In-situ SEM study of fatigue crack growth behaviour in IN718

Fatigue crack growth experiments on Inconel 718 have been performed in a scanning electron microscope at room temperature. The investigated cracks were approximately 1-3 mm long and the samples used were thin 10 mm wide plates. The crack opening stress was found to vary significantly, even in the Paris region, for a constant stress ratio. Crack growth was predominantly found to be either growth on slip planes in a shearing mode, or linking after formation of microcracks ahead of the main crack, which do not require detectable crack opening. A rough crack surface indicates that at room temperature roughness induced crack closure is significant even in the midst of the Paris region, at higher stress intensity factor levels than anticipated. A post experimental examination of the fracture surfaces revealed that a striation forming mechanism was not active at short cracks, but as crack length increased, the occurrence of striations increased in the interior of the specimen. No striations were found in the grains at the specimen surface. As multiple crack growth mechanisms are active, fatigue crack rate predictions need to take all different mechanisms into account to improve accuracy.