Dear Yves,
Thank you very much for your answer.
It's OK for scalar variables but I do not really understand how Grad_u is built when u is a displacement vector field of 3 components. I thought Grad_u would represent the vector du/ds ([dux/ds,duy/ds,duz/ds]) with s the local curvilinear abscissa (so that Normalized(element_K).Grad_u would give the linearized longitudinal deformation) but it seems that Grad_u is actually a 3x3 matrix field. Then I do not see how to build the longitudinal deformation. What would be the best way please? And by the way, what would be the right syntax to get the second derivative of the transverse displacement by means of Hermite elements and the Hessian?
Thank you again for your help.
Best regards
Jean-François