What is the significance of mild steel being the only steel with a "true" yield point, as opposed to a yield strength?

In the mechanical tensile test there is a lesser (smaller) yield point that dips and continues climbing like a normal test graph but quickly drops off on the stress axis but "accelerates" on the strain axis as it necks down.

My question is: above ~.45 to .60% carbon content steel this graph is identical but for the first "upper" and "lower" yield point, excluding the more quickly reached "taffy" effect of low and med carbon steels.
What is happening mechanically at these initial lower and upper yield points that makes them so significant as to be valued above the same material's elongation and ultimate strength?

If it is grain structure, I would think that grain boundries would grow quite slowly and result in almost instantaneous fatigue breakage rather than the curve demonstrated. But that feels incorrect.

Any tips from you eggheads?

[Edited on July 30, 2016 at 3:30 AM. Reason : GIF]

7/30/2016 3:26:57 AM

what are you scared of a community college fag?
FAGS

[Edited on July 30, 2016 at 9:25 PM. Reason : fags]

7/30/2016 9:25:18 PM

We're not doing your homework for you. But it's probably an emergent phenomenon due to a combination of interatomic & intermolecular interactions within the carbon that isn't expressed outside of that concentration.

8/1/2016 9:05:03 AM

just buy better quality steel

8/1/2016 11:59:26 AM

8/2/2016 2:30:31 PM

The lower yield point is where it swaps from elastic (reversible) deformation to plastic (permanent) deformation. For most applications, you don't want to get into the plastic region. We generally like for parts to remain the same size and shape as they were intended to be.

8/2/2016 10:11:44 PM

I suppose i mean, to be more specific, the usefulness between the upper and lower yield points at THE yield point of low carbon steel.
There seems to be a correlation or ratio between the two that determines the ultimate strength of the whole.
Not so much chemical, in this question, but in practical.
1040 has a high/low initial difference of about 50/50
1015 has one of about 10/90 low/high. the chemical composition is not linear nor rational.
What aspect am I missing.
(yes I admit I am missing an aspect)

8/5/2016 3:56:34 AM

I do not understand your question.

8/5/2016 7:22:42 AM

i think he understands what yield is, but doesn't understand why different alloys exhibit different characteristics entirely.

Post some yield curves up, Fermat.

8/9/2016 7:54:10 PM

Ok let me give this a shot. Mods: please forgive me if I double-post. I am old now, as are you and need time absorb the booze that keeps us alive.

My question is, more precicely, what is it about the carbon content of mild steel that grants it this "grace" period in the the stress/strain graph.

More important to me, it appears, is what the significance of the lower yield point followed by necking?
I am reading a VERY old book. It is not described past "this is important" as the graphic will show.

What is it about the carbon content, physically, that makes this an impossibility in other ferrous metals?

9/16/2016 1:19:20 AM

I always assumed it was like mild vs sharp cheddar.

9/16/2016 9:24:55 AM

Here is another with more emphasis put on stainless steel and how it elongates in comparison to duplex and low hydrogen steels.



Chemically I have no clue. But on a rudimentary level I can see how lower carbon mild steel might behave the way it does.
Maybe there isn't enough carbon to construct regular cementite boundries to break all at once and so it behaves less like a single material and more like, say, a "rope" constructed of glass filaments with high modules of elasticity with the rest of the rope being made of, say, rubber.
The strain is super low till the glass fibers break, at which point the rubber strands just stretch through the rest of it's "plastic" phase till ultimate failure.

Reckon it has any salt to it?

9/17/2016 1:13:50 AM

This thread is performative.

9/19/2016 8:33:11 AM

I always order the hot or blazin' habanero. The mild just lacks flavor.

9/19/2016 9:24:35 AM

No one is going to 4 frame zoom one of those pictures?

TWW, I am disappoint...

9/19/2016 3:41:58 PM