PSI - Issue 13

Andrzej Neimitz et al. / Procedia Structural Integrity 13 (2018) 285–291 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 3a. Distribution of η , L, ε pl_e ; specimen C04, material HW, temp. +20  C, last step of integration

Fig. 3b. Distribution of η , L, ε pl_e ; specimen C1, material HW, temp. +20  C, last step of integration

Fig. 4b. Specimen C04; material N, temp. –50  C, cleavage fracture surface

Fig. 4c. Specimen PN; material N, temp. –50  C, cleavage area shape and size; left side of specimen

Fig.4d. Cleavage surface; specimen PN; material N, temp. –50  C

Fig. 4a. Specimen C04; material N, temp. –50  C, ductile zone inside

Evolution of the cleavage failure mechanism and the analysis of this process in terms of mechanical field parameters is not unique, at least authors are not able, at the moment, propose a unique not controversial hypothesis. It strongly depends on a history of plastic deformation evolution and evolution of voids' growth. According to accepted so far hypotheses [1, 2], this process is controlled by normal stress tensor component and a scale parameter (critical length or area). In the case of the N material and –50  C, two specimens failed by cleavage after intensive evolution of voids. They were PN specimen and C04 specimen. The specimen C1 failed according to ductile failure mechanism. The normal to the fracture surface stress tensor components are shown for all four specimen geometries C04, C1, PN, PR in Fig. 5. Since the specimen C1 failed due to the ductile mechanism the normal stresses within this specimen should be lower than in the C04 and PN specimens. It happens in between two vertical lines 1 and 2 in Fig. 5. Along the abscissa the normalized distance from the specimen centre is measured. Thus, the critical stress should be sought in between vertical lines 1 and 2. The observations of the cleavage surface in PN specimens suggest that the cleavage started at the distance about 6.6 mm from the specimen centre. If this estimation is correct the critical stress is about 1240 MPa and the origin of cleavage is at the distance 1.29 mm from the centre of C04 specimen. The values of other parameters at the critical moment for two specimens which failed according to cleavage mechanisms are shown in Fig. 6. Observing evolution of mechanical field parameters with external loading some questions arise: 1) Why caverns are not observed on a cleavage fracture surfaces? Are voids not nucleated in the domain where cleavage fracture is nucleated at the last step of loading? 2) Why cleavage is not initiated in the domain where ductile failure is observed, even though the opening stress level is higher than the presumed critical stress. It is easier to give a qualitative answer for the second question than to the first one. The cleavage can happen when the opening stress is greater than the critical value. It was estimated at the level of 1240 MPa. In the case of PN specimen such a level of the opening stress may happen at 13/17 or 14/17 step of loading (Fig. 7 and 8) at the centre of the specimen. In the C04 specimen it may happen at 11/15 step of loading also in the centre of the specimen. However, in both specimens the