As interesting as obscene gratefulness may sound, I doubt ill be much help here.
Hehe, that sounded
so wrong. Er, I mean grateful to an obscene degree, not, um, grateful in an obscene way!
I asked myself a lot of similar questions when making a number of materials meant to represent medieval irons and steels, taking into account the fact that those were inconsistent in composition, heat treatement as well as confusing materials like laminated and pattern welded steel. Didnt go so well, while I found a variety of information on the composition of various medieval artifacts I had no idea how that would translate into the materials behavious much less how to put that in DF raw numbers.
I know totally what you mean. Fortunately, I started researching it stubbornly and found that it's actually a lot simpler than I was thinking. (Unless I think I understand it and I don't!) No horridly complex formulae necessary -- at least most values. Just use the stress values straight off the SS diagrams, converted to KPa. (Shear and torsion
do seem horridly complex, though, and I have no grasp on them at all -- but Urist Da Vinci's clarification about metals having the same characteristics for all stress types at least eliminates that confusion for metals, again if I understood correctly.)
The biggest mystery for me was the weird STRAIN_AT_YIELD value, for which the wiki and most everything else I was finding on the forums was pretty cryptic and unhelpful. Turns out it's extremely simple and straightforward how Toady rendered it -- just a direct rendering of the percentage strain such that: 1% = 1000, 0.3% = 300, etc.
I tested my understanding of Young's modulus and strain at yield as Toady is using them by recalculating iron's strain at yield from the YM that Toady listed, and got the same number he did, so I did a little dance of joy. Baby steps, baby steps.
I vaguely recall charts like that one, and think that the point where it becomes permantely deformed is the part where the line stops going steeper and starts to flatten off (in the rhino skin case around 12MPa I guess?), and that the point where line stops is where the material also stopped and went snap. Which means you cant realy get much useful information out of that chart for cat skin and tendons, beyond which is more or less elastic.
Yep, that's the normal rule as I understand it as well. BUT THEN! Just when you thought it was safe to enter the water! You get a
new complexity with collagen-based organics (most of them) -- "viscoelasticity."
Basically it means that with viscoelastics there's an additional stage of semiplastic elasticity inbetween simple elastic deformation and full-on damaging plastic deformation, where the collagen fibers straighten out and reorient themselves and sort of rapidly creep, but in a recoverable way. Meaning that in an hour or two they'll regain their original shape, though there will be micro-level damage.
It's really cool, actually. Nature is amazing.
But anyway, this means, if I understand it correctly (reasonably level of confidence) there are essentially
two yield points with stretchy organics. One that is a yield-to-viscoelastic-deformation point, where simple elasticity ends and we go into this plasticity that isn't permanently damaging but will take a while to recover, and then the yield point we are familiar with from inorganic materials -- the yield-to-beginning-of-failure point where we get tearing, necking, etc. onto fracture/failure.
What this means in DF terms, I'm not completely sure (and now mentioned, I am in need of advice on that too), but it does imply that it
might be functionally inaccurate to use the yield-to-viscoelastic-deformation point as our Yield for stretchy organics. I'm very spotty on this, but if I understand correctly (low confidence here), if we use that earlier yield point, we'll get materials that look to DF like they're not stretchy at all, because the STRAIN_AT_YIELD values will be super low, like 1-2% as opposed to the 30-50% that skin
does stretch before failure. (Skin elastin really only gives very little macro-level skin elasticity.)
So I do strongly suspect that the implication of that is that we should be using the higher yield-to-beginning-of-failure point. But most sources don't seem to look at that. (A lot of them are cosmetic surgery ones where the big concern is the earlier viscoelastic yield point, or studies with live subjects where for obvious reasons they couldn't strain to failure.) Thus, my question.
As a side note, apparently even hair, which is keratin, not collagen, does this too, albeit with a different mechanism -- if you stretch it out into its the early part of its plasticity slope, it stretches plastically (i.e., gets damaged), but then if'n you put it in water, it will recover (wholly or partially, not sure -- I didn't care to read scientific papers on hairstyling any further on the point, bleah).
I should also mention I dont think DF tendons even use the material properties in case you were trying to collect that data as well.
I'm doing everything else, so, since the data shows up incidentally as I look up other things, I thought I might as well keep it and throw it in.
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Topic: Material science and biomechanical questions (Read 1874 times)