A crack is characterized by its shape, crack front/tip, crack discontinuity plane, crack normal, and crack direction. A crack front in three dimensional analyses represents the line of separation of the discontinuous crack surface. The same is represented by a crack tip in two dimensional analyses. A crack inside Mechanical can be defined using a Semi-Elliptical Crack object, an Elliptical Crack object, a Ring Crack object, or an Arbitrary Crack object, a Corner Crack object, an Edge Crack object, a Through Crack object, a Cylindrical Crack object, or a Pre-Meshed Crack object. These objects can be inserted under the Fracture folder.
Arbitrary Crack objects use a surface body selection to define an arbitrary crack shape in three dimensional analyses. The exterior edges of the surface body define the crack front and the surface itself defines the discontinuous crack plane.
Analytical cracks: Semi-Elliptical Crack, Elliptical Crack, Ring Crack, Arbitrary Crack, Corner Crack, Edge Crack, Through Crack, and the Cylindrical Crack are analytical cracks.
Analytical crack objects use the geometric parameters to define the crack shape and crack front in three dimensional analyses. These geometric inputs along with additional input parameters on the arbitrary crack and analytical crack object defines the region and shape of the generated crack mesh. Internally, the crack mesh generation is performed after the creation of the base mesh. By default, the crack mesh generation automatically creates a node-based named selection for the crack front under the respective crack object.
A Pre-Meshed Crack definition assumes that the crack meshes, representing the discontinuity or flaw in the structure, have already been generated. That is, the pre-meshed crack does not internally generate the crack mesh using Fracture Meshing, as other crack objects do, but instead assumes that the crack mesh has been generated beforehand. A Pre-Meshed Crack object uses a node-based named selection to analyze crack front; this nodal named selection is required for the computation of fracture parameters. If a geometric edge represents a crack front, you must first convert it to a node-based named selection using the Worksheet criteria before it can be used by the Pre-Meshed Crack object. See the next section, Defining a Pre-Meshed Crack, for more information on the Pre-Meshed Crack.
The orientation of the crack plays a vital role in the fracture parameter calculations. All analytical cracks, except the Cylindrical Crack, support only planar cracks where the crack normal can be defined using a single coordinate system. The orientation of all analytical crack objects is reflected by its Crack Coordinate System, while the orientation of a Pre-Meshed Crack object is reflected by its Coordinate System property setting. The orientation must be defined such that the y-axis is normal to the crack surface while the x-axis helps align the crack extension direction. The Arbitrary Crack on the other hand supports both planar and non-planar cracks. The orientation of an Arbitrary Crack object is reflected by its Coordinate System property setting. A single coordinate system cannot define the crack normal for non-planar cracks which is varying along the crack front. Hence, the Y axis of the Coordinate System property setting is directed towards the crack top face normal and X axis helps align the crack extension direction.
Note: When you insert a Commands (APDL) object under a crack, the object provides a Crack Front Selection property that enable you to execute commands specific crack front definitions. Property options include (default), , , and . When you select the option, an additional property displays: Crack Front Number. The default value for this property is . You can use it to specify a desired value using the entry tool that displays when you select the field.
Tip: To achieve coordinate system alignment to the face normal, create your coordinate system as described in Creating a Coordinate System Based on a Surface Normal and assign the created coordinate system to the Semi-Elliptical Crack object. You can also set the Align with Face Normal property to to generate a crack mesh whose coordinate system is aligned to the face normal and that is reflected by the SECrack Coordinate System object that is added under the Semi-Elliptical Crack object. Otherwise, the Semi-Elliptical Crack object's coordinate system can be at an inclination to the face normal direction. For the Pre-Meshed Crack and Arbitrary Crack objects, the origin of the coordinate system must be located on the open side of the crack.
Note: The graphical view of the semi-elliptical crack may differ from the mesh generated. Possible reasons include:
A crack definition unsuitable for valid mesh creation may result in some layers being "peeled off" to create a valid mesh.
The crack contour may be shrunk to fit into the mesh domain.
The crack coordinate system may be changed to align it to surface normal.
The center of the crack may be changed to create the crack on the surface.
The crack is meshed with gradation from the contour center to the outside results in difficulty distributing the crack mesh.
The offset of the crack is not suitable for the crack contour, resulting in a contour that must be reduced to ensure all element contours fit into the template.