"In case of a failure, the shell may fail due to large deformations (buckling) or due to material nonlinearity (cracking and crushing) or by a combination of both (so-called inelastic or plastic buckling). ... Opposite to columns and plates, shells experience a sudden decrease in load carrying capacity after the bifurcation point (which can be obtained by a simple linear buckling analysis). ...compound buckling ...refers to several buckling modes associated with the same critical load. In the postbuckling range the modes... start to interact resulting in a significantly reduced load carrying capacity. As discovered by Koiter... geometrical imperfections in the shell cause the bifurcation point never to be reached and lead to... buckling at a considerably lower load. The size of the imperfections determines the limit load at which the shell fails. In case of plastic buckling, the fall-back is further intensified by material nonlinearity."