PSI - Issue 23

A. Eremin et al. / Procedia Structural Integrity 23 (2019) 233–238 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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whereas the crack growth duration until fracture and total crack length achieved is much higher for all tested specimens of UFG Ti (approximately 2.5 mm for CG and 3 mm for UFG).

(a)

(b)

Fig. 4. Fatigue fracture kinetic diagrams (a) and change of local strains at the crack tip during crack propagation (b) for course-grain and ultrafine-grain titanium.

Fig. 4a demonstrates fatigue fracture kinetic diagrams; it is seen that when the crack is small the propagation rates are near the same, but when the crack reaches ~0.7 mm (SIF range Δ K ≈ 20 MPa √ m) it leads to the higher growth rate of the crack in CG titanium.

(a)

(b)

(c)

a ≈ 0.37 mm

a ≈ 1,9 mm

a ≈2 ,9 mm

(d)

(e)

(f)

a ≈2 ,37 mm

a ≈3 ,45 mm

a ≈ 0.34 mm

Fig. 5. Fatigue fracture surfaces for CG (a,b,c) and UFG (d,e,f) titanium specimens at different crack lengths.

Evolution of strains at the crack is shown in Fig. 4b and they demonstrate nonlinear behavior. The last measurement points (prior to fracture) are not presented in the plot due to high deformations between the last adjacent images and DIC technique could not perform calculations. The SEM images of the fracture surface texture of CG and UFG titanium are presented in Fig. 5 for different crack length. The fracture patterns for both specimen types consist of four main textures representing different crack propagation processes – origin area (not presented in the paper); area of microcrack growth (Fig. 5a,d), area of stable crack growth (Fig. 5b,e) and static fracture area (Fig. 5c,f).