I have a uniaxial tensile test data for, let say, a material X (graph in attachment). Now, principally it is not recommended to manipulate the data. However, it is equally true that negative strain does not make sense practically, and its just a sample mounting err. So, will it be technically correct if I use the part of the tensile stress-strain curve above the red dashed line for the calculation of toughness values (J/m ³). I am especially convinced of chopping off the data below the 0-Y-axis because, after failure, there is only 1 data-point that is pushing the graph again in the 4th quadrant.
Any thought is deeply appreciated? Thank you.
If you have balanced the load cell before sample mounting it is reasonable to shift it. But if you have changed jaws and didn't balance the load cell I would repeat the experiment. Some machines have option to keep the load at 0 during sample mounting so called "sample protection" it helps to start from 0.
I am agree with Grzegorz Cios comment to adjust the load cell setting for correcting the zero at start of your experiment. Removing negative strain portion from your tensile data is not correct and recommended for calculations of fracture toughness or any other mechanical properties of material. However as you mentioned the last part after material failure you can certainly remove points in forth quadrant.
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