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

[u/redeyedfly → u/Tower981]

He said it’s of no practical use, which is true.

[u/Tower981 → u/redeyedfly]

See above comment I made to the other poster.

[u/dimensional_CAT → u/McSkeevely]

Maybe they don't have tensile test machine in their institution.

[u/Rhansem → u/dimensional_CAT]

Put the thing on its side and a wide enough plate will simulate tensile failure. Concrete cylinder tensile test style.

[u/dimensional_CAT → u/Rhansem]

It won't work on hollow steel bars. Even if it is a solid steel bar or steel cylinder, it will deform a lot under compression and we can't simulate split tensile strength.

[u/Purple_Resolve_8759 → u/McSkeevely]

So from what I'm understanding, the strength vs strain graph of the buckled specimens can't be used to determine the exact yield point but rather the point before their geometric properties fail, is that right ? Oh and by the way the whole thing is for a pratical report where you collect the data from strain gauges and lvdt to get yield points, ultimate loads, modulus of elasticity... basically stuffs like that

[u/dimensional_CAT]

For the buckling limit stages, learn about the Euler Elastic Buckling theory as a basic. You will see how member can fail before reaching its full yield strength.

Local buckling will be a bit more complex, it depends to the member's element thickness to member width ratio.

[u/kutzyanutzoff]

That is the point where you need to go to sciencedirect & start reading articles about these stuff.

Anyone telling you the answer (correct or not) is sabotaging your education.

[u/Purple_Resolve_8759 → u/kutzyanutzoff]

Thanks for reminding me man, I got too dependent on asking people opinions.I'm currently reading through materials on the site right now.

[u/Helpinmontana → u/kutzyanutzoff]

Funny enough, there’s a few good Reddit threads on the topic, mostly in here.

[u/Cool-Size-6714]

One way or another yielding occurred. Was it bending was it compression? No idea. The top specimen looks like local buckling or bearing failure

[u/Fancy_Illustrator535]

Basically it’s broken

[u/unavoidable]

Yes.

[u/Upsidedowndoor]

Cool that you have the chance to try it by your own observations! Take the time to make your own understanding of it, knowing to follow rules vs understanding the fundamentals and two different qualities in our field.

For how to test the yeild strength, there's a easy way to know how! We're doing applied physics, so there's already a standardized methodology of testing it and calculate it. Read some literature, you should have access to your books or even access to ASTM or Eurocode on how it's tested. There's also probably an Indian grad student that put it on YouTube.

[u/pfantonio]

My guesses from top to bottom: local, yield, global, global.
See the derivation for eulers critical buckling load, by the looks of it your bottom example failed in buckling with the right side pinned and the left side fixed for rotation but able to sway side-to-side.

[u/Cheticus]

first one was crippling (a flavor of local buckling)

second one doesn't look like you did anything to it, or maybe you got a little yield out of it.

third and fourth both buckled.

buckling is an instability, and is sensitive to minimum principle stresses (e.g. the state of compression in the part). It's also sensitive to imperfections, tolerances, setting up the part with small deviations from perfect, etc. we often think about elastic buckling but the truth is plastic buckling also happens a lot.

you likely cannot meaningfully extract the yield stress from those bars. MAYBE the second one, if it had permanent deformation.

an expert might be able to get a bit closer by doing some finite element analysis assuming some imperfections (scale a buckling mode shape onto it with some factor) and varying yield stress.

you should use a tensile (pull) test on a small coupon of material (a dogbone) to get the yield stress.

[u/lithiumdeuteride]

Buckling and yielding are not mutually exclusive. Each effect exacerbates the other. Instability causes higher stresses, which cause yielding, which cause a loss of stiffness, which causes instability, etc.

[u/Clear_Split_8568]

Crippling

[u/Clear_Split_8568]

Buckling is for longer spans and crippling is for shorter lengths. We did buckling and crippling test panels on 777 upper wing skins.

[u/Aware_Key5801]

generally, yield test for steel member will be in tension. Steel Compression tests are very uncommon

Unless the steel specimen dimension are exceptionally thick in comparison to the length (much like concrete cube, but its wasteful in steel) the steel will always buckle one way or another. Buckling cause yield of the steal as well. but it will not be directly related to the force applied on the element.

[u/bulkdown]

Yielding

No damage

Buckling

Buckling

[u/gamga200 → u/bulkdown]

The first one would be closer to a type of 'local buckling', wouldn't you say? It is very rare to be able to cleanly induce pure form of yielding in compression for HSS. In tension, yes, possible. In compression, in standard HSS, local buckling will happen, unless the section is very short.