PSI - Issue 21

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

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Fig. Configuration of three-sided slit Charpy specimen

2.1. Test Steel Two kinds of steel plates were prepared as test steels. The first is a 590 MPa class TMCP steel with a plate thickness of 40 mm, which possesses considerable texture (crystal orientation bias). The second is the same base steel plate as first one but it has been heat-treated to eliminate the texture. The heat treatment conditions are described in detail in Table 2.

Table 2 Heat treatment conditions of the test steel #2 (mark NQT)

Peak temperature [℃]

Holding time [min.]

Process

Cooling process

Normalizing

1000

90

Air cooling

Quenching

900

90

Water cooling

Tempering

620

90

Air cooling

2.2. Experiment and result Three-sided slit Charpy test specimens were collected from the two types of steel plates described above. All specimens were cut from 1/4 t position of the plate. From each steel type, test specimens in which cracks develop in the plate thickness direction (marked : “L - Surface notch”) and test specimens in which cracks develop in the plate width direction (Figure 2) were prepared (marked : “L - Cross section notch”). The results of the experiment are shown in Fig. 3. In order to purely discuss the propagation and arrest of brittle crack, this time the entire brittle fracture temperature range is especially focussed on as ductile crack propagation does not generate in this range. It was confirmed that in both materials, the absorbed energy in L-Surface notch tends to be larger than that in L-Cross section notch , and the results are consistent with the experimental facts of the large-scale test simulating real container ship described in previous chapter. In the figure, “Surface” represents a specimen with L-Surface notch , and “Cross” represents a specimen with L-Cross section notch . This test result agrees with the previous test result where EB welded ESSO tests were carried out using TMCP steel (H. Hiramatsu, 2007). In addition, the crack propagation plane was subjected to EBSD analysis, and the crystal orientation orientations were investigated. IPF map and Grain map analysed by OIM software version 6 (TSL, 2016). Results are shown in Fig. 4. It is observed that flattened Grains are arranged in the rolling direction of TMCP, and it is confirmed that NQT has an equiaxed crystal arrangement.