PSI - Issue 16

Eugene Kondryakov et al. / Procedia Structural Integrity 16 (2019) 43–50 Eugene Kondryakov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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The values of components of the total specific energy of crack propagation depend on the fracture mechanism; moreover, their values change in the variation of the fracture mechanisms from cleavage to ductile dimple fracture. The fractographic methods of investigation together with the analysis of the diagrams 'force vs displacement' make it possible to compare the values of energy in various segments of the crack propagation with the fracture mechanisms observed within the corresponding fracture zone. To determine the areas of the characteristic zones of the specimen fracture, a computer-aided method of quantitative fractographic was applied. The given procedure was implemented in the specially designed program for the processing and analysis of bitmap images for an accurate assessment of the percentage of areas of grains, phases, zones of the structure and surfaces of fracture. Fig. 5 illustrates the temperature dependences of the specific energy for three types of specimens and its components: energy of the ductile crack growth, energy of the brittle crack jump, and energy of the ductile rupture area that differ significantly. The specific energy of the crack propagation within the zone of the stable growth for the side-grooved specimens is considerably lower as compared with that one for Charpy specimens. The specific energy values of propagation of the unstable crack are practically the same for three types of the specimens, while the specific energy values of rupture area differ significantly affecting the general specific energy of the main crack propagation. The differences in the values of specific energy of deformation and fracture in standard Charpy specimens and side-grooved specimens can be explained by the formation of share lips in standard specimens. In compliance with the investigations in paper Kondryakov et al. (2015), the crack initiates in the centre section of the standard Charpy specimens with its subsequent propagation to the lateral surfaces. As is seen from images of macrofracture (Fig. 6) the shape of the ductile crack growth in standard Charpy specimens is nonuniform and has a large elongation in the central part, while in the side-grooved specimens the crack front is uniform. Thus, it is reasonable to use the side grooved specimens to eliminate the influence of share lips in the determination of different characteristics (energy of the crack propagation, moment of the crack initiation).

Fig. 5. Temperature dependencies of the total specific energy of fracture (а) of three types of specimens and its components: energy of the ductile crack growth (b), energy of the brittle crack jump (c) and energy of the ductile rupture area (d).