Meshing procedure - overview

The generation of 3D finite element models suitable for analysis of cracks requires special attention to the meshing in and around the crack region. The crack front itself introduces a singularity into the stress and strain fields. The use of energy release rate and j-integral methods requires that there are "rings" of elements surrounding the crack front. To address both of these items properly requires the use of a focused mesh of hex elements (rather than tets) around the crack front.

In addition to these finite elements issues, the crack front presents topological problems. The initial crack front may be straight, a simple elliptic section, or a general curve in space.

To address these issues, Zencrack removes the onus of modelling the crack region from the analyst and requires instead that an uncracked mesh is supplied. A crack-block approach is then used to introduce one or more crack fronts into the uncracked mesh.

The term crack-block refers to a collection of brick elements stored as a unit cube. The arrangement of these crack-blocks is such that in their unit cube form they contain either a quarter circular or through crack front on one face. Part of this face is allowed to open up under loading giving the opening crack face within the crack-block.

The meshing procedure is one of replacement of one or more 8 or 20 noded brick elements in a user supplied uncracked mesh by crack-blocks. During the mapping process to introduce the crack-blocks the user can control the size and shape of the generated crack front section for each crack-block. The initial crack front derived from a quarter circular crack-block may be elliptic, for example. Crack-blocks can be connected together to form distinct crack fronts of the required size in the cracked mesh. In the example shown here, in which only one side of the crack front is modelled, two crack-blocks have been merged together to form a single crack front. In a more general case there may be multiple distinct crack fronts in a model.

For crack growth analysis an adaptive meshing algorithm is used. The updated crack front position and any non-planar crack history are used to re-insert the crack-blocks to create an updated crack position in the mesh. The crack shape that develops as the crack grows is a function of the initial shape, specimen geometry, applied loading and materials data.

Summary of meshing capabilities

• Rapid generation of 3D finite element meshes of components containing multiple cracks
• Automatic introduction of crack fronts into an existing mesh of the intact component, replacing user-defined target brick element(s) with crack-blocks, each modelling a section of crack front
• Use of "large" crack-blocks and surface based tying to give greater flexibility in crack modelling
• Introduction of cracks in 8 or 20 noded brick meshes
• Library of crack-blocks in "unit cube space" for introduction of starter cracks
• Mapping from "unit cube space" to distorted shape of target element
• User defined initial crack shape
• Quarter point or midside nodes in crack front elements
• Single or multiple nodes at each crack front position
• Automatic merging of crack-blocks into the mesh
• Automatic generation of element properties, boundary conditions & loading for crack-blocks
• Generation of complete finite element analysis input file for cracked component including virtual crack extension data
• Option to "reverse" the open and closed parts of a crack face
• Interpolation of specified nodal temperatures in the uncracked mesh to nodes in the cracked region of the cracked mesh
• Option to shift crack-block boundaries and relax the surrounding mesh to minimise element distortion.