When setting up a fracture analysis for 3D structures, consider the following hints and recommendations:
The recommended element type for 3D fracture models along the crack tip is the 20-node brick element SOLID186.
A fine sweep mesh along the crack front yields more accurate results. Create a torus along the crack front in the model geometry for this purpose.
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 plane normal (CINT,NORM).
For multiple cracks, use unique crack tip IDs and separate sets of CINT commands for each.
To determine SIFS and J-integrals for the same crack, use two separate CINT command sets.
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 Understanding Fracture Mechanics in the Fracture Analysis Guide.