Geometric nonlinearity ==== This part treats the problems most frequently encountered when calculating with geometric nonlinearities. Flambage ---- The operator STAT_NON_LINE makes it possible to calculate a stability criterion *via* using the keyword CRIT_STAB =_F (TYPE =' FLAMBEMENT '). The documentation for using buckling [:external:ref:`U2.08.04 `] gives a lot of advice on setting up a stability calculation. One gives here some general advice. In general, the stability analysis is carried out on updated stiffness matrices. In practice, calls to CRIT_STAB should be limited for reasons of calculation costs, by limiting the times at which the stability analysis is carried out. In addition, it is a good idea to use CRIT_STAB only on time intervals where there is a suspicion of the possibility of instabilities. It is also important to refine the time step when approaching this zone. By default, 3 critical loads are calculated (NB_FREQ). Often the first one can suffice. One of the particular points related to instability is the choice of the technique for controlling the algorithm. In fact, conventional effort control is not suitable. As you approach a limit point, you need to reduce the load increment and increase the maximum number of iterations. It is also recommended to use arc-length control. Large deformations ---- The keyword DEFORMATION under COMPORTEMENT makes it possible to define the hypotheses used to calculate the deformations. By default, small displacements and small deformations are considered. (DEFORMATION = 'PETIT'). This means that we remain on the Small Perturbations Hypothesis: small displacements, small rotations, small deformations (less than about 5%). When this hypothesis is no longer true, the deformation model must be changed. For slender structures (shells, plates, beams), it often happens that we are in large movements, large rotations but small deformations. We then use DEFORMATION = 'GROT_GDEP'. The treatment of large deformations differs according to the type of element and the law of behavior. The large deformation model of Simo and Miehe (DEFORMATION = 'SIMO_MIEHE') is recommended for the behavior relationships VMIS_ISOT_LINE, VMIS_ISOT_TRAC, ROUSSELIER and all isotropic work hardening behaviors only, associated with a material undergoing metallurgical phase changes (relationships META_X_IL_XXX_XXX and META_X_INL_XXX_XXX). For other laws of behavior, we recommend the model of large deformations by Miehe and Apel (DEFORMATION = 'GDEF_LOG'), theoretically applicable to any law of behavior for 3D and 2D models. More detailed explanations can be found in the instructions for using non-linear behaviors [:external:ref:`U4.51.11 `] in paragraph DEFORMATION.