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Here this is a Load-Displacement curve from a compression test I did earlier with my colleges, Below is the resulted stress strain curve from the test from the lab computer why did the curve not start from zero is this just a zero error from the testing equipment or is there something else behind it?

enter image description here

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  • $\begingroup$ displacement sensor was not zeroed? $\endgroup$
    – Solar Mike
    Commented Dec 11, 2022 at 16:02
  • $\begingroup$ I'm assuming yes, it doesn't make sense that the displacement has value without load $\endgroup$ Commented Dec 11, 2022 at 16:07
  • $\begingroup$ Assuming? Surely you know from all the setting up and recording that you did for the test? ambient temperature, calibration records for equipment etc etc. $\endgroup$
    – Solar Mike
    Commented Dec 11, 2022 at 16:09
  • $\begingroup$ Displacement without a load measurement also occurs when the load is not measured. Unlikely in this case as it is likely slippage of some sort (could be anything from specimen to displacement sensor mounting). $\endgroup$
    – Abel
    Commented Dec 11, 2022 at 18:59

1 Answer 1

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In general

In most cases, the norm is that you don't get zero starting values.

Normally the test procedure has the following steps:

  1. move the testing machine crosshead to a place that it can accommodate the specimen
  2. place the specimen between the jaws
  3. secure the specimen either by : a) tightening it or b) in the case of self locking jaws applying small tension
  4. perform zeroing for displacement and load
  5. start the test

Normally in you need to make sure that both displacement is set to zero and that there is no inherent tension while you tighten the specimen to the testing jig (this is for not damaging the specimen). During that stage, normally you set to zero the displacement

However, because in most cases that (especially displacement) can be zeroed during the analysis stage, that can be easily forgotten/omitted.

(Removing the displacement is as easy are subtracting the first value from all the rest)


Particular case

Sometimes the testing machines have an autozeroing feature. If that is the case, and the fixture is enabled, then you would expect that there might be a problem with the testing.

However, for this particular case seeing the shape of the curve (I don't know what type of material it is but I will guess some type of aluminium or other metallic alloy), I would guess that the specimen was not tightened properly, and there was some slippage in the grips between 1.5 and 1.7 mm.

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