Geometric nonlinearities refer to the nonlinearities in the structure or component due to the changing geometry as it deflects. That is, the stiffness [K] is a function of the displacements {u}. The stiffness changes because the shape changes and/or the material rotates. The program can account for four types of geometric nonlinearities:
Large strain assumes that the strains are no longer infinitesimal (they are finite). Shape changes (e.g. area, thickness, etc.) are also accounted for. Deflections and rotations may be arbitrarily large.
Large rotation assumes that the rotations are large but the mechanical strains (those that cause stresses) are evaluated using linearized expressions. The structure is assumed not to change shape except for rigid body motions. The elements of this class refer to the original configuration.
Stress stiffening assumes that both strains and rotations are small. A 1st order approximation to the rotations is used to capture some nonlinear rotation effects.
Spin softening also assumes that both strains and rotations are small. This option accounts for the coupling between the transverse vibrational motion and the centrifugal force due to an angular velocity.
All elements support the spin softening capability, while only some of the elements support the other options. Please refer to the Element Reference for details.