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The tensile test specimen used for tension test has an enlarged cross sectional area at the ends. The sources I'm referring to, suggest that this is the case so that the failure occurs in the central region of the specimen where the stress distribution is uniform and is more easier to calculate (using P/A).

Now, what I conclude is that if the c/s area of the specimen were to be uniform throughout the length, failure would've occured near the ends where the stress distribution was non uniform, and that is the reason why we increase the area near the ends so that failure doesn't occur near the ends. My question is if the c/s area were to be uniform throughout the length why the failure occurs near the ends?

I mean if we are enlarging the ends, we would've been doing so because failure is occuring near the ends (with uniform c/s), so we're trying to avoid that by enlarging the ends.

If what I have written isn't making sense, then if anyone could explain what would've been the problems faced with a uniform c/s area specimen, that would be enough too.

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The stress is not uniform near the grip because there is a biaxial state of stress.

There is

  1. the axial force
  2. the compression force from the grips.

That is the reason that if the Cross-section was uniform the failure would start from the grip.

the mental image is the following: If you have a ballon that you apply force from one end, and at the same time you compress it laterally then it will break easier.

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As far as I know, the enlarged part is used to grip the specimen.

The fillet and smaller cross-section area is where the necking happens and that smaller cross-section is designed to avoid necking on the grip.

I think the failure does not happen at the end. If its completely axial load, the whole specimen will elongate and become thin, then the failure would still happen around the middle. However, if it does uniform in cross-section, then it will have no place to grip the specimen.

enter image description here

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  • $\begingroup$ For the last part of the answer, if the failure is happening in the middle only (with a uniform c/s), then why do we need to enlarge the ends. I can simply take a uniform c/s area specimen, install it on a UTM and apply the loads. I could still grip the specimen with uniform c/s. $\endgroup$
    – Harshit Rajput
    Commented Sep 16, 2021 at 9:03
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    $\begingroup$ Because then there will be no place to grip it.. If we make it uniform the end will also neck (reduced in cross-section). Please refer to this and search 'enlarged': asminternational.org/documents/10192/3465262/… $\endgroup$
    – el-cheapo
    Commented Sep 16, 2021 at 9:09
  • $\begingroup$ The smaller cross section should be weaker right, and hence it will break first. If its uniform in cross section, the only way you could keep gripping it is by making the grip tighter in the middle of testing. Correct me if I'm wrong, anyone. $\endgroup$
    – el-cheapo
    Commented Sep 16, 2021 at 9:13
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One does not want the rupture in the grips; that makes elongation and reduction of area measurements very difficult. Also, notch sensitive materials will rupture in the grip area unless the the grips are significantly larger cross-section than the gage length. For notch sensitive materials even the shoulders must have a long gradual taper to avoid shoulder ruptures.

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