PSI - Issue 13

Kejin Zhang et al. / Procedia Structural Integrity 13 (2018) 1047–1052 Kejin ZHANG / Structural Integrity Procedia 00 (2018) 000 – 000

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(a)

(b)

(c)

Shear lip

Shear lip

Shear lip

Square failure (stable ductile)

Square failure (stable ductile)

Square fracture (quasi-cleavage)

stretch zone

shear crack

Shear crack

Fig. 6. Schematic illustrations of the failure surfaces of (a) honed specimens; (b) PH6 Punched specimen; (c) PH8 Punched specimen.

As shown in Fig. 6 (b), in the failure surfaces of PH6 punched specimens, an inclined failure area along the hole corresponding to the SAZ (henceforth, “shear crack”) existed. The failure area is formed by initiation, growth, and coalescence of voids. As mentioned in subsection 3.1, a strain localization is likely to occur in the processed specimens under tension [Hosokawa et al. (2013)]. It is thought that the stress concentration generates strain localization in the SAZ due to the hole, and the shear band initiates in the direction inclined from the tensile axis, a mechanism similar to the shear-lip formed in the slant failure area. Since the coalescence of voids forms this slant failure area, it is thought that the stress concentration at the edge of the slant failure area is not so much like at the edge of a crack, so after the slant failure area formed, the following failure progress of the material did not show the same behavior as a material with a crack. The following failure process is thought to be similar to the honed specimen shown in Fig. 6 (a). As shown in Fig. 6 (c), the failure surface of the PH8 punched specimen is composed of a slant fracture area along the holes, square fracture area, and final slant failure (shear-lip) area. The slanted fracture area along the hole corresponding to the SAZ is thought to be generated in the shear band caused by the localization of the strain, and the fracture surface shows a quasi-cleavage, then exhibits as brittle failure. This fracture surface is the same as the one obtained by the fracture toughness test of a pre-cracked specimen exhibiting brittle fracture [Kaufman et al. (1970)]. 4.2. Characteristics of the crack in the SAZ From the engineering stress-stroke diagram shown in Fig. 3, the brittle fracture of the PH8 punched specimen seems to be due to the shear crack initiated in the SAZ. Since plastic strain localizes easily for PH8 steel, a crack shape initiated in the PH8 SAZ is sharper than in a PH6 specimen. Therefore, the engineering stress-stroke curve and the failure surface are considered the same as a specimen with a pre-crack and resulting in a brittle fracture. Moreover, even after cracks initiated in the SAZ, plastic strain in PH8 steel is localized, and plastic deformation occurs in a narrow range, thus the brittle fracture is expected to occur. It is conceivable that shear cracks initiated in the SAZ could lead to failure. It can be confirmed in Fig. 5 that under tensile loading, the cracks of PH8 punched specimens propagate and finally become a brittle fracture. Additionally, we observed shear cracks near the hole in the SAZ of both PH6 and PH8 punched specimens, and failure surfaces in all specimens other than the PH8 punched specimen showed traces of ductile failure. Therefore, for the punched specimens, we conclude that shear cracks initiate via the same mechanism as shear-lips, and the SAZ therefore promotes the initiation of the shear cracks. 4.3. Stretch zone at the shear crack tip and influence on failure As shown in Fig. 5, in the PH6 punched specimen that exhibited a ductile failure, a notable stretch zone was formed between the shear crack area and the square failure area whereas there is no such stretch zone in the PH8 punched specimen, which exhibited a brittle fracture.