PSI - Issue 18

Prokhorov A. et al. / Procedia Structural Integrity 18 (2019) 802–808 Author name / Structural Integrity Procedia 00 (2019) 000–000

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5. Structural analysis of fracture surfaces The fracture surface of two tested specimens were investigated using electron-scanning microscopy. In both samples, the foci of nucleation of fatigue cracks is located under the surface of the sample and has the appearance of a “fish eye” with a large inclusion in its center, which initiated the nucleation and the onset of propagation of the fatigue crack. In a sample tested under normal conditions, the foci of nucleation of a fatigue crack concentrates around a large inclusion (up to 100 μm) (Fig. 7b) located under the sample surface, from which several deep secondary cracks run through the entire zone of slow crack growth (area 1). According to the results of micro X-ray analysis, it was determined that the inclusion contains 51% Al, 43% O and 4% Ti, which makes it possible to attribute it to aluminum oxide and titanium. Area 1 in diameter is ~ 730 microns and has a virtually structureless macro relief (Fig. 7c). However, at the micro level in this zone, finely dispersed elements of viscous fracture are observed similar to small holes. (a) (b) (c) (d)

area 2

area 3

area 1

Fig. 7. (a)Fracture surface general view. (b) Inclusion in initiation area. (c) Surface relief in area 2. (d) Surface relief in area 3.

The macro relief of the area of accelerated crack growth (area 2) consists of viscous and brittle fracture elements, which can also be traced on the micro relief of this area with greater magnification in combination with more pronounced plastic fatigue grooves (Fig. 7d). In a sample tested under the influence of an external alternating magnetic field, a similar scenario of the development of destruction is observed (Fig. 8a). The source of nucleation and growth of fatigue cracks also served as an agglomerate of inclusions up to 100 µm in size, which, according to the results of micro X-ray analysis, is an accumulation of aluminum oxide and titanium (up to 40% O, 56% Al, 60% Ti) (Fig. 8b). (a) (b)

area 2

area 3

area 1

Fig. 8. (a)Fracture surface general view. (b) Initiation area.

A distinctive feature of this sample from samples tested under normal conditions is significantly (2-2.5 times) smaller area 1, area of slow crack growth (350 μm), with equal inclusion sizes and approximately equal values of test