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Stress-strain curve i am working with

This stress-strain curve presents a linear elastic range, followed by yielding and accompanied by a decrease in toughness the material goes into the plastic region. As it strains more and more the material then suddenly snaps! Without necking...

Fracture occurring without necking isn't something I would expect from a ductile material, in fact it's a trait of a brittle material. But, the material can't be brittle with all that strain in the plastic region.

So which is it? Question: Would this material fall into a "mostly ductile" category instead of ductile?

I have four stress-strain curves i am working with at the moment. Three of them are similar to this one however, they show a decline in stress when necking before fracture. This diagram does not decline in stress before fracture. They were all tested on the same tensile stress machine (a Shimadzu machine i believe). Part of the assessment is to determine the possible types of materials each curve could be.

Question: Because this material doesn't neck so i'm wondering if there are any materials which have a stress-strain curve such as this. I was thinking some metals may strain and then just snap or perhaps timber. I'm really not sure.

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  • $\begingroup$ If you also have another account (@zantheor?) please combine them. If not, to anyone posting edits, please keep this question as the asker intended. $\endgroup$
    – hazzey ♦
    May 15, 2016 at 13:03
  • $\begingroup$ Were these diagrams collected by you (or a colegue) or did you find them somewhere? Are you sure they are displaying engineering stress, as opposed to true stress? Have you seen the ruptured elements, are you sure they don't display necking? $\endgroup$
    – Wasabi ♦
    May 16, 2016 at 11:04
  • $\begingroup$ Here is the Help Center page about merging accounts. $\endgroup$
    – Air
    May 16, 2016 at 14:49
  • $\begingroup$ This may be helpful, if late. If we assume volume is conserved in a material during plastic deformation (which is true for virtually all useful engineering materials) then any extension along one direction must cause contraction along at least one other independent direction to conserve volume. In isotropic materials contraction occurs in the plane normal to tension, i.e. necking. $\endgroup$
    – do-the-thing-please
    Aug 19, 2017 at 13:56

2 Answers 2

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The diagram is representing true stress, as opposed to the engineering stress. So as not to copy-paste, see my answer to a related question for a description of these different stress measurements. Basically, true stress takes into account the reduced cross-sectional area due to necking, therefore the stress measurement never drops. Meanwhile, engineering stress considers the original area throughout the test, even though necking has reduced the actual area, leading to a reduction in the apparent requisite stress to further deform the member.

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Not all ductile alloys neck. See, for example, Haynes 25 (L605).

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    $\begingroup$ Do you have a source for this claim? I cannot find that information at all. $\endgroup$
    – JMac
    Aug 17, 2017 at 21:07

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