PSI - Issue 52

Ding Zhou et al. / Procedia Structural Integrity 52 (2024) 430–437 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

431

2

Surveillance programs were initially organized to monitor fracture toughness typically with the Charpy impact technique, which is an indirect measurement method (Terán et al. 2016). The Master Curve (MC) approach for assessing the fracture toughness, which was first proposed and developed by Wallin (Wallin 1993; Wallin 1999), has been gaining acceptance throughout the world since it is a direct toughness evaluation approach rather than a semi empirical one based on Charpy measurements (Rosinski and Server 2000; Joyce and Tregoning 2001; Odette et al. 2003). The MC procedure was standardized by the American Society for Testing and Materials in 1997 and has undergone several revisions up to the current version: ASTM E1921-22a (2022). One of the greatest challenge in assessing irradiation embrittlement is related to the fact that only relatively small specimens are used in surveillance specimen matrix. However, it is well known that brittle fracture in the ductile to brittle transition is strongly specimen size dependent. Thus, it is imperative to develop either models to account for this effect or to evaluate the reliability of small specimen in comparison with large ones. A great deal of studies have been carried out in the last decades to address the issue of specimen size effect on fracture. For example, Kasada et al. evaluated the fracture toughness of blanket structural materials with the Master-Curve method using ½-CT specimens (Kasada et al. 2006). Chaouadi et al. discussed the reliability of miniaturized CT specimens (thickness 4.2 mm) in comparison to large specimens (Chaouadi et al. 2016). Miura et al. also measured fracture toughness of two RPV steels with 4 mm-thick specimens (Miura and Soneda 2012). Mueller et al. reported fracture toughness data for the tempered martensitic steel Eurofer97 with two different sizes of specimens and pointed out that Master-Curve shape adjustments may have to be considered when analyzing the lower part of the transition region (Mueller et al. 2009). Currently, the European H2020 FRACTESUS project aims at validating subsized compact tension specimens to determine the reference temperature T 0 of the Master-Curve. Indeed, there is a strong interest to use such subsized specimens in reactor pressure vessel surveillance programs. FRACTESUS consortium proposes an innovative approach using subsized compact tension specimen that could be machined from a previously tested broken Charpy specimen. Typically eight to ten specimens could be extracted from a Charpy specimen (Cicero et al. 2020). In this study, undertaken in the frame of FRACTESUS, we estimated the reference temperature T 0 of the Japanese Reference Quality (JRQ) ferritic steel based on the Master-Curve approach with sub-sized compact tension specimens. In addition, we looked at the possibility to incorporate low temperature toughness data into the Master-Curve analysis, which under the current version of the ASTM-E1921 standard would be rejected. 2. Experiments 2.1. Material and specimen preparation The material investigated is the low-alloy reactor pressure vessel ferritic steel JRQ (Japanese Reference Quality) (2005). It is a A533B steel produced within the IAEA coordinated research project on the optimization of reactor pressure vessels and surveillance programs. The steel was produced by Kawasaki Steel Corporation. The chemical composition is given in Table 1. After rolling, the plates were heat treated by normalizing at 900 °C, quenching from 880 °C and tempering at 665 °C for 12 hours, then stress relieving at 620 °C for 40 hours.

Table 1. Chemical compositions of JRQ steel (mass%).

•

Composition

C

Si

Mn 1.35 1.43

P

S

Cu

Ni

Cr

Mo 0.49 0.51

V

Al

min max

0.16 0.20

0.24 0.26

0.017 0.019

0.003 0.004

0.13 0.14

0.85 0.82

0.12 0.12

0.002 0.003

0.012 0.014

Sub-sized compact tension (CT) specimens were designed for fracture tests having a thickness B of 4.5 mm, which was referred as to 0.18T according to the specifications of the ASTM-E1921 standard (2022). Note that 1T specimen is used to refer to fracture specimen having a thickness of 25.4 mm. The width W of the 0.18T CT specimens was equal to 2 B . The specimens were sampled from the inner half of the original block 5JRQ56. Fig. 1 shows the geometry of the CT specimen and the comparison between the standard (1T) with the sub-sized (0.18T) CT specimens, highlighting the drastic reduction of specimen volume. Specimens were fatigue pre-cracked until the crack length a was controlled within the range 0.45 < a/W < 0.55 according to the requirement of the ASTM-E1921 standard.