Detecting Fatigue Crack Closure and Crack Growth Delays After an Overload Using DIC Measurements

The closure and crack growth behaviors of a fatigue cracked DC(T) 4340 steel specimen before and after a single 100% overload on the stress intensity factor (SIF) peak, applied over an otherwise mode I loading with quasi-constant SIF range and load ratio, was studied using Digital Image Correlation (DIC) and strain gage techniques. A significant retardation in the post-overload fatigue crack growth rate was observed, and it recovered its pre-overload value only after the crack grew a distance much larger than the Irwin plastic zone induced by the overload. This behavior is attributed to discontinuous closure, since high values of crack opening loads were observed even outside the overload plastic zone. Full-field displacements and strains determined ahead of the crack tip and crack flank opening displacements measured at points along the crack faces from DIC analysis, as well as redundant back-face strain-gage measurements, support this claim.