PSI - Issue 36

Petro Yasniy et al. / Procedia Structural Integrity 36 (2022) 197–202 Petro Yasniy, Ihor Okipnyi, Oleksandr Dyvdyk et al. / Structural Integrity Procedia 00 (2021) 000 – 000

199

3

crack growth direction each 0.5 mm. The lengths of fatigue crack growth on the surface с and along the plate thickness a are measured by means of MBS-10 binocular mic roscope with accuracy 14 μm. For hole cold expansion mandrel with central calibration part with diameter 8, 10 and 12 mm, made of steel grade 40X and after heat treatments to 35-38 HRC hardness is used. The mandrel speed was 0.1 mm/s. The cold expansion degree is determined by the formula = ( 1 − о )/ ∙ 100% , (1) where d 0 , d 1 is the diameter of the plate hole before and after deformation. The dimensions of the holes before and after cold expansion holes are given in Table 1.

Table 1. D imensions of the plate’s hole before and after cold expansion d 0 , mm d 1 , mm і , % N rc , cycles N rc /N r

Strengthening method

8

8

0

12500 77000 12000 37000 9000 53700

without cold expansion hole cold expansion hole without cold expansion hole without cold expansion hole cold expansion hole cold expansion hole

7.761

7.970

2.7

6.6

10

12

0

9.728

9.960

2.4

3.08

12

12

0

11.635

11.960

2.8

5.96

3. Results and Discussion The dependence of crack lengths c і a on the number of load cycles in the specimens after cold expansion holes of different diameters with cold expansion degree і = (0 – 2.8) is shown in Fig. 2. The residual lifetime of the plate with preexisting near the hole fatigue crack is determined by the number of load cycles when the crack becomes through the-thickness one. The increase of cold expansion degree slows down the fatigue crack growth and respectively increases the residual lifetime of the plate with the hole.

6

6

5

5

4

4

3

3

c (8) а (8) c (10) а (10) c (12) а (12)

c, I = 2,7 (8) a, I = 2,7 (8) c, I = 2,4 (10) a, I = 2,4 (10) c, I = 2,8 (12) a, I = 2,8 (12)

2

2

a, c, mm

a, c, mm

1

1

0

0

0 2000 4000 6000 8000 10000 12000 14000

0

20000

40000

60000

80000

N, cycles

N, cycles

a

b

Fig. 2. The dependence of crack length c and a in the plate on the number of load cycles without (a) and after cold expansion of holes і = (2.4 – 2.8) %(b). Δσ = 147 MPa, R = 0.05

Due to the uneven growth of the crack on the surface с and along the specimen thickness а , its shape changes. The dependences of the shape factor a/c on the number of load cycles (Fig. 3a) and the crack length along the plate thickness а (Fig. 3b) in specimens with holes d = 8, 10 і 12 mm in diameter are shown in Fig. 3. For plates with holes of different diameters, the dependence of the shape factor a/c on the number of load cycles has three areas: descending, associated with crack delay; growing and permanent area (Fig. 4a). Similar dependences are shown by