====== FAIN3 ======
3D fault interface
===== Description =====
Type: 217 \\
\\
{{ :elements:cfi3d.png?400|}}
The element consists of 4 or 8 nodes (DoF 1, 2, 3, and 4) relating to one side of the fault (= the structure), and 4 or 8 additional nodes to discretize the flow  inside the fault (DoF 4). It also relates to a predefined foundation. \\
The normal (red side in DESFIN) must go out of the structure. \\
\\
The foundation, defined by a series of triangular or quadrangular sides, is defined in the "FOUND" section of the *.lag file. If the foundation is defined by a series of sides, the normal is given by the product of vectors $\vec{12}$ and $\vec{13}$ built on the nodes of the foundation $\vec{n}=\vec{12} \wedge\vec{13}$ of each side. This normal must be directed inwards the foundation. \\
\\
This element is dedicated to soil mechanics and in particular to structure-structure contact (fault). It includes pressure, friction, flow axial to the fault, and 2 flows transversal to the fault (along the thickness), and two additional local flows in the fault plane. \\ It is actually a combination between a contact element [[elements:cfi3d|CFI3D]] and a spatial plane of flow [[elements:plath|PLATH]]. \\ 
The element can work in flow, mechanical, or coupled analysis. \\
\\
This element can use mechanical law [[laws:intme|INTME3]] or a classic law and flow law [[laws:intec3|INTEC3]].
\\ \\
__Warning__: When using renumbering of the DoFs, only directional renumbering works. \\
\\
Implemented by: J.P. Radu, 2001
==== Files ====
Prepro: FAIN3A.F \\
Lagamine: FAIN3B.F

===== Input file =====
^Title (A5)^^
|TITLE|"FAIN3" in the first 5 columns|
^Control data (3I5)^^
|NELEM|Number of elements|
|INSIG|= 0  no initial stresses|
|:::  |= 1 → initial stresses|
^Initial stresses - only if INSIG = 1 (6G10.0)^^
|The pressure varies as: $PRESS = PRES0 + (Z*DPRES)$ \\ Tau varies as: $TAUn = TAUn0+(Z*DTAUn)$||
|PRES0|Pressure of the contact at the axis origin|
|DPRES|Coefficient of variation of the pressure along Y (= 0 → constant pressure)|
|TAU10|TAU along direction 1 at the axis origin|
|DTAU1|Coefficient of variation of TAU1 along Z (= 0 → constant TAU1)|
|TAU20|TAU along direction 2 at the axis origin|
|DTAU2|Coefficient of variation of TAU2 along Z (= 0 → constant TAU2)|
^Definition of the elements (5I5/8I5/8I5)^^
|NINTE|Number of integration points (1, 4, or 9)|
|LMATM|Mechanical law number|
|LMATF|Flow law|
|IFOND|Number of the foundation or tool \\ If the foundation number is equal to 0, the boundary thermal flow is calculated with the ambient, without mechanical contact with any foundation.|
|IRIGF|Type of contact|
|:::| 0 → rigid foundation or tool |
|:::| 1 → uncoupled solid/solid contact \\ One contact element on each structure, the interpenetration distance is divided by 2. \\ Suitable for solids with similar stiffnesses.|
|:::| 2 → coupled solid/solid contact \\ Only one contact element must be defined on a solid, the other being its foundation. The computation of MBAND and NHICO must be actualized (see [[lagamex:auto#4th_line_14i5|ISTRA(4)]]). \\ Suitable for solids of different stiffnesses, with one (the foundation) can be more roughly approximated.|
|:::| 3 → coupled solid/solid contact \\ One contact element on each structure, the force is divided by 2. None of the structure is privileged. The computation of MBAND and NHICO must be actualized (see [[lagamex:auto#4th_line_14i5|ISTRA(4)]]). \\ Suitable for solids of different stiffnesses, both must be properly represented.|
|NODES(1 to 8)|List of nodes on the structure side. Nodes 5 to 8 can be 0; in that case the element is of linear degree|
|NODES(9 to 16)|List of nodes inside the fault. Nodes 5' to 8' can be 0; in that case the element is of linear degree|
===== Results =====
**Stresses**: \\
  * Pressure
  * Tangent stress in direction 1
  * Tangent stress in direction 2
  * 0
  * Flow in the plane of the element $f_{x,loc}$, 
  * Flow in the plane of the element $f_{y,loc}$,
  * Stored flow, 
  * "FI" (between Foundation and Inside) transversal flow
  * "IS" (between Inside and Structure) transversal flow

**Internal variables**: \\ 
The first values are the ones relating to the mechanical law \\ 
For [[laws:intme|INTME3]], this will be: 
  * Plasticity indicator, 
  * Dissipation, 
  * Fault closing $V$, 
  * Fault opening $D$
The 2 next values correspond to the contact geometry. They are: \\
  * The segment number of the foundation in contact
  * The interpenetration distance
The last values are those relating to the flow law; for [[laws:intec3|INTEC3]], they are:
  * The pore pressure inside the fault
  * The intrinsic permeability in the plane of the element
  * Porosity
  * Saturation
  * Specific mass of the fluid
  * The transversal transmissivity