Hello,

to evaluate correcly the Young modulus from a tensile tests, an extensometer is required to measure the elastic deformation (with minimum 03 repetitions). In the case of your material (1.4512 steel), the first elastic part of the curve should be linear, except if your specimen wasn't well fixed between the jaws and an eventual slipping was occured during tension.

For the determination of the A, B and n material parameters of the JC constitutive model, effective stress vs effective strain curve should be used instead of nominal stress-strain curve. This could be determined from true stress-true strain data. Therefore, the elastic strain should be eliminated from the total strain and the true stress should be corrected using bridgman equation.

DE TRIA

Something appears wrong. Why there is no strain hardening past the yield point (the intersection of orange colored line with blue colored graph)?

In addition to what Djalel Eddine Tria has mentioned, have you done any metallography on the (1.4512) alloy used for the tensile specimens to check the microstructure is free of defects, such as casting defects that can distort the deformation of the alloy and you would not achieve the true mechanical properties.

Thank you for your answer Djalel Eddine Tria, maybe you have some references about how to make this Bridgman correction corretly? I never did it before and I have problems with finding articles or videos about it.

Ameer K Ibraheem I didn't but I think this is not a problem in this case becasue I also have tensile test results for another alloy and it looks quite simillar. I think the problem is slipping of specimen at the begginign of the test as Djalel Eddine Tria sugested.

Dear Mr. Lukasz Lojek,

Please find bellow some articles that help you to proper perform and explore your tensile data.

https://doi.org/10.1016/j.ijimpeng.2008.12.003

https://doi.org/10.1016/S0997-7538(01)01157-3

https://doi.org/10.1016/j.ijimpeng.2016.10.011

I will, thank you.

I recommend that you measure Young's modulus using ASTM E111, not ASTM E8 or the equivalents used in your country.

I think you could probably get a value for "Young's Modulus" by measuring off the upper half of the elastic region. However, your results are very atypical of a tensile test so I am thinking something is wrong in your method or equipment. As was stated by Isaac, there should be some sort of strain hardening component once you hit yield. Since you reference that you have other tests for a different alloy with similar curves, I suggest you start there (evaluating your method). I would also suggest you reach out to an accredited laboratory near you for some Round Robin comparison testing to get yourself in alignment. If you keep the number of samples simple and small, I believe they would be happy to assist you. I know I would at my lab.

Also, as pointed out by Doug, the "standard" tensile testing performed by the vast majority of labs cannot produce a Young's Modulus accurate enough for material qualification/comparison. Young's modulus requires special methods and very accurate extensometers (Class A, iirc) for determination. In fact, you will find no reference to the term Elastic Modulus or Young's Modulus in ASTM E8, ASTM B557, ISO 6892, JIS Z 2241, or other tensile testing standards. These standards can only be used to produce Ultimate Strength, Yield Strength, Elongation, and Reduction of Area. For determining YS, they describe taking a line parallel to the linear portion at the beginning of the curve but without making any reference to Modulus.

Yes, everyone commonly reports the slope of the line tangent to the linear portion as THE modulus. It makes a good red flag that testing went wrong when the modulus isn't near what was expected, so it is worth keeping around. Nobody gives it much notice, though.

Could you explain clearly how you are measuring "strain"? This shape of curve is sometimes seen when the strain is really cross-head displacement and not measured by extensometry.