After you have defined the crack front, the next step in the procedure to configure the contour integral is to define the crack tip or crack line by selecting entities from the assembly. The modeling space of your assembly governs whether you need to define a crack tip or a crack line for a contour integral analysis.
Two-dimensional
If your assembly is two-dimensional, you must define the crack tip by selecting a vertex or a node. The crack tip is the point on the crack front where you define the crack extension direction, 
. In some cases the desired vertex or node will not exist, and you must create it by partitioning the crack front.
If you selected a vertex or a node to define the crack front, the same vertex or node defines the crack tip.
Three-dimensional
If your part is three-dimensional, you must define the crack line by selecting edges or element edges that form a continuous line. The crack line is a series of connected edges along the crack front where you define the crack extension direction, . In some cases the desired edges will not exist, and you must create them by partitioning the crack front.
If you selected edges or element edges to define the crack front, the same edges define the crack line.
Selected edges (from an ABAQUS/CAE native part or from an orphan mesh) must be connected, must connect one side of the crack front to the other, and must be included in the crack front.