If switch is set to -yes the stress becomes zero in 2D and the and stress become zero in 1D (in combination with axi-symmetry in 1D, only the stress becomes zero since is the axi-symmetric direction). If group_materi_membrane is not used the plane strain conditions are used. Always the -thickness is 1. in 3D, and the , and -thickness are 1. in 2D; see however also volume_factor.
The group_materi_membrane option cannot be used in combination with group_materi_elasti_compressibility, group_materi_hyperelasticity and group_materi_viscosity.
The index specifies the element_group, see element_group.