Fatigue crack growth analysis in porous ductile cast iron microstructure

Fatigue failures in ductile cast iron components usually initiate from defects or porosities, where the stress can locally exceed the notch fatigue limit. Since porosities are practically impossible to eliminate from the casting process, it is essential to define a limit or admissible size under which the component is designed in safety. This admissible size will depend not only by the volume of the porosity itself, but also by its shape and orientation with respect to the loading direction. This work reports the study of the fatigue strength of a ferritic porous ductile iron in the HCF regime. The fatigue strength is experimentally determined via fatigue tests on specimens machined from huge casted parts. The specimens are designed in order to maximise the volume tested, and the surface-to-volume ratio. Fracture surfaces deriving from fatigue failures are examined, looking for initiation sites and related features. The fatigue crack propagation is then simulated using the advanced finite element Virtual Crack Closure Technique. In this way, a set of featuring microstructural parameters are identified and associated with the conditions of propagation or non-propagation of a crack.