So I am running a bias extension test on Abaqus for an orthotropic composite. I am getting different results for the model when using S8R and S8R5 elements. I understand that S8R is for thick shells and S8R5 is for thin. I also know that these elements use Kirchoff and Mindlin shell theories for calculations. However, my problem is that the thickness of the shell should not impact the results of an in-plane loading. Therefore, theoretically S8R and S8R5 should give the same results. Can anyone explain how these elements converge to a solution in the case of planar stress and strains?​​​​​​​