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?
In most cases, the norm is that you don't get zero starting values.
Normally the test procedure has the following steps:
- move the testing machine crosshead to a place that it can accommodate the specimen
- place the specimen between the jaws
- secure the specimen either by : a) tightening it or b) in the case of self locking jaws applying small tension
- perform zeroing for displacement and load
- 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)
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.