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Define a crack or a 2D or 3D interface using level functions.

For cracks, two level functions (level sets) are defined to characterize any crack (flat crack or not). For interfaces, only one level-set is required. Level sets can be defined in four different ways:

• or from two groups of cells (the group of cells of the crack and the group of elements of the bottom of the crack);

• or from two analytical functions.

• or from a catalog of predefined shapes.

• or by re-reading fields to existing nodes.

For the 2D case, the cells at the bottom of the crack are POI cells; they should not be given as a node group but as a group of elements.

For the 3D case, the crack bottom is automatically oriented. The ordered list of background points is automatically displayed in the message file when the DEFI_FISS_XFEM command is passed. The orientation method and the specific case where it does not work are described in the documentation [R7.02.12].

This operator does not make it possible to define a crack coincident with an edge of the structure.

The characteristics associated with the contact on the lips of the crack are not given here but in a specific contact load with operator DEFI_CONTACT. It is also possible to define cohesion forces between the lips of the crack, by this same operator. The crack is then generally declared to be of type COHESIF.

The theoretical aspect of the X- FEM method is discussed in [R7.02.12], and it is recommended that the user who is not familiar with the use of level sets read the few explanatory lines relating to the representation of surfaces by the level set method.

In addition, this operator can also be used with mesh cracks, prior to the calculation of stress intensity factors in 3D by the operator CALC_G [U4.82.03] with the option CALC_K_G.

The operator produces a fiss_xfem concept.