Panasenko O. Peculiarities of fracture of carbon and heat resistant steels at the crack initiation and propagation in ductile-to-brittle transition zone

The main goal of this work is to obtain new data and establish relationships between the parameters of the fracture surfaces structure of different specimen types and physico-mechanical characteristics (velocity and specific energy) of crack initiation and propagation in structural and heat-resistant steels in the ductile-to-brittle transition zone. The object of research is the process of crack nucleation and propagation in different specimen types under various types of thermo-force loading. For this reason, modern experimental and numerical research methods are used in the work. Experimental and numerical investigation methods of the crack initiation, propagation and arrest under different types of thermal force loading were used in this work. Experimental methods for investigation of crack initiation and propagation in disc-shaped specimens under thermal shock and in Charpy specimens, sub-sized specimens and side-grooved specimens under impact loading were developed. Impact tests were carried out on an instrumented drop-weight vertical impact testing machine equipped with a high-speed digital recording system of deformations and forces. Techniques and software for metallographic studies using methods of quantitative analysis of digital bitmap images and their segmentation algorithm have been developed, which is a special case of a group of cluster analysis methods. Metallographic tests were carried out using a Carl Zeiss AxiotechVario direct-type optical microscope equipped with Carl Zeiss Epiplan lenses and a Canon PowerShot G9 camera. Using methods of quantitative metallography and fractography (using the image segmentation algorithm), the lengths of brittle crack jump on the surface of a disk-shaped specimen and the relative parts of the separation crests area on fracture fractograms along the path of crack propagation were determined. Methods for determining the moment of crack initiation and the instantaneous velocity of its propagation using special fracture gages were developed. New data of variation of instantaneous crack propagation velocity in Charpy specimens from steel 45 and disc-shaped specimens from steel 20, steel 45 and steel 15Х2NMFA were fulfilled. The influence of the share lips formation in the standard Charpy specimens on the determination of the crack initiation moment was confirmed, in contrast to the side-grooved specimens. Based on the results of numerical calculations using the model of material damageability, an analysis of the processes of crack nucleation and propagation was carried out, which showed that the results of experimental and numerical modeling are in good agreement and the share lips formation affects the moment of crack initiation. The results of the studies and the developed methods have found their application during work on life-time assessment and searching of additional reserves of strength of the 1st circuit equipment elements of the Rivne and South Ukrainian NPPs.