Meshing procedure >
Initial crack definition
Initial crack definition
The initial crack is defined by replacing one or more elements by "crack-blocks" to build up one or more crack fronts in the model. Each crack-block models a section of crack front and the combination of multiple crack-blocks allows modelling of a complete crack. The replaced elements are identified by their element numbers and the orientation of the crack front section in each is defined by two node positions on each replaced element.
The initial crack size can be specified in three ways:
- using ratios along two edges in each replaced element
- using absolute sizes along two edges in each replaced element
- using a spline that defines a complete crack front
The first two methods are appropriate for simple meshes in which the elements are fairly regular. The user only has control over key points along the crack front section with intermediate points determined by the element shape functions.
The spline method is extremely powerful and gives total control over the entire crack front shape. Crack front nodes are placed onto the spline and the crack-blocks are mapped into the element space taking account of the crack definition.
The Zencrack input file contains a number of keywords to define the required input data. The initial crack front is specified using one *CRACK FRONT keyword for each distinct crack front in the model. For the example of an elliptic corner crack shown here the spline method is used. The input is:
*CRACK FRONT, INITIAL=POINTS, SPLINE=STARTER
s01_q103x4 826 1203 1204
s01_t88x5 824 1203 1255
0 2.451963E+00 9.902455E+00
0 2.500000E+00 1.000000E+01
0 2.451963E+00 1.009755E+01
0 2.309699E+00 1.019134E+01
0 2.078674E+00 1.027779E+01
0 1.767767E+00 1.035355E+01
0 1.388926E+00 1.041573E+01
0 9.567086E-01 1.046194E+01
0 4.877258E-01 1.049039E+01
0 0.000000E+00 1.050000E+01
0 -4.877258E-01 1.049039E+01
This input specifies that element 826 is replaced by crack-block s01_q103x4 with crack orientation using nodes 1203 and 1204. Element 824 is replaced by crack-block s01_t88x5 with crack orientation via nodes 1203 and 1255. The initial crack shape is cross-referenced to a spline named STARTER.
This screenshot shows an example of the process for defining an initial crack front in the Zencrack GUI. This is the simplest way to define the crack front as it allows creation of the keyword input data without the need to manually extract element and node numbers for the crack-blocks. Instead, the target elements and nodes are picked from the uncracked mesh.
For deep initial defects the Zencrack deep crack option can be used to separate pairs of standard elements that are on the crack faces. To use this option the user replaces some elements by crack-blocks and also defines element split pairs. Each element split pair contains two elements that straddle the crack face. Zencrack separates these elements.
Example of a deep crack.
This example also uses boundary shifting (see below).
Boundary conditions and loading
All merging of crack-blocks with one another and with the uncracked mesh is carried out automatically to create the new finite element mesh containing the required crack fronts. During this mesh creation, Zencrack updates information related to the crack-block region e.g.:
- Pressure loads on the outside of the crack-blocks
- Body loads such as centrifugal loading
- Applied boundary conditions
- Temperature distributions
The initial cracked mesh can be analysed as a one-off crack size or as the first step in a crack growth analysis.