PSI - Issue 21

Taiko Aikawa et al. / Procedia Structural Integrity 21 (2019) 173–184 Taiko Aikawa/ Structural Integrity Procedia 00 (2019) 000 – 000

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thickness increases in principle of strength of materials. From the viewpoint of safety design, it is required to have propagation arresting characteristics against brittle fracture. As shown in Fig. 1, two scenarios, which are established from the discussion in past ten years, are considered for arresting a brittle crack propagation in upper member of container ships. Experimental evidence dictates that scenario 1 is more prone to crack arrest than scenario 2, and the required K ca is higher for scenario 2. This finding was proposed in the rules of the International Classification Society (IACS), as shown in Table 1, and is now being discussed. However, the cause of the characteristic differences due to this scenario is unknown (Matsumoto et al., 2018). The steel plates used in important members of container ships are now mostly produced by a process known as Thermo Mechanical Control Process (TMCP). TMCP is a method of producing fine crystal grains by a combination of rolling and cooling after rolling, and it is known that the orientation of crystal grains is nonuniform in the thickness direction and the longitudinal direction of the plate (Kozasu, 1997). It has been reported that the distribution of crystal orientations affects the arrest of the brittle crack propagation, and the energy required for the development of brittle cracks depends on the difference in grain boundary orientation (Nakanishi et al., 2018). In this study, based on this knowledge, three-sided slit Charpy specimens with different crack propagation directions were prepared and tested. Then, EBSD analysis was carried out on the test piece in order to quantify the characteristics of crystal orientations in both directions, and finally the relationship between the orientation data and the toughness test results was examined. In addition, the drop-weight tear test (DWTT) which is a representative test method for brittle crack arrest were carried out in order to complement the easier three sided slit Charpy test.

Fig. 1 Brittle crack propagation stop scenario in container ship

Table 1 Fracture toughness requirement by container ship fracture scenario (IACS, 2015)

Required K ca at -10 ℃ (N/mm 3/2 ) Thickness, t (mm) t ≤ 80 80 ≺ t ≤ 100

Applicable Structural number

Scenario

#1 #2

Upper deck

6000 6000

6000 8000

Hatch side coaming

2. Three-sided slit Charpy test and analysis focusing on crystal orientation The three-sided slit Charpy test requires less energy for crack initiation due to the sharpened slit than the conventional V-notch Charpy test. This is the so-called shear-lip, generated in top and bottom surface. Therefore, this specimen can focuses more attention on crack propagation behaviour (Deguchi et al., 1990). In order to discuss the propagation and arrest of brittle cracks in the thickness direction and the width direction of the steel plate, the three sided slit Charpy test was selected.