PSI - Issue 42

B.Aydin Baykal et al. / Procedia Structural Integrity 42 (2022) 1350–1360 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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conditions. As an example, the reactor pressure vessels of nuclear power plants (NPPs) can be as thick as 40 cm (16 in) thick and loading a 1:1 scale specimen with any significant deformation would be very difficult. For the purpose of providing convenient specimens for research purposes, a series of standardized geometries have been developed, the most popular of which are the compact tension (CT) specimen (see Figure 1) used in this study and the bend bar (BB) specimens. These specimens allow the scaling of displacement, load and volume proportional to appropriate powers of the width of the specimen, designated B (in inches).

Fig. 1: Side view of 0.18T-CT specimen geometry. Features of note are the sharp crack and the notches on the top and bottom of the specimen, designed for a clip extensometer to facilitate a strain-controlled experiment. Improving the understanding of the effects of miniaturization is desirable because of the wide range of applications, such as determining the effect of temperature on fracture toughness and the ductile-brittle transition temperature, or the effect of neutron embrittlement. For a high-value, safety-critical application such as irradiated structural materials in a nuclear reactor, it is crucial that the operating temperature is in the ductile (high toughness) regime. The state of the art for examining the temperature dependence of toughness is the master-curve method (ASTM E1921), which accounts for experimental scatter by defining a failure probability parameter and building separate K jc vs. T-T 0 curves for different failure probabilities based on experimental data ("ASTM E1921-21a Standard Test Method for Determinationof Reference Temperature, To, for Ferritic Steels in the Transition Range," 2021). The master curve method initially proposed and developed by Wallin (Wallin, 1993; Wallin, 1999), while widely accepted and utilized, has some important limitations. These include the requirements of strict testing and loading conditions, namely quasi-static loading of 1T-size specimen, mode I fracture, homogeneous material structure, deep crack with highly constrained crack tip, that are not always representative of potential loading conditions. There are also open issues like overestimation of fracture toughness in miniaturized specimens or effects of operational environment such as irradiation and mixed loading. One important issue that this paper aims to address is the overestimation of fracture toughness in miniaturized specimens. The effect of miniaturization on the stress intensity can be modeled in simplified form (without minimum stress intensity term) using a correction factor: 2 = . 1 The ASTM-E1921 standard proposes a crack front length (B) adjustment to deal with the effects of specimen size: = ( 1 2 ) 1/4 = 1 4