Crack
Add a Crack node to indicate that certain boundaries represent a crack. A crack can either be infinitely thin, and represented by a single boundary, or being represented by disjoint surfaces in the geometry.
A crack can have any number of branches and corresponding crack fronts.
The Crack node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).
Crack Front
This section is only shown if Crack surface is set to Symmetric in the Crack Definition section. In this case, it is not possible to deduce the number of crack fronts uniquely from geometric analysis, so you must explicitly select the crack front or crack fronts.

Crack Face 1
This section is only shown if Crack surface is set to From geometry in the Crack Definition section. In this case, the main selection will contain both sides of the crack. In this section, select one of the crack faces, so that it is possible to discriminate between the two faces of the crack.
Coordinate System Selection
The coordinate system selection is only used when a user defined crack orientation is selected in the Crack Orientation section.
Crack Definition
In this section, you select how the crack is represented in the geometry.
Select a Crack surfaceSlit, From geometry, or Symmetric.
When the crack is represented as an internal boundary, select Slit. This will cause the displacements of the two sides of the boundary to be decoupled from each other so that a crack is formed.
If only half of a symmetric structure is modeled, so that the crack is an external boundary, select Symmetric. In 2D axisymmetry, this is only meaningful for cracks that are perpendicular to the axis of rotation. There are two cases:
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The crack is infinitely thin and located in the symmetry plane. Other boundary conditions like Fixed Constraint or Symmetry will be overridden on the crack surface, so that the crack can be opened.
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Crack Orientation
The direction of the crack growth is important if you are going to compute a J-Integral.
Select a Crack orientationFrom crack, or User defined. In most cases the crack growth direction can be automatically deduced from the geometry of the crack. If this is not the case, select User defined and then enter a vector for the Crack growth direction. The vector is represented in the coordinate system selected in the Coordinate System Selection section.
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics selected: