PSI - Issue 59

Eugene Kondryakov et al. / Procedia Structural Integrity 59 (2024) 50–57 Eugene Kondryakov et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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predicting its life-time. The surveillance specimens (SS) program is developed to monitor the degradation level of the RPV metal during operation due to the influence of high temperatures and radiation. According to the SS testing results obtained by different methods, the fracture toughness temperature dependence is calculated. The life-time of the RPV is estimated by the shift of this curve relative to the initial state. The Master Curve method is the most accurate, but for a qualitative determination of the reference temperature T0 by this method, quite a large number of CT specimen tests are required Wallin et al. (1999). Before the start of exploitation of operating WWER-1000 type reactor plants, a rather small amount of SS was loaded into the active zone. Thus, the use of miniature compact CT-specimens that can be made from the destroyed parts of standard Charpy specimens can significantly increase the amount of experimental data and improve the reliability of the fracture toughness assessment of metal by direct method Das et al. (2023). The specimen size significantly affects the fracture toughness by changing the crack tip constraint and the stress state conditions, which leads to a change in the fracture behavior. Reducing the specimen size leads to an overestimation of the material's fracture toughness characteristics. Therefore, there are still many questions connected with the correct processing and interpretation of results, adjustment of experimental equipment, and test procedures. One of the problems is also the difficulty of realizing a plane strain state in compact CT specimens; therefore, specimens with side grooves have recently been used to obtain reliable results. However, there are still many questions about the correct processing and interpretation of the results, setting up the experimental equipment, and testing methods. The use of modern apparatus of experimental and calculation methods, which includes instrumented units equipped with high-precision registration systems, methods of quantitative fractographic analysis using the capabilities of modern computer and software technologies, and the use of numerical methods for modeling the crack propagation and arrest in structural elements with various failure mechanisms, allows carrying out complex studies and investigate the features of materials deformation and fracture with high accuracy, taking into account the influence of various factors Kravchuk et al. (2021).

Nomenclature σ c

critical principal stress; critical principal strain;

ε с σ b G c l cr V cr

yield stress; fracture energy; crack length; crack velocity;

LLD L norm

load line displacement;

normalized distance from crack tip.

2. Experimental Technique For fracture toughness tests, four types of compact specimens made of 15Kh2NMFA steel were used (Fig. 1a): 0.5T С ( Т ), 1T C(T), side-grooved CT-0.5T and miniature 0.16T C(T). The specimens were made from a forged piece sized 240x140x2000 mm. To carry out the tests, a special construction of fork grippers was developed. Special heat-resistant steels were used as the material for the grips and pins, which allowed us to avoid fatigue failures due to multi-cycle fatigue. The following experimental equipment was used for the research: a Carl Zeiss Axiotech Microscope for fatigue crack measurement, a Servo hydraulic machine BISS 25 kN for growing cracks and conducting fracture toughness tests, and a climatic chamber was developed for cooling the specimens. To measure fatigue cracks on specimens with side grooves, the grooves were polished. To grow sharp fatigue cracks on miniature 0.16T C(T) specimens, selection of special cyclic loading parameters was carried out. Fracture toughness tests of four types of specimens were conducted. Based on the test results, load diagrams were obtained (Fig. 1b, c, d) Determined fracture toughness K Jc were converted to K Jc 1T equivalent values, according to standard ASTM 1921-03 (2015).