PSI - Issue 68

A.F Perez et al. / Procedia Structural Integrity 68 (2025) 439–445

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A. F. Perez et al. / Structural Integrity Procedia 00 (2025) 000–000

Examination of fracture surfaces shows that when crack growth is limited, the extension of the crack is greater at the centre of the specimen. This phenomenon, where a crack extends faster at the centre than at the edges of a material, is called crack tunnelling (Calvet, 2024). Both blunting of the crack and ductile tearing were observed. At the edges of the crack fronts in all CT specimens, there is limited crack extension, with crack blunting being notably significant.

Fig. 2. Fracture surfaces of the three CT samples.

3.2. J-R curves The DCPD method was used for the three 600°C fracture tests. Each fracture toughness test was terminated once the plastic collapse load was attained. For each loading value, the stress intensity factor was calculated. In particular, the critical stress intensity factor # was calculated using $" (which stands for “limit crack load” and corresponds to the maximum value of the load during the test). The ASTM E1820-23a standard (2023) gives the expression for all the fracture parameters, including the J-integral value. Figure 3 shows the three J-R curves obtained following the three tests and Table 5 summarises all the parameters useful for characterising the toughness of a material. The three tests yielded values within the same order of magnitude, with an average fracture toughness value, !" , of 321 kPa·m. Table 5. 12 and 32 values – DCPD method. . / 0 42 [kN] 38.8 39.6 40.9 5 [MPa ×Ö m] 67.1 68.4 70.6 12 [kPa ×Ö m] 330 323 310 312 [MPa ×Ö m] 217.1 214.8 210.4

Fig. 3. J-R curves – DCPD method – 600°C.

3.3. DIC analysis A 2D DIC study was carried out during the last test. In the image analysis software, the deformation parameter along the X axis was observed. The DIC analysis confirms that deformations are greater around the crack propagation zone, as shown in Figure 4. However, it appears that the loading was not symmetrical during the test, since the zones are not symmetrical with respect to the crack axis.