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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Therefore, we found that, the PH steels – and especially the PH8 steel – show plastic strain localization. We anticipate that this strain localization is caused by the PH steel characteristic which shows almost no work hardening occurrence. The stress-strain curve of PH steels for plane specimens are shown elsewhere [Hamada et al. (2018b)].

(a)

(b)

Fig. 4. Punched specimen surfaces of (a) PH6; (b) PH8 at the stroke of 3.85mm. [Hamada et al. (2018b)]

3.2. Failure surface observation Figure 5 shows SEM pictures of the tensile fracture surfaces near the hole of the honed and punched specimens, whereas Fig. 6 shows the schematics of the whole fracture surface. We can divide the failure surface of the PH6 and PH8 honed specimens into a stable ductile square failure area and shear-lip area; the PH6 punched specimen into a slant failure area, stable ductile square failure area, and shear-lip area; and the PH8 punched specimen into an unstable quasi-cleavage square fracture area and shear-lip area.

RD

RD

TD

TD

PD

PD

hole

1 mm

hole

1 mm

1 mm

hole

a) Honed (example: PH6) (c) PH8 punched Fig. 5. SEM micrographs near holes of specimens after tensile fracture. PD: punched direction. [Hamada et al. (2018b)] (b) PH6 punched

4. Discussion 4.1. Fractography analysis

As shown in Fig. 6 (a), the overall appearance of the failure surface of honed specimens is similar to the failure surface obtained by fracture toughness testing for a pre-cracked specimen that showed ductile failure [Kaufman et al. (1970)]. In other words, initiation, growth, and coalescence of voids, which initiate from inclusions in triaxial stress conditions at the hole edge which has the same effect as a pre-crack tip, take the shape of a stable ductile failure area. The failure surface formed at this stage is the square failure area. After that, the stress concentrates at the end of the stable ductile failure area and the shear band [McClintock et al. (1966)] is formed. In the shear band, the plastic deformation is localized in the direction in which the slip is inclined from the loading axis, and the failure is called a void sheet [Rogers et al. (1960)] due to the initiation, growth, and coalescence of voids in the shear band. The failure surface which is formed in the process is a slant failure (shear-lip) area.