4130 stress-strain curves

[tomsing]

In the Atlas of Stress-Strain Curves, 2nd ed, in chart AS.005 (

https://books.google.com/books?id=u...in Curves&pg=PT99#v=onepage&q=AS.005&f=false)

they present 3 charts of tensile stress-strain curves up to yield for 4130 at room and elevated temps for three different heat treatments. Now, first of all, the labeling of the charts is a bit odd - they refer to the "Left", "Center", and "Right" charts, but they're arranged on two rows and labeled like this:

(a) (b)
..(c)..

Based on the nominal strengths listed in the caption - Left is 150 ksi, Center is 180 ksi, and Right is 200 ksi - I'm pretty sure that the order is

(150) (200)
...(180)...

So that's one thing. The other thing is, check out the modulus on chart (c). The RT curve crosses right at .004 strain, 100 ksi, for a modulus of 25 Msi, when I'm expecting something closer to 29-30. If you check charts (a) and (b), they go through .004 strain, 120 ksi, for a modulus of 30 Msi, just as expected. So I think chart (c) is in error.

The caption references WADC TR 56-340, as published in Aerospace Structural Materials Handbook, Vol 1, Code 1201. Unfortunately, I've been unable to find either of these. Does anybody have a copy?

 

[EdStainless]

I'll bet that in the original layout they were side by side, and then they moved the enter one below to make more room.
You are correct on the order.

You can't take an accurate modulus off of a typical tensile test curve, and these have been 'smoothed' on top of that.
25Msi is plenty close for this work, after all the properties for a different heat of steel will be different from these anyway.

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P.E. Metallurgy, Plymouth Tube

 

[tomsing]

I do expect that properties of different heats will be different, but I don't expect elastic modulus to drop by 20% and then come back up. That's a relatively stable property, in my (limited) experience.

 

[EdStainless]

These curves were not to measure modulus. That is a more delicate measurement.
These also look like 'artists conceptions' and not exact test curves.

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P.E. Metallurgy, Plymouth Tube

 

[tomsing]

Definitely not an exact test curve, in the sense of data pulled from a single stress-strain coupon. I assume they're "typical"/average, although it's tough to tell without access to the source reference; in any case, it certainly involves a bit of art to make a smooth curve.

My concern isn't pulling a modulus off of the chart. It's that, if the indicated modulus is so far off from what it should be, the whole chart is wrong, probably scaled incorrectly in one or both axes. Which is a problem if I want to calculate area under the curve, or ... do anything with it, really.

 

[EdStainless]

You have to normalize these curves, they are only to be used for determining temperature factors (change in strength with temp) and nothing else.
I wouldn't even try to compare them to each other in absolute terms, unless you can find the raw data.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

It is a lousy calculation for modulus from SS curve, if you are picky to define modulus of elasticity. people still do that and report results, but needed to take into consideration two points:
1. based on true SS, not on an engineering SS.
2. focus on the linear portion of the curve. 100-120ksi points OP used are off elastic deformation. if using .002 strain, you will get a bit higher E.
Anyway that is not a good method to get an accurate E, but may be good enough for an educational purpose.

 

[EdStainless]

Good comments Ben,
When we have to report a modulus we will load the sample to and estimated 50% or yield, then unload to about 10%, and then re-load slowly. This will get you a good section of load curve to calculate from.

We do calculate it for every lab test, and if it is within 25-32Msi we are ok. If it comes out higher or lower we redo the tensile test because it will throw the yield calculation off by too much.

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P.E. Metallurgy, Plymouth Tube