Quasistatic and Dynamic Fracture of Pearlitic Steel

The processes of cleavage crack initiation and characteristic features of fracture surfaces in pearlitic hypo-eutectoid steel (the chemical composition is given in Table 1) have been investigated using Charpy impact specimens and CT specimens for fracture toughness KIc assessment. The results demonstrated that initial cracking in both dynamically and quasistatically loaded specimens was mainly shear cracking of pearlite, e.g. localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths (Fig. 1). Inclusion triggering cleavage exhibiting as facet where cleavage river lines extended from the particle in its centre was observed as a second fracture micro-mechanism. In CT specimens cleavage occurred by multi-initiation due to the sharp stress peak ahead of the crack tip throughout the whole specimen thickness. In Charpy specimens, the cleavage initiation has been preceded by the ductile tearing changing the stress distribution at the notch root. In consequence of the ductile crack tunneling, the single cleavage triggering site situated in the centerline of Charpy specimens was found.