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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where 1 refers to the simulated specimen and 2 refers to the reference (miniaturized) specimen, and r is the miniaturization ratio (or B 1 /B 2 ). Note that in the standard ASTM-E1921, the adjustment reads: 1 = + ( 2 − ) ( 1 2 ) 1/4 K min is usually accepted as 20 MPa.m 1/2 . For the sake of simplification, K min has not been considered in this work. This crack front adjustment has been validated for non-self-similar BB specimens and wide range of crack front lengths B (Rathbun H. J., 2006). According to the ASTM standard E1921, this correction factor is valid provided that the measured toughness K Jc is lower than K Jc, limit given by: , = √ ′ . 0 . where E is the Young’s modulus, ′ = (1− ) 2 , b 0 is the ligament length (b 0 = B in this case) σ YS is the yield strength and M lim is a constant. The generally accepted value for M lim is 30. In previously published works, some challenges have been made to the range of applicability of the ASTM correction factor. An important factor to consider is whether the determined temperature depends on M lim . As a constant determined by the user, if varying M lim has an effect on the calculated DBTT, then M lim must be increased until the effect disappears to avoid false results. Joyce and Tregoning (Joyce & Tregoning, 2005) and Odette et al. (Odette et al., 2004) found that the minimum value for M lim was 50-80 for CT specimens and 100-300 for BB specimens. This was later confirmed by Mueller (Mueller, 2009). While corrections to the limiting factor are useful, they also mean a further restriction of the applicability range of the ASTM correction factor by a factor of (30/M lim ) 1/2 . Since this range is already restricted by the aforementioned assumptions behind the correction factor, the range falls below typical stress intensity values of concern, which will be demonstrated by the results of this study. Due to this limited range of application, it becomes imperative to relax the assumptions in order to mitigate the underlying issues that prevent the response to a wider range of loading from being modeled accurately. This paper aims to address the issue of constraint loss in miniaturized specimens, which is not considered in the ASTM correction factor, by adding a second correction factor to compensate for the effect and gain a better understanding of the relationship between equivalent stress intensities in standard and miniaturized specimens over a wider stress intensity range. 2. Theory/Calculation 2.1 Approach For the purposes of this paper, we modeled brittle fracture with a local approach, which states that a cleavage fracture occurs when a critical stress σ * is exceeded in a critical volume V*. A summary of the fundamental model and mathematical/physical background for this approach has been published by Odette et al. (Odette et al., 2003). 2.2 Material The material chosen for the CT specimen the high-chromium reduced activation tempered martensitic steel Eurofer97 developed for fusion reactor applications (van der Schaaf B., 2003). The temperature was chosen as -100 o C, which is below the reference temperature T 0 of the master-curve (T 0 = -78 °C) of unirradiated Eurofer97 (Mueller P., 2009)(the DBTT can increase by hundreds of degrees after irradiation and defect accumulation(Spätig et al., 2009)).