The primary variable (xd) of the mechanical calculation

[Jiahui Chen]

Dear MRST team,

Could you please explain to me the variable 'xd' in the ad-mechanics module? I have been thinking 'xd' is displacement, but what is the difference between 'xd' and 'u' in variable states? I know 'u' is the displacement located at the nodes of each grid cell.

Besides, in function 'addDerivedQuantities', the other mechanical fields are calculated from 'xd', why the new stress is the product of global stress and displacement, as shown in the image below?

Look forward to your reply, thank you!

Best regards,

Jiahui