Characterisation of A508 steel cleavage initiation sites and local plasticity using surface matching technique

Cleavage fracture in ferritic pressure vessel steels is preceded by local plastic deformation that initiates microcracks at carbides or second phase particles within the highly stressed region ahead of a crack tip. The objective of this study is to locate initiation sites of failed Compact Tension fracture mechanics specimens, tested at temperatures within the lower ductile to brittle transition region. A surface matching approach was developed to identify regions of local plastic deformation on the fracture surfaces of tested specimens, using confocal microscopy to acquire accurately mapped topographic images of both fractured surfaces, and then subtracting each pair of images in a virtual environment. This methodology is conceptually similar to the fracture surface topography analysis (FRASTA) technique but uses a home developed MATLAB software based on image correlation. The residual mismatch (interference) between the two datasets was used to identify the regions of local plasticity on both fracture surfaces of each test specimen studied that is believed to be associated with the cleavage initiation sites. The size and location of the localised plasticity was found to be consistent with scanning electron microscopy observations of cleavage initiation sites located directly ahead of the fatigue pre-crack tips in tested specimens. Finite element modelling was used to identify the magnitude of stress at the identified regions of cleavage initiation for the specimens studied, providing new insights into the mechanism of cleavage initiation in the lower transition region. This study suggests that this is a promising methodology for the identification of the initiation sites of cleavage fracture. Copyright © 2013 by ASME.