Interface elements represents an interaction between points, edges or surfaces. They are used to model debonding between surfaces or more general fracture processes through the use of cohesive zone (cz) models. They can also be used to model contact between elements. Specific interface material models needs to be used  consult the matlibmanual for supported models.
Ordering convention: All inerface elements have plusside and a minusside and all nodes should first be specified for the minusside and then the plusside. The normal to the interface is defined to point from the minusside to theplus side. Direction of normal vector on an element specifies normal stress/cohesion across the element. It is assumed that normal jump, normal traction are at the first position of corresponding vectors. Stiffness matrix in local coordinates has always on position 1,1 normal stiffness and then shear stiffness.
Implementation of one dimensional (slip) interface element. This element connects two separate nodes and the interaction is governed by a onedimensional slip law. This law determines the force acting between the nodes as a function on their relative displacement in the slip direction. The element can be used in 1D, 2D, and 3D (default) and its features are summarized in Table 39.
Keyword  IntElPoint, Interface1d (deprecated) 
Description  One dimensional (slip) interface element 
Specific parameters  [refnode #(in)] [normal #(ra)] 
Parameters  refnode: determines the reference node, which is used to specify a reference direction (the direction vector is obtained by subtracting the coordinates of the first node from the reference node). 
normal: The reference direction can be directly specified by the optional parameter normal. Although both refnode and normal are optional, at least one of them must be specified. 

Unknowns  One, two, or three DOFs (udisplacement, vdisplacement, wdisplacement) are required in each node, according to element mode (determined from domain type). 
Approximation   
Integration   
Features   
CS properties   
Loads   
Nlgeo  0 
Status  Reliable 
Note  Element requires material model with _1dInterface support. 
Implementation of a two dimensional line element with a linear approximation of the displacement jump. The element can be used to tie together two element edges and is defined by four nodes  two on each edge. Note that, the nodes along the interface are doubled, each couple with identical coordinates. Nodes on the negative side are numbered first, followed by nodes on the positive part. Requires material model with _2dInterface support. The element features are summarized in Table ??.
Keyword  IntElLine1, Interface2dlin(deprecated) 
Description  2D interface element with linear approximation 
Specific parameters  [axisymmode #()] 
Parameters  axisymmode: Flag controlling axisymmetric mode (integration over unit circumferential angle). 
Unknowns  Two dofs (udisplacement, vdisplacement) are required in each node. 
Approximation  Linear approximation of displacements and geometry. 
Integration  Full integration of all strain components using two point integration formula. 
Features   
CS properties   
Loads   
Nlgeo  0 
Status  Reliable 
Note  Element requires material model with _2dInterface support. 
Implementation of a two dimensional interface element with quadratic approximation of displacement field. Can be used to glue together two elements with quadratic displacement approximation along the shared edge. Note, that the nodes along the interface are doubled, each couple with identical coordinates. Nodes on the negative side are numbered first, followed by nodes on the positive part. Requires material model with _2dInterface support. The element features are summarized in Table ??.
Keyword  IntElLine2, Interface2dquad (deprecated) 
Description  2D interface element with quadratic approximation 
Specific parameters  [axisymmode #()] 
Parameters  axisymmode: Flag controlling axisymmetric mode (integration over unit circumferential angle). 
Unknowns  Two dofs (udisplacement, vdisplacement) are required in each node. 
Approximation  Quadratic approximation of displacements and geometry. 
Integration  Full integration of all strain components using four point integration formula. 
Features   
CS properties   
Loads   
Nlgeo  0 
Status  Reliable 
Note  Element requires material model with _2dInterface support. 
Implementation of a three dimensional interface element with linear approximation of displacement field. Can be used to glue together two elements with linear displacement approximation along the shared triangular surface. Note, that the nodes along the interface are doubled, each couple with identical coordinates. Nodes on the negative surface are numbered first, followed by nodes on the positive part. The numbering of surface nodes on positive surface (+) determines the positive normal (right hand rule). Requires material model with _3dInterface support. The element features are summarized in Table 42.
Keyword  IntElSurfTr1, Interface3dtrlin (deprecated) 
Description  3D interface element with linear approximation 
Specific parameters   
Unknowns  Three dofs (udisplacement, vdisplacement, wdisplacement) are required in each node. 
Approximation  Linear approximation of displacements and geometry. 
Integration  Full integration of all components using one point integration formula. 
Features   
CS properties   
Loads   
Nlgeo  0 
Status  Reliable 
Note  Element requires material model with _3dInterface support. 