What is the difference between shell 63 and shell 181?
SHELL63 and SHELL181 are different in many aspects:
1. SHELL181 is first order shear deformation shell, while SHELL63 does not account for transverse shell energy
2. SHELL181 has finite strain capability, while SHELL63 does not.
3. SHELL181 can have all applicable nonlinear materials
4. Stress stiffness in SHELL181 is done in a more consistent way, with both membrane stresses, enhanced strain effects, and transverse shear stresses accounted for.
5. SHELL181 has a more advanced warping correction algorithm, making the element stiffness less sensitive to warping. This may be the reason why we don't have the similar warping warning messages for SHELL181
Therefore, SHELL181 and SHELL63 will have close results only when
1. homogeneous linear elastic material
2. small deformation
3. very small shell thickness
4. small warping