The numerical evaluation of fracture-mechanics parameters in 3D cases depends to a great extent on the finite element meshes used in your analysis.
While structured hexahedral meshes are commonly used to accurately evaluate fracture-mechanics parameters, some analyses may require unstructured hexahedral meshes or tetrahedral meshes. In such cases, the conventional method used for parameter evaluation results in either approximate values or values that tend to oscillate around some mean value.
The unstructured-mesh method (UMM) is a numerical tool for evaluating fracture-mechanics parameters more accurately in cases where unstructured hexahedral meshes and tetrahedral meshes are present. The accuracy of the resulting parameters is generally comparable to that of parameters evaluated using structured hexahedral meshes.
The following additional UMM topics are available:
The unstructured-mesh method supports the following fracture-mechanics parameter calculations:
When evaluating these parameters, you can change the default UMM setting (via the CINT command). If disabled, the calculation result is the same as that of the conventional calculation method.
The unstructured-mesh method is controlled via the CINT,UMM,ON/OFF command. UMM is enabled (ON) or disabled (OFF) by default according to the element type, as follows:
| Element Type | Description | Default UMM Setting |
|---|---|---|
|
3D 8-Node Structural Solid |
OFF | |
|
3D 20-Node Structural Solid | ||
|
3D 10-Node Tetrahedral Structural Solid |
ON | |
|
3D 4-Node Tetrahedral Structural Solid with Nodal Pressures |
If issued, the CINT command overrides the default UMM setting.
The following assumptions and restrictions apply to UMM:
The portion of crack face inside the region near the crack front where contour integration is performed is assumed to be flat. Ansys, Inc. recommends disabling UMM (CINT,UMM,OFF) for crack surfaces with significant curvature. Also, the results of fracture-parameter calculations near the ends of the crack front may be affected when curved boundary surfaces exist in that region; therefore, carefully evaluate the numerical results before use.
Discontinuous crack-surface pressure or body temperature in the vicinity of the crack front may cause path-dependency in the fracture-parameter calculations.
The two crack faces should be coincident (having no gap between them).
For nonlinear elastic-plastic materials, Ansys, Inc. recommends using a finer mesh in the region near the crack fronts.