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Mesh controls

Within the options to specify the crack front the user has control over several aspects of the generated mesh. For example:

  • The radial midside nodes in the crack front elements may be specified at the quarter-point position or the midside position (or anywhere in between).
  • The crack front nodal positions may contain a single node or multiple nodes that can open up to simulate blunting.
  • The element rings around the crack front may be biased towards the crack front or evenly spaced.
  • The outer size of the element rings around the crack front can be controlled.

These options are easy to specify in the input file. For example, the combination of quarter-point nodes and a single node for each crack front nodal location, which is suitable for the r-0.5 LEFM singularity, can be requested using:

*TIP MODEL, LEFM

The combination of midside nodes and multiple nodes at crack front nodal locations gives an r-1 singularity and is more suited to EPFM. This is requested using:

*TIP MODEL, EPFM
Tip model for LEFM
Tip model for LEFM
Quarter point radial nodes and
a single node at each crack front position
Mip model for EPFM
Tip model for EPFM
Midside radial nodes and
multiple nodes at each crack front position

Boundary shifting and mesh relaxation

When standard crack-blocks are used there are additional options available to control the overall size of the replaced elements. This is to help minimise distortion within the crack-blocks. This also allows analysis of a wide range of crack sizes from the same uncracked mesh.

The options available include boundary shifting and mesh relaxation. These options allow cracks to be placed in meshes where element distortion would otherwise preclude the analysis of the mesh. The boundary shift algorithm adjusts the external boundaries of the crack-blocks such that the element distortion inside the crack-block is reduced. The mesh relaxation algorithm then attempts to re-distribute the nodes outside the crack-blocks to reduce any distortion introduced by the boundary shifting.

A simple example of the effect of boundary shifting and relaxation is given below. These features are extremely useful in parametric studies of different crack sizes in the same uncracked mesh. They can also be significant for crack growth analyses.


Meshing example
Mesh without boundary shifting and relaxtion can produce
distorted elements in the crack region
Meshing example
Application of boundary shifting and relaxtion reduce
element distortion in the crack region
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