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Crack Closure Calculations

When a mesh in an FEA calculation has an explicit crack, this command will use crack closure methods to calculate mode I and mode II energy release rates. Select the command and a sheet will open on the results window. The calculation proceeds as follows:

Crack Tip Node
Enter the node at the crack tip. You can plot the mesh and move the mouse over the node to find the node number. Alternatively, you can use the mesh preferences to show node numbers and zoom in on the crack tip location in a mesh plot.
Crack Surface Tractions
The crack closure calculations allow inclusion of some constant crack surface traction normal or tangential (shear) to the crack surface. These tractions are normally applied with constant stress boundary conditions, but the crack closure calculation does not look of the applied stress for you. Enter any constant tractions added to the crack surfaces in these fields.
Crack Closure Calculation
To do the crack close calculation, click the "Calculate" button. The calculation will appear in the text field. If the node does not appear to be a crack-tip node, an error message will appear instead. You can copy and paste (or drag and drop) the text to any text-editing application.
Exit Crack Closure Mode
If there are multiple cracks, you can enter a series of crack tip nodes and click the "Calculate" button for each one. When all calculations are finished, click "Done" to close the sheet.

Notes

  1. Two types of explicit cracks are supported. The first is an internal crack as illustrated on the right. Enter the crack tip node. The elements behind the node must share an edge. The elements ahead of the crack must have separate edges. All element edges in the crack plane must be collinear and of equal length (Δa). Other edges may have any length and be at any orientation. Finally, any number of elements might share the crack tip nod, although four of elements must have crack plane edges.
        A symmetry crack will only have half of the crack tip elements and should appear as illustrated on the left. Here the element behind the edge must be fixed for zero displacement in the direction normal to the crack plane. The crack closure calculation will not check for the zero-displacement conditions; it is your responsibility to set them up and pick the correct node. Symmetry cracks are only appropriate for symmetric or mode I loading.
  2. If you pick a node that cannot be a crack tip node (i.e., does not conform to either of the above two geometries), the crack closure calculation will fail and an error message will be printed. A bad node will always be recognized for internal cracks. For symmetry cracks, the calculation will proceed for any edge node (with collinear and equal length elements). The calculation will only be meaningful if the boundary conditions are correctly set up as illustrated above.
  3. The crack closure calculations follow literature methods for common elements, but have a new method for quarter-point elements. The details are in Nairn, 2007c.