F is a symbolic expression. It is not an array of values.
You would have to implement the polar decomposition in UFL. ie. take the logic from: scipy/scipy/linalg/_decomp_polar.py at v1.15.2 · scipy/scipy · GitHub
and implement it with ufl operators, similar to:
Calculating local eigenvalues and eigenvectors for use in weak form
(There are analytical formulas for svd of 2x2 matrices at): Singular value decomposition - Wikipedia
1 Like