9.15. Identify Fragments

The IDENTIFY FRAGMENTS capability provides data on unique contiguous mesh fragments in a simulation. Fragments are identified based on mesh connectivity. The fragment identification algorithm accounts for topological changes including element death, element separation in an XFEM simulation. Fragment identification element field elem_fragment_id and nodal field node_fragment_id indicate the fragment number for each element and node in the simulation, respectively, and may be output in a results block, allowing for fragment visualization and other post-processing operations. Fragment numbers range from 1 to the total number of fragments, sorted in order of increasing fragment mass. Additional fragment data, including fragment mass and momentum, may be written to an optional output text file.

A nodal vector field fragment_force is provided to compute force and moment on a per-fragment basis. If using an applied pressure (Section 7.7.1), the fragment_force field can be used as the external force contribution output name. In a Zapotec coupled analysis the field force_to_apply might coupled with fragment_force to report force and moments on the fragments. Otherwise, fragment_force can be populated through user output methods.

Fragment identification is controlled by the BEGIN IDENTIFY FRAGMENTS block in the region scope:

BEGIN IDENTIFY FRAGMENTS
   OUTPUT FILE = <string>output_filename
   INCREMENT = <real>increment
   MINIMUM FRAGMENT SIZE = <int>min_element_threshold
   START TIME = <real>
END [IDENTIFY FRAGMENTS]

The optional OUTPUT FILE command triggers output to a text file named output_filename containing fragment data. The data written to the text file include the mass, momentum, center of mass, moment of inertia, angular momentum, and names of element blocks present in each fragment. The optional MINIMUM FRAGMENT SIZE command groups fragments containing fewer than min_element_threshold elements into a single fragment with ID 0. This feature is useful, for example, in a brittle fracture simulation that results in a significant number of isolated elements. By default, the fragment identification algorithm is executed once per output step. If the optional INCREMENT command is included in the IDENTIFY FRAGMENTS block, the fragment identification algorithm is executed only at the corresponding simulation times. For example, if INCREMENT is assigned a value of 0.23, the fragment identification algorithm will be executed only when the simulation time reaches a multiple of 0.23. Limiting the number of times the fragment identification algorithm is executed reduces the associated computational expense. The optional command START TIME controls when the fragment data will start to be printed to the output file.

Warning

Fragment identification is performed based only on node-to-element connectivity. If two elements share a node, those two elements are in the same fragment. Other mechanisms to connect elements, such as tied contact, MPCs, spot welds, etc., are not considered in fragment identification at this time.