I am not sure I understand correctly your question but I hope this helps :

1) the tensile stress is the ratio of the tensile load F applied to the specimen to its original cross-sectional area So

2) You have already the strain variation from your DIC setup

then

3) if instead of the original cross-section So area of the specimen you use the cross-section area Si at a given time i during the experiment then you can draw the Streess - strain at a local point.

There are several ASTM ort ISO standards for tensile testing,

for metals for instance you can read ASTM E8/E8M or ISO 6892, there is also a standard for DIC testing.

Dear Mr. Corniani,

Thank you for your reply. I was also thinking for doing it this way. However, do you have any idea how to calculate the cross sectional area over the test duration. Shall I take the lateral strain into consideration for this?

a way to estimate the cross section area at a certain time t is to make the assumption of constant volume and uniform deformation:

Ao Lo=At Lt, With At cross section at time t of the experiment and Lt the lenght at time t.

You can also read this document at pag. 7

https://www.asminternational.org/documents/10192/3465262/05105G_Chapter_1.pdf/e13396e8-a327-490a-a414-9bd1d2bc2bb8

Regards