Question - Why the SIFS value is not matching after the unloading for a frictional contact problem?
Answer - If your problem setup is linear and conservative, ideally the Stress and SIF should give you same result at no loading and unloading, however the definition of sliding friction (Mu) makes your system non conservative and thereby path dependency comes into picture.
Please refer to the attached model.
For more details, please refer to the document below: