Digital image correlation (DIC) techniques are used to obtain the stress intensity factor (SIF) via experimental J-integral evaluations in an AISI 4340 steel disk-shaped compact-tension (DC(T)) specimen subjected to mode I loading conditions. The combined analytical and experimental methodology is based on the computation of the J-integral over an arbitrary but elastic contour path enclosing the crack tip. The DIC displacement field around the crack is used to directly determine the displacement gradients, the stress components, and the total strain energy for points along the selected path employing stress-strain linear elastic relations. The proposed analytical-experimental hybrid approach is then applied to investigate the influence of crack closure on SIF measurements under constant amplitude loading conditions, after and before the application of overloads. The effects of the closure-induced nonlinearities observed in the experimental results are analyzed and discussed.