PSI - Issue 42

Toru Yagi et al. / Procedia Structural Integrity 42 (2022) 702–713 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction 1.1. Background

Fracture toughness is a resistance to a failure of a material with a crack or a flaw. The evaluation method of fracture toughness was begun to be propose in the middle of 19 th century. The CTOD test is one of the universal fracture toughness tests which measures the opening displacement of the fatigue crack of a specimen. During the CTOD test a phenomenon which is called pop-in sometimes occurs. A pop-in is an arrested brittle crack manifests itself with a drop in load and increase in clip gauge displacement. When pop-in occurs during CTOD test, if that pop-in is judged as a fracture, the fracture toughness of the material is determined by the point of the pop-in which is the point A of the P-Vg curve shown in Fig. 1.2. If pop-in is judged as insignificant, the fracture toughness is determined by the point of a complete fracture which is the point B of the P-Vg curve shown in Fig. 1.2.

Fig. 1.1 Crack surface of CTOD specimen with a pop-in

Fig 1.2 P-Vg curve of specimen with pop-in occurrence

1.2. Previous Study The study done by Willoughby in 1989 describes the mechanism of a pop-in crack formation with the theory of the stress wave. In the standard three-point bending test, when a small crack suddenly originates at the tip of the fatigue crack, a stress wave which breaks down the static balance originates at the crack tip as shown in Fig. 1.3. The stress wave then reflects at the lower jig which is a fixed-end reflection. The reflected stress wave then approaches to the tip of the propagating crack and decreases the driving force of the crack. Therefore, a running crack which is small enough to be arrested before the reflected stress wave arrives will also arrest in real structure. The velocity of the stress wave is that of the longitudinal wave which is about 5000m/s. The crack velocity of the cleavage fracture is 500m/s –