PSI - Issue 28

V.V. Sudin et al. / Procedia Structural Integrity 28 (2020) 1637–1643 Sudin V.V./ Structural Integrity Procedia 00 (2019) 000–000

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the critical point of the cleavage formation and the distance between the notch root and the cleavage facets. This relationship is shown in figure 3. It should be noted that the nature of the correlation does not change significantly either during the transition between different microstructures of welded joints or during the transition to the base metal. Measuring the distance between the notch root and the brittle facets by scanning electron microscopy showed that the described relationship is also performed for very small deflections of the pendulum up to 0.5 mm, in which the depth of the initial ductile crack is 200-300 μm. The study of load-displacement curves has shown that in some cases, the formation of a cleavage fracture, accompanied by an instant drop in load, is followed by an extended area of final ductile fracture, accompanied by a large spend of energy (figure 4). During impact bending tests, this energy is taken into account in the resulting value of the fracture work along with the work performed before the brittle crack is formed. By measuring the energy spent before the brittle crack was formed, it was shown that for some specimens this energy is up to two times lower than the total energy of fracture. At the same time, there may be a situation in which the value of the total work of fracture will be greater than the value required for the acceptance of the material as serviceable according to the results of tests, while a significant part of the work performed is accounted for by the formation of a ductile final fracture, and the energy spent before the formation of a brittle crack is less than the required standards. The frequency of occurrence of the Charpy test result in which the specimen has a CVN value of more than 40 J while the energy spent before the cleavage is less than 40 J was estimated. This is the most likely outcome for specimens of welded joint No. 4, for which 24.6% of specimens with a brittle fracture site show this type of failure. At the same time, the metal of welds made by methods 2, 3 and 5 is also prone to this type of destruction, but with a lower probability. Only specimens of 17G1S steel show a tendency to early brittle destruction from the specimens of the base metal and the heat affected zone. The described type of fracture occurs due to the high energy spent on the specimen final fracture after the occurrence of a brittle crack. The largest share of the energy of the final fracture in the total energy of destruction has the metal of welded joints 3, 4 and steel 17G1S. 4. Discussion Analysis of load-displacement curves of specimens tested at temperatures above the brittle-ductile transition

Fig. 3. Correlation between pendulum displacement in moment of cleavage and depth of initial ductile crack.