In the context of solving a linear elasticity problem, is there a way to extract the linear operator that converts displacement into strain tensor? For example, if there are n nodes, we would have a displacement vector of size 3*n, if the strain tensors are reshaped into a 6*n vector (assuming isotropic material), I would like to build the 6*n x 3*n matrix that convert displacement into strain.
I have tried a finite difference approach where strain is written as the symmetric gradient of displacement, and the gradient of displacement is approximated by finite difference. However, this does not yield the same result as the stress tensor computed in src/getfem/getfem_derivatives.h