I need to have exact access to the blocks of stiffness matrix

[dokken]

You should then figure out what degrees of freedom each element contain, Which can be accessed via
dolfinx.fem.FunctionSpace.dofmap.cell_dofs(cell_index) this will give you want degrees of freedom (not unrolled with the block size of the problem, 3 for 3D elasticity) are in the cell. You can then use this to extract the rows and columns of the matrix, by either converting the PETSc mat to a dense matrix, or a csr matrix.

Transform dolfinx assemble matrix to numpy array

It’s probably the dense mode that’s killing you. You can convert to a sparse CSR matrix using something like A = assemble_matrix(a, [bc]) A.assemble() assert isinstance(A, PETSc.Mat) ai, aj, av = A.getValuesCSR() Asp = csr_matrix((av, aj, ai))