PSI - Issue 39

Andrey Shanyavskiy et al. / Procedia Structural Integrity 39 (2022) 327–332 Author name / Structural Integrity Procedia 00 (2019) 000–000

330

4

It was established using scanning electron microscopy that along the total surface of the slot for a blade the cut of disk material was found with a depth of 0.1 mm. From this cut, the multiple origins of fatigue crack are formed (Fig. 4a). It is worth emphasizing that the length of the slot surface along which the origins were observed was established to be equal to 110 mm. The fatigue crack path was performed by the mechanism of crack meso-tunneling with facetted pattern relief and fatigue striation formation on the fracture surface (Fig. 4b). Such a mechanism of crack propagation is typical for the fatigue fracture of forged titanium alloys due to the creation of filaments with different abilities to sustain the plastic deformation as it was described and discussed in Shanyavskiy et al. (1998), Shanyavskiy and Losev (1999), Pilchak and Williams (2010), Kocanda et al. (2001), Pilchak (2013), Shanyavskiy et al. (2021). The main regularities of the crack meso-tunneling mechanism are expressed in the following. The crack propagation is realized by two alternating steps. At the first step, the fracture on the quasi-brittle filaments of titanium alloy structure created at forging takes place with the formation of meso-tunnels and facetted pattern relief in them. At the second step, the connection of meso-tunnels along the bridge between them leads to the formation of local areas with groups of fatigue striations on the fracture surface. Then the steps alternate during subsequent cyclic loading.

b

a

Fig. 4. (a) Multiple origins of fatigue crack; (b) block of fatigue striations at the distance of 0.94 mm from the surface of the slot for blade. The direction of crack growth is indicated by the arrow.

The fatigue striation spacing δ was measured for different values of crack length in a depth direction along the three different sections (Fig. 5a). Section 1 corresponds to the maximum crack length of 48 mm. Sections 2 and 3 are parallel to section 1 and located at a distance of 35 mm from section 1. Based on the performed spacing measurements the number of cycles N p was calculated depending on the crack length a in a depth direction (Fig. 5b).

a

b

Fig. 5. Dependences of (a) fatigue striation spacing δ and (b) the number of cycles N p on the crack length a in a depth direction for 3 sections of the main fracture surface.