Evolutionary Morphology of Short Fatigue Cracks of 1Cr18Ni9Ti Weld Metal

Evolutionary morphologies of the short crack propagation of 1Cr18Ni9Ti weld metal are investigated on specimen surface and in depth direction.To be agreement with the previously effective short fatigue crack（ESFC） criterion,attention is paid to the formation zone of the dominant ESFC（DESFC）,which results in the specimen failure,in the micro-structural short crack（MSC） regime and then the tip zone（s） ahead of the DESFC in the physical short crack（PSC） regime.Results show that the ESFCs imitated from the bounds between the ferrites and the austenite matrix.The ferrites are distributed randomly in an angle range less than 45 degree to the cross-section perpendicular to the loading axle.The initiated ESFCs on surface propagate to be inclined to the direction perpendicular to the loading axle due to the stronger microstructural constraints.In the depth direction,the initiated ESFCs grew perpendicular to the loading axle first and then to be inclined to the direction perpendicular to the material columnar grain structure formation.When the sizes of some longer ESFCs,with closer tips ahead the growth paths and similar directions,get the material maximum barrier size,coalescence may occur to form at a true DESFC.In the PSC regime the formed DESFC grows to be more and more inclined to the direction perpendicular to the loading axle on the surface.But in the depth direction,it grows to the direction perpendicular to the formation direction of the material columnar grain structure first and then to be inclined to the direction with 45 degree angle to loading axle.Finally,the specimens are fractured mostly in direction perpendicular to the loading axle.It revealed that there is significant difference between the evolutionary morphologies of long and short fatigue cracks.