Contact-friction nonlinearity ==== The definition of the touch-friction load is the subject of a specific operator: DEFI_CONTACT [:external:ref:`U4.44.11 `]. In addition to choosing the entities in contact, this operator makes it possible to choose the type of formulation of the contact problem (discrete or continuous), parameters for the matching operation and parameters for the resolution phase. Numerous data layout tips are available in the document [:external:ref:`U2.04.04 `], in particular on the choice of master and slave surfaces. Taking into account contact conditions with or without friction leads to non-linear problems, which can be a source of difficulties in solving STAT_NON_LINE. One of the first difficulties is the blocking of rigid body modes. In fact, the contact and friction conditions do not intervene in the blocking of rigid modes. It is therefore necessary to block rigid modes without taking into account contact conditions (with rare exceptions). Poor blocking of rigid body modes leads to the appearance of zero pivots during matrix factorization. Tips are given in the documentation [:external:ref:`U2.04.04 `]. Attention, some contact formulations require a particular solver. In particular, in discrete formulation, method CONTRAINTE requires a direct solver (MULT_FRONT or MUMPS). In general, the use of linear contact research is discouraged. In case of coupling with other nonlinearities, it is advisable to ensure that each nonlinearity taken separately converges. If problems only occur when the nonlinearities are coupled, it is advisable to work with the updated tangent matrix and an elastic prediction. A comparative benchmark between*Code_Aster* and other commercial software was carried out in 2010 on 5 touch-friction problems [:ref:`2 <2>`]. In particular, this benchmark made it possible to highlight oscillations in contact pressure for continuous formulation in the event of incompatible meshes, a defect that also affects the commercial software used. It is then recommended to extract the contact pressures from the stresses on the edge, or else to use compatible meshes. .. _Ref374957480: .. _Ref375039773: