Ansys Employee
nShell 181 with KEYOPT(1) = 1 has no bending stiffness, a condition that can result in solver and convergence problems. For example if your circular membrane is along xz and the pressure is along the y direction, you would have a moment at the first iteration but no reaction forces (since the element are laid out in the normal plane) and no bending moments (since bending stiffness is zero). This results in a pivot error. You could try adding 2 load steps (under analysis settings), the first one with an in-plain force. Then the second one with the actual pressure and the in-plain force reduced to zero. Also try using a curved membrane (this worked for me) instead of the flat membrane.nFor additional documentation regarding shell181 refer the help documentation SHELL181 (ansys.com)nnRegards,nIshan.n