I am running simulations of a cantilever beam where it is fixed on one end and negative force applied to the other end. The first simulation is with 4-node linear quadrilateral elements and the other is simulation with 9-node quadratic quadrilaterial elements. At first I ran the simulations with a poisson's ratio of 0.3, then with a Poisson's ratio of 0.499 and calculated the convergence rate.
I got the convergence rate for the 4-node as 1.7 but it should be 2 and for the 9-node it is 3. My question is why they are different than each other is it because 4-node is linear and 9-node is quadratic.
In addition, what would be the reasons that cause the convergence rate to be 1.7 not 2? when I asked my professor he said your algorithm is right but you need to know why its not 2. When I changed the Poisson's ratio to 0.499. everything messed up it the 4node simulation but it in it is fine for the 9node, why is that? why the 9-node can handle 0.499 material and 4-node can'