PSI - Issue 33

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

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1.3. Objective Considering the history of the establishment of the pop-in criterion, which is explained in detail in the previous section, a more thorough investigation and experimental evidence of the mechanism of pop-in cracks is desirable. However, such an investigation would be accompanied by the following difficulties. • Difficulty to produce a test specimen that can stably generate pop-ins. • Difficulty to analyse the behaviour of a pop-in crack since a pop-in crack is a microscopic high-speed brittle crack which originates, propagates, and arrests within the plate. • Difference in the loading mode between bending test and the real structures. The objective of this study is to investigate of mechanism of formation of pop-in and improve the pop-in acceptance criteria with novel experimental method and numerical analysis which overcomes the difficulties above mentioned.

Nomenclature B

thickness of test specimens [mm] width of test specimens [mm] length of test specimens [mm]

W

L S

length of span in three-point bending test [mm] initial notch depth of test specimens [mm] initial ligament length ( W - a 0 ) [mm] curvature radius of test specimens [mm]

a 0 b 0

R

P load in fracture toughness test [kN] g clip gauge opening displacement [mm] ν Poisson’s ration YS yield stress [MPa] E young’s module [MPa] d dynamic stress intensity factor [MPa √m ] universal function of crack speed V crack velocity [m/s] 1 longitudinal wave speed [m/s] 2 shear wave speed [m/s] r Rayleigh wave velocity [m/s] closure side ligament closure effect [MPa √m ] LBZ width of local brittle zone [mm] d,c fracture toughness of material [MPa √m ] δ pop critical pop-in CTOD [mm] 2. Experiment 2.1. Material

To observe in detail the behaviour of pop-in, specially manufactured welded joints that can ensure the occurrence of pop-ins are prepared. NRL-S which is a brittle material usually applied for NRL test s welded on the centre of base metal, WELTEN780, and then covered by MGS-80 which is a tough material. The chemical composition of these materials is shown in Table 1. The NRL-S is used to represent the local brittle zone of the welded-joints. The local brittle zone (LBZ) is discrete microstructural regions in a weld heat affected zone that exhibit significantly lower resistance to fracture initiation than surrounding material. The joint geometry of this “brittle bead specimen”