If the geometry of the crack region defines a sharp crack, the strain field becomes singular at the crack tip, as described in “Constructing a fracture mechanics mesh for small-strain analysis” in “Contour integral evaluation,” Section 11.4.2 of the ABAQUS Analysis User's Manual. Including the singularity in your model for a small-strain analysis improves the accuracy of the contour integral and the stress and strain calculations.
To control the singularity at the crack tip:
Display the Crack editor using one of the following methods:
To configure a new contour integral, follow the instructions in “Defining data for contour integrals,” Section 20.1.11.
To edit an existing contour integral, select Special
CrackEditcrack name from the main menu bar in the Interaction module.
To include the strain singularity in your contour integral estimates, click the Singularity tab in the Crack editor, and do one of the following:
To create a 
strain singularity for an elastic fracture mechanics application:
From the Second-order Mesh Options field, enter a value of 0.25 for the Midside node parameter to move the midside nodes to the 
points.
From the Degenerate Element Control at Crack Tip/Line field, choose Collapsed element side, single node.
To create a 
strain singularity for a perfect plasticity fracture mechanics application:
From the Second-order Mesh Options field, enter a value of 0.5 for the Midside node parameter to keep the midside nodes at the midside points.
From the Degenerate element control at Crack Tip/Line field, choose Collapsed element side, duplicate nodes.
To create a combined and strain singularity for a power law hardening material:
From the Second-order Mesh Options field, enter a value of 0.25 for the Midside node parameter.
From the Degenerate element control at Crack Tip/Line field, choose Collapsed element side, duplicate nodes.
Click OK to include the singularity in the contour integral estimation and to close the editor.
Note:
In most cases you must assign second-order triangle or wedge elements to the crack front region to include the singularity in your contour integral estimates. If you assign first-order elements, ABAQUS ignores the value of the midside node setting in the Singularity tabbed page. However, if you assign first-order elements and choose Collapsed element side, duplicate nodes from the Degenerate Element Control at Crack Tip/Line field, you will create a strain singularity.