XFEM module

An entity subject to XFEM modeling is described by an EnrichmentItem. Such an entity can be e.g. a crack or a material interface. The EnrichmentItem is responsible for the geometrical description as well as the enrichment functions. The EnrichmentItem is also responsible for the necessary updates if the interface is moving (e.g. if a crack is propagating). The EnrichmentItem has different data members to fulfill these requirements:

The enrichment functions are represented by classes derived from the pure virtual EnrichmentFunction class. In particular, the following methods have to be overloaded by all subclasses of EnrichmentFunction:

The enrichment items are stored in an array in the XfemManager class, that also provides initialization and access to individual enrichment items. The purpose of the XfemManager is to encapsulate XFEM specific functionality. This avoids to a certain extent the need to modify and penetrate the original code.

When a certain node in the problem domain is subjected to enrichment, the corresponding enrichment item has to introduce additional degree(s) of freedom in that node. Since several enrichment items can be active in a node, all DOFs are assigned with a label, that allows to distinguish between them (in traditional FE codes labels are also used to distinguish physical meaning of particular DOF representing displacement along x,y,z axes, rotations, etc.). The XfemManager class maintains a pool of available labels and is responsible for their assignment to individual enrichment items.

Each enrichment item keeps its assigned DOF labels (provided by XfemManager). These are used by individual enrichment items to introduce and later identify related DOFs in individual nodes. The physical meaning of these DOFs depends on the enrichment item. For example, a single DOF is needed to represent slip along a slip line and two DOFs are required for a displacement discontinuity in 2D modeled with Heaviside enrichment.

If the enrichment items evolve in time, the geometrical description needs to be updated and new enriched DOFs may need to be created. This is done by the updateGeometry method in the XfemManager class. The subtriangulation of cut elements may have to be updated as a result of the geometrical update (e.g. previously uncut elements may be cut by a crack and therefore need to be subdivided). The solver class XFEMStatic handles this problem by creating a new domain with a new subtriangulation and mapping state variables as well as primary variables from the old domain to the new domain.

The subtriangulation of cut elements as well as computation of B-matrices for 2D XFEM elements are to a large extent encapsulated in the XfemElementInterface class. Methods needed for cohesive zones are also implemented here. The 2D elements with XFEM functionality are therefore derived from the standard Element base class, as well as from the XfemElementInterface.

When a cut element has been subdivided by the XfemElementInterface, a PatchIntegrationRule is used to represent the integration scheme, where Gauss integration is employed on individual triangles. When initializing the integration points, their local coordinates (related to the triangle geometry) are mapped into parent element coordinates and the corresponding integration weights are scaled accordingly. There is no need to store individual patch geometries, as the parent element geometry is used to correctly evaluate transformation Jacobians (we may however wish to store the them for debugging or visualization).

If a cohesive crack model is employed, additional integration points along the discontinuity are needed. These are stored separately, see XfemElementInterface.

The following test cases involve the XFEM module:

Borek Patzak 2018-01-02