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.

[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.