The DIC technique was used to determine displacement and strain fields surrounding the tip of a fatigue crack in DC(T) (disk compact tension) specimen of normalized AISI 4340 steel, before and after an 120% overload was applied over otherwise fixed loading conditions in mode I ΔK, =, 31, MP√m and R, =, Kmin/Kmax, =, 0.1. The specimen has thickness of 4.6, mm, crack length to thickness ratio of 1.3 and width of 55.55, mm. From the DIC analysis, important parameters could be extracted, such as stress intensity factors, crack opening displacements, J-integrals, crack tip blunting, crack opening loads, and plastic zone sizes. These parameters were used to investigate the crack closure behavior before and after the application of the overload event, which produced a significant retardation in the subsequent crack growth rate. The role of the non-linearities observed in the experimental results during the crack closure phase, which are most important to correctly measure the above mentioned parameters, are analyzed and discussed.