Load Displacement curve for XFEM in ABAQUS

I am using Maxps Damage criteria for an XFEM crack for a three point bending case. I am using displacement loading on the specimen and wish to plot reaction load v/s mouth opening displacement. I would like to know how to obtain the crack tip opening displacement in ABAQUS. All the journal papers I read show a slump in reaction force after the crack starts to propagate. I plot the results and contour plots show me clearly that the crack has started to propagate. Please help.