Transverse loading on Shell Elements

Why don't SHELL181 mid-surface results reflect plastic stress-strain curve? Due to transverse shear the mid-surface equivalent stress is higher than the top or bottom surface and the value being reported is far beyond the yield stress. 

The SHELL181 treats the transverse shear linearly. The plasticity is not considered only in the transverse shear direction. If transverse shear needs to be modeled with plasticity a 3d element is required. Possibly a 3d submodel could be used. It is rare in thin shells for the transverse shear to be the dominant stress.