Dear Aline...what you mean by deformation its not clear for me..but i think since the crack in specimen was developed so you are already in plastic zone.

First it depends on your material.

The the reading of the strain gage contains elastic part and plastic part of deformation. 

You should know how to separate them.

Professor Mohannad Husain

What paper or author do you recommend for this analises (to separate elastic part to plastic part)? 

Thank you so much.

Dear Amjad Albayati

The deformation that i mentioned is that recorded by strain gage during the fatigue crack growth.

Professor Al-Sherrawi,

My material is Al-Li alloy.

Thank you so much.