Green strain calculated with deformation gradient vs Green strain calculated with stretch ratio

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Hi.

I just wondering why the Green strain values calculated by the deformation gradient and stretch ratio are different.

the deformation gradient of each facet are extracted by the automation coding with pyautogui, and the stretch ratio are extracted by using Python script imbedded in GOM.

After I read the manual of the gom, I thought the 'stretch ratio' at GOM GUI,,, you know,,, surface inspection GUI,,,, comes from U (right stretch ratio), but actually, the stretch ratio from the right stretch tensor  and surface inspection GUI stretch ratio are different.

Can you tell me the reason why this difference is happened?

Thanks.