PSI - Issue 13

3

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

287

a

b c Fig.2. Microstructures of HW material (a), N material (b) and NW material (c)

3. Failure mechanisms During the experimental tests several fracture mechanisms were observed, depending on the specimen shape, material and test temperature. The failure mechanism observed in almost all materials, specimens and test temperatures was due to voids nucleation–growth–coalescence. In some cases ductile fracture was not observed at very low temperatures (–80  C or –100  C; not shown in this paper). Voids were growing along surfaces perpendicular to the external loading and inclined to the external loading. The purpose of this stage of research was to determine the values of the critical effective plastic strain (ε pl_e ) as a function of computed parameters: η , L at the critical moment. It was assumed that the initiation of the rapid failure process due to the voids coalescence started at last or at next to last step of integration (loading). Rapid cleavage fracture was observed in some cases and it started at the last step of integration. No irregularities were observed along the force–elongation curves indicating the sudden jumps of cleavage fracture during the loading process. The locations of cleavage fracture initiation spot could be identified, to some extent, analyzing the fracture surface images and orientation of river pattern on the cleavage planes. The locations of the ductile fracture initial spot were usually not possible or not unique by observation of fracture surface by the scanning electron microscope. In the most cases, both within cylindrical and PN specimens, the differences between the sizes and shapes of caverns in a fracture surface were not noticeable. Thus, some working hypothesis had to be assumed to localize the critical spot. The origin of this hypothesis is Rice and Tracy [5] result concerning the rate of growth of the isolated spherical void surrounded by an ideally plastic material. Their numerical results were well approximated by the formula:     0 0 0 / 0.263 exp / 2 m R R       . Since the whole critical cross–section of loaded specimen is stretched at the same time, it is proposed to compare the quantity representing, in a very rough approximation, the extension of the voids’ radii, recorded along the fractured surface at the presumed moment of the rapid evolution of damage. The simplified formula is as follows:   _ exp pl e R      (1) The results concerning two cylindrical specimens with different radii of the circumferential notch are shown in Fig. 3 and Table 2. It was concluded from the results listed in Table 2 that the final ductile failure process started at the centre of the C1 specimen and next to the notch in the C04 specimen.

Table 2. Values of the mechanical field parameters at the critical moment ε pl_e_cr η L

σ

ε pl_e_cr ·exp(η)

Specimen centre

0.24

1.6

0.99 0.54

1443

1.19 1.53 1.36 0.86

R=0.4

Next to the notch 0.93

0.497

789

0.996

Specimen centre

0.36

1.33 0.42

1298

R=1.0

Next to the notch 0.57

0.78

619

With lower temperatures one expects failure mechanism to change from ductile to cleavage. However, it did not happen in all specimen tested. Specimens C1 and PR never failed by cleavage mechanism in the temperature range from +20  C to –50  C (they did at lower temperatures). The examples of cleavage surfaces are shown in Fig. 4.