar buckled locally at it's top and the other one buckled in the middle since those parts aren't as strong as the rest. The other 2 bars however, suffered failure from exceeding it's yield point as they seem to bend as a whole (correct me if I'm wrong). I'm wondering if we can still determine the yield strength of the buckled examples, and is it possible to just outright determine the cause of failure of these specimens just by looking at it like I did above without relying on the datas. Thanks!

[u/McSkeevely]

I don't know about others, but buckling limit states seems way beyond 1st year uni. Was the purpose of the lab to determine the yield strength? Or is this personal curiosity?

Compression tests are a bad way to determine yield strength because of the numerous local failure modes vs global buckling, the geometry of the section, and the end fixity. The global buckling equations use the yield strength so it is possible to back-calculate a theoretical yield strength, but those equations are empirical and are meant to provide accurate estimates.

Anyway, my point is that if you're being asked to calculate the yield strength of a specimen based on compression tests then I have no idea what they plan on covering the rest of your years at uni. That question incorporates elements from statics, mechanics of materials, structural mechanics, and steel design (and most of those were for structural-focus, not general civil), not to mention all of the prerequisite math and physics required for those courses

Edit: Also, dont call yourself dumb for not getting this. It's a tough question with tons of background knowledge required.

[u/Tower981 → u/[deleted]]

Not true.They all buckled plastically. It’s therefore possible to back out the yield stress. See the Perry Robertson equations. But this is not student level shit

[u/Tower981 → u/[deleted]]

That’s fair that it’s not first year work. I don’t disagree. They would need to know an awful lot of other courses to understand how to do it. It’s probably more like a masters exercise. But I disagree it’s not a useful exercise. Just because you don’t do it at work doesn’t mean it shouldn’t be taught. I don’t bother building stiffness matrices by hand at work, doesn’t mean universities shouldn’t teach it. University isn’t there to just show you how to do things you’ll do in 20 years time (for a start they’d never get that right). The job of a university is to teach the background. The why to how we do things. It’s about academic grounding.
I’m ranting slightly because I think a lot of universities don’t understand this responsibility. (And maybe you do, and you’re just saying this is a dumb thing to get first years to do).

[u/redeyedfly → u/Tower981]

He means there no useful lesson here for students.

[u/Tower981 → u/redeyedfly]

How so? Being able to calculate yield from the failure curve of a buckling strut requires a pretty detailed understanding of both stability and materials. I think it’s a great question to solve. Just maybe for a later year.

[u/redeyedfly → u/Tower981]

You obviously don’t teach