Generalities
===========

Information describing contact options is stored in the sd_contact data structure (SD) (operator DEFI_CONTACT).

This SD stores two types of information:


* Global contact options, which do not depend on zones;


* The contact's local options, defined zone by zone.

Some options are common to all methods, others are specific to each formulation.

Access to the SD is done*via* specific routines that encapsulate the definition of JEVEUX objects. They should be used exclusively.

A touch/friction zone includes two surfaces whose interpenetration is sought to be prevented. There are three formulations for touch/friction (keyword FORMULATION):


* The discrete formulation (DISCRETE)


* The continuous formulation (CONTINUE including formulation LAC)


* The continuous formulation applied to XFEM (XFEM)

We combine formulation DISCRETEet, formulation CONTINUEdans, what we will call **meshed** formulations. So method XFEMn is not a mesh formulation.

In addition, there is a specific formulation (LIAISON_UNIL), dedicated to simple unilateral conditions (without pairing). This formulation, derived from discrete methods, makes it possible to impose a unilateral condition on a degree of freedom. For example: :math:`\mathit{DX}<4` or :math:`\mathit{PRES}<3`. It is used in particular in THM, to impose so-called oozing conditions.

In the case of mesh formulations with NODALou MAIT_ESCL pairing, there are two surfaces whose composition is given under the keywords GROUP_MA_MAIT/MAILLE_MAIT and GROUP_MA_ESCL/MAILLE_ESCL. In the case of the formulation LIAISON_UNIL, there is only one surface whose composition is given under the keywords GROUP_MA/MAILLE/GROUP_NO/NOEUD. In the case of the formulation XFEM, there is no mesh and the cracks XFEMsur are given which will apply the contact/friction conditions (*via* the keyword FISS_MAIT).