Brittle Fracture Initiation and Propagation-Arrest Characteristics in Ferritic-Pearlitic Steels

Abstract

The effects of the microstructure and alloying elements on the initiation and propagation-arrest properties of brittle fracture in ferritic-pearlitic steels have been investigated by means of COD (Crack Opening Displacement) and tapered DCB (Double Cantilever Beam) tests.
It was observed that in both cases the temperature at which brittle fracture was initiated or arrested at a given toughness was linearly related to the square root inverse of the mean pearlite colony diameter.
This implies that the pearlite colony which lay in the plastic zone in the vicinity of the fatigue crack tip or the propagating main crack tip was cleaved by the applied stress, and that the critical condition whether this pearlite crack cleaved the adjacent ferrite as a Griffith's flaw dominated the initiation or the propagation-arrest properties of the brittle fracture.
The difference between the result of the static COD and that of the tapered DCB test might mainly exist in the strain rate, the strain rate at the tip of the advancing cleavage crack for the DCB test being much larger than that of the fatigue crack tip for the COD test.
It was also confirmed that the addition of nickel greately improved the low temperature toughness for the tests with high strain rates.