When setting up a fracture analysis for 3-D structures, consider the following hints and recommendations:
The recommended element type for 3-D fracture models along the crack tip is the 20-node brick element, SOLID186.
A greater number of crack front nodes yields more accurate results.
If any component in the global coordinate system is not orthogonal to the crack surface, create one local coordinate system with one component normal to the crack surface. This action is necessary to define the crack coordinate system (crack plane normal).
For multiple cracks, use separate crack coordinate systems for each.
The interaction integral method gives accurate results because the contour integral is evaluated at points far away from the crack-tip. Disregard the first contour results, however, as they are less accurate than the other results due to the nearness of the crack tip.
For more information, see Fracture Analysis in the Mechanical User's Guide.