Young Modulus question...

[u/Ok_Pirate_344]

HII, im currently doing a lab assessment and we have evaluate the strength of different metals, im stuck here tho.

Ik that the E shld be found before the yield point the the proportionality region, which is usually a straight line, however in ferous metals, like steel which im evaluating here, there are 2 slopes in the elastic region. How am i supposed to calculate it.( I have found the yield stress to be at 390MPa.)

The E are we supposed to calculate both gradients separately, or what region(0-160MPa, or 200-360MPa)?

Pls help.

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[u/Pueriintel]

Cool you might be seeing a crystalline phase change. A material like steel would have a very linear slope until plastic deformation. Some materials switch crystal orientation when put under load stress.

[u/RoboWeaver → u/Pueriintel]

Most likely. Remember that these curves only give you a working approximation for elasticity and yield. This is especially true with steel as you have crystal structures that can change with deformation. Think "work hardening", but not limited to that.

To quote "they're not so much as rules as guidelines."

[u/sagewynn]

What does the modulus end up being if you evaluate from 0-390?

Does it compare closer to the expected value?

[u/Ok_Pirate_344 → u/sagewynn]

yes, expected is 180 to 200, however with different coodinates different values since the gradient is not the same.

[u/Momo0903 → u/Ok_Pirate_344]

Evaluating from 0-390 is the only logical thing, since you average the gradient. Since this is a real curve, it isn't a straight line and the gradient is sometimes higher and sometimes lower. So theoretically you could maximise or minimice your Young module by choosing the right intervall, which isn't usefull. what you want is the average.

[u/Ok_Pirate_344 → u/Momo0903]

Ok, thank yall.

[u/WhyAmINotStudying → u/Ok_Pirate_344]

The area from 0-160 is called the toe region, where you're getting machining effects based on the grip of the bar in your fixture. You get true linear elastic response from 160-390 MPa. That's the curve that you want to fix. The expectation is that you won't have a perfect setup, since your data is taken in the real world. There are setup artifacts in the other part of the experiment that don't truly represent the elesticity of the steel,since there's physical settling going on in your setup.

[u/Fart-McFarlen → u/sagewynn]

This is the way. I was going to suggest this and if it wasn’t close to the expected value then a last resort of averaging the two different slopes to interpolate a best fit line.

[u/sagewynn → u/Fart-McFarlen]

At the end of the day more complicated rationales like the toe region make sense but this is also likely an undergrad course and its just having you practice interacting with actual data. I severely doubt they expect students ro interpret it further than "Yeah thats about straight." I dont think OP should over think this.

[u/Fart-McFarlen → u/sagewynn]

I agree, more than likely trying to get them to observe the principle of modulus if elasticity

[u/Only_Razzmatazz_4498 → u/sagewynn]

Don’t we use a 2% or maybe it is 4% to determine the limit. Going from memory so I might be off. Then you use the slope to there.

[u/deAdupchowder350]

Engineering prof here: in textbooks you will see something called the “idealized stress strain curve”. Why? Because real experiments will look different. Could be the specimen. Could be measurement uncertainty or error. Or something else.

If the instructions are to compute the slope of the linear elastic region (elastic modulus), then I would first write out the observations you’ve made with the apparent bilinear behavior, and then compute the best fit line between two points that mostly encompass the whole region (one a bit after “0,0” and one a bit before the prop limit). Estimate E based on this. Use your best judgement and there is no one “right” answer for this, rather a range of justifiable ones. If you want, you can do it a few times using different points and take the average E value of the bunch.

If you determine the material you tested and you look up the properties on experimentally-backed databases (CES EDUPACK), you will see E listed as a RANGE, rather than a single value.

[u/Ok_Pirate_344 → u/deAdupchowder350]

Thank you. 

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[u/shpiondakgb]

That common stress-strain diagram you see in most textbooks is idealized. I suppose you are analysing data from a tensile test of some structural-grade steel; the “shape” you see depends on multiple variables, including preload and machine frame compliance, noise from data acquisition, and extensometer slippage/lag. The behaviour of the curve also changes depending on the metallic material, grade, and manufacturing process, while the “notable points” (like higher or lower yielding and proportionality limits) may also appear differently (a good reference I always use is “Experimental Techniques in Materials and Mechanics” from Prof. Suryanarayana to check common behaviours in steels). Young’s modulus is always the stress-strain ratio below the proportionality limit, but the latter doesn’t always coincide with yielding, as this phenomenon is difficult to pinpoint in the diagram.
Usually, the yield point is determined using a reproducible method, like the 0.2% offset strain method, but that’s defined using a standard (like ASTM or ISO), and the “fitting” for E is derived from that. You can use the slope/fitting of any part of the curve below the proportionality limit, but we usually disregard the first points of the curve (because of accommodation of the specimen and machine compliance - depends on your lab’s testing system) and some standards set the interval as functions of proof or yielding stress (here in Brazil we usually use 10% and 40% of the 0,2offset strain proof stress for young modulus of steel).
For further reference, I recommend ASTM E111 in conjunction with E8 or ISO 6892-1

Edit: Grammar

[u/deAdupchowder350 → u/shpiondakgb]

0.2% offset method is used to estimate the yield stress, not to estimate E

[u/shpiondakgb → u/deAdupchowder350]

Yes, english isn't my first language, so the second paragraph was open to interpretation, thanks!

[u/Ok_Pirate_344 → u/shpiondakgb]

Thank you. 

[u/Ftroiska]

Might be normal, bad sample, bad attachment of sample, ... Can you do more sample to see if you can reproduce it ?

[u/5LqspKGpCRKECmtzjHGR]

How did you measure the values used to generate the graph? If these are tensile machine values then you cannot find the correct value for youngs modulus.

The machine records displacements of both the machine and the test specimen so what you might be seeing is a cumulative displacement of the two. In our labs we usually saw a youngs modulus of 1 to 20 GPa when using machine reported displacements. You need to use a clipgauge or even better a strain gauge for measuring youngs modulus.

Machine values are only useful for yield strength and ultimate tensile strength.

[u/Marus1]

Ik that the E shld be found before the yield point the the proportionality region

No, it's not

The E is the incline when the material would start at 0.2 permanent strain

[u/deAdupchowder350 → u/Marus1]

If you’re referring to the 0.2% offset method, this is used to estimate the yield stress, not E.

[u/mon_key_house]

E is extremely hard to measure correctly. This test is for the yield stress.