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.