PSI - Issue 5

A. Prokhorov et al. / Procedia Structural Integrity 5 (2017) 555–561 A. Prokhorov et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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does not have time to reach the surface of the specimen, therefore it can be measured only after the rapture of the sample.

Figure 5. The direct component of the measured signal.

4.3 Scanning electron microscopy

The structural analysis was carried out using scanning electron microscope Quanta 200. The analysis was directed to fracture surfaces of specimens which tested with air cooling and specimens which tested with water cooling.

a

b

Fig. 6. Fracture surface of specimen tested with air-cooling (a), the specific area (b).

The areas with specific square (fig. 6b) about 0,005 mm 2 were found on the fracture surfaces (fig. 6a). These areas characterize the brittle fracture on the grain volume. The emergence of these area could lead to temperature increasing during the test and change the specimen resonant frequency. The water cooling specimens had qualitative different character of fracture. Four types of areas were found on the fracture surfaces of specimens which tested with water cooling. There are three areas of fatigue fracture and rupture area (fig. 7a). All of detected areas have different fracture types. The specific dimensions of first area (fig. 7b) much bigger than the area detected on the specimens which tested with air cooling. This areas has the fundamentally various types of fracture. First area in figure 7b is characterized by fracture on the grain boundaries and evolution using disclination motion. Authors suggest that this area is zone of fatigue crack initiation. The structural analysis does not allow us to explain the source of fracture, but it is expected that this is inclusion. The second zone of fracture (fig. 7c) is zone of brittle fracture on the grain volume with partial fracture on the grain boundaries (red arrow). The third area is characterized by fatigue fracture on the grain volume.