PSI - Issue 81

V. Sidyachenko et al. / Procedia Structural Integrity 81 (2026) 123–128

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fracture mechanics approaches makes it possible to account for both in-plane and out-of-plane constraint under biaxial loading in the presence of shallow cracks (Chen et al. (2022)). Therefore, the objective of the present study was to develop an experimental methodology for investigating the fracture toughness of reactor pressure vessel steels under biaxial loading using small-scale specimens within the ductile-to-brittle transition temperature range, as well as to further develop two-parameter approaches for the analysis of the obtained results. 2. Methodology 2.1. Experimental procedure Fracture toughness was determined using the following specimens: a CT-1 specimen (standard compact tension specimen with a relative crack length a/W = 0.5 ), SENB specimens of 10 × 18 mm (single -edge notched bend specimens subjected to three-point bending, a/W = 0.1…0.2 ), and CRSEN specimens (cruciform specimens subjected to five-point bending, a/W = 0.17…0.39 ) (Fig.1). The investigations were performed on model steel 15Kh2NMFAA with mechanical properties σ 0.2 = 462.08 MPa and σ u =715.75 MPa. The design of the CRSEN specimen (Fig. 1) was developed by analogy with the specimens investigated in Kim et al. (2016) and Hohe et al. (2011). Fracture toughness tests under biaxial bending were carried out under displacement-controlled loading of the central part of the specimen 2 by means of the loading rod 1 (Fig. 1). Using a Biss digital controller the total load applied to the specimen (P), the displacement of the active grip (LLD), and the crack mouth opening displacement measured by an extensometer (CTOD) 3 were recorded. To initiate a starter fatigue crack from the central notch, the cruciform specimen was subjected to cyclic three-point bending at a frequency of 15 Hz in accordance with the recommendations of ASTM E1820. The change in crack depth was monitored by tracking the variation in specimen compliance, following an approach similar to that described by Pokrovs’kyi et al. (2022). For this purpose, a relationship between the relative crack depth a/W= 0.1…0.25 of the CRSEN specimen, CTOD and load P was established on the basis of linear elastic finite element analysis (Fig. 2). Control of the fatigue crack depth a/W during crack initiation was performed using relation (1), based on CTOD data at a given load P, with subsequent refinement after final fracture of the specimen. a W ⁄ ( , ) ≈ + 5.205036⋅10 −4 +2.6458333⋅10 −2 1.849855⋅10 −2 +2.2916667⋅10 −1 (1)

Fig. 1. Loading scheme of the small-scale cruciform CRSEN specimen under five-point bending during static fracture-toughness testing:1 – loading punch; 2 – CRSEN specimen; 3 – extensometer for recording crack mouth opening displacement.

Fig. 2. Dependence of crack depth on load and CTOD.

The J-integral for the CRSEN specimen was calculated using relations (2), obtained numerically in accordance with the methodology proposed by Hohe et al. (2011), followed by conversion to K JC in compliance with ASTM E1921 – 17a: = + = 1− 2 ( 3⁄2 ( ⁄ )) 2 + ( − ) ( ⁄ ) , (2) where , – elastic and plastic parts of the J-integral respectively, ν – Poisson’s ratio, E – Young’s modulus, W , B – specimen dimensions according to Fig. 1, – crack depth, , – calibration functions: