Stress shielding and fatigue crack growth resistance in ferritic–pearlitic steel

The effect of pearlite morphology on stage IIb (Paris regime) fatigue crack growth behavior in ferritic-pearlitic steel was investigated. Networked and distributed pearlite structures were prepared. Constant-(delta)K fatigue crack growth tests were performed in situ in a scanning electron microscope. The results revealed that a distributed pearlite structure had better fatigue crack growth resistance than a networked pearlite structure. From the in situ observations, the distributed pearlite structure indicated a tortuous crack path, which induced crack interlocking as well as crack closure. For the networked pearlite structure, some crack branching was found on the crack path. The crack growth curves for the two microstructures, plotted using the effective stress intensity factor range (delta)Keff, where crack closure behavior is taken into consideration, did not coincide. The crack growth curves plotted using the crack tip effective stress intensity factor range (delta)Keff,tip, where crack tip shielding behavior as well as crack closure are taken into consideration, successfully coincided on one line.