PSI - Issue 33

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Toru Yagi et al. / Procedia Structural Integrity 33 (2021) 1225–1234 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4 Schematic illustration of pop-in and stress wave propagation in a three-point bending test specimen.

ASTM was the first to include a pop-in judgment method in the CTOD test method, which incorporated BS5762 established in the UK. As shown in Fig. 5, a point where the load drops by 5% from the load just before pop-in occurs was identified, and the line connecting this point and the elastic unloading point was extended to form the criterion line. It is thought that this line was easily accepted because the figure of 5% is the same as that used in the KIC test to determine stable ductile crack extension. According to Dawes (1991), the length of a pop-in crack is equivalent to about 2% of the initial crack length a 0 , and if the crack propagates beyond this length, the effect of stress wave reflection and the update of the crack tip position to the pop-in crack tip should not affect the fracture toughness evaluation. Considering Willoughby’s conclusion, that is

Fig. 5 95% secant line in ASEM E399

4%, this judgment condition, 2% is conservative. Authors guess this judgment condition was determined mainly by this stress wave reflection aspect but also the fact that the crack front unevenness problem for the fracture toughness evaluation is considered after the crack tip position is updated to the pop-in crack tip. In 1991, BS established more detailed judgment condition describing the change of compliance was specified in BS7448 part1. Here, almost same judgment condition as ASTM was adopted. In Japan, Arimochi and Isaka (1986) proposed another opinion in 1980’s. they conducted experiments and analytical investigations using the finite element method on the propagation of stress waves and showed that the main stress waves propagate almost at the perpendicular to the crack growth direction. Furthermore, the crack velocity is experimentally measured using a clever method, and it is found that the slowest crack velocity is 200 m/s in normal structural steel and its welds. The limit of arrival of the reflected wave is up to 48% of the ligament, and the pop-in crack length is very small, which is usually observed in three-point bending tests. In addition, from the comparison between the tensile type stress field, which is often observed in real structures, and the stress field in the bending case for mild and 500 MPa class steel and its welds, it is noted that the stress field at the crack tip of the bending specimen becomes more severe below a certain value of crack growth, and that the pop-in seen in the bending test The critical pop-in crack length to arrest as a pop-in crack is given as 20% ( δ pop = 0.05mm) and 33% ( δ pop = 0.10mm) based on a study of experimental data, which is quite large compared to the conditions proposed by 5% drop line (2% b 0 ) and Willoughby ’s study(4% b 0 ). Since 2002, ISO standards (ISO1235 and 15653) has been activated. In these standards, the 5% compliance change line which was specified in BS7448 part1 is continuously being used until now. However, significant concerns remain regarding the validity of this criterion, as there are no definitive comprehensive studies with a sufficient amount of experimental data.