PSI - Issue 13

1654 4

Ya. Khaburskyi et al. / Procedia Structural Integrity 13 (2018) 1651–1656 Author name / Structural Integrity Procedia 00 (2018) 000–000

a

b

10 -6

10 -6

Aqueous solution of special technological environment (2 g/l)

10 -7

10 -7

Air

Air

10 -8 da/dN , m/cyc le

10 -8 da/dN , m/cycle

Aqueous solution of special

Spirituous solution of spec ia l technological environment (20 g/l)

technological environment (20 g/l)

10 -9

10 -9

10

20

30 40 50

10

20

30 40 50

 K , MPa m 1/2

 K , MPa m 1/2

Fig. 2. Fatigue crack growth curves for steel 20 ( f = 1 Hz, R = 0): ( a ) – in aqueous solution of special technological environment; ( b ) – in spirituous solution.

It is suggested that (electro)chemical interaction of the proposed technological environment with metal in a crack is more intensive when this metal is devoid of oxide films. An intensity of the chemical reactions on juvenile surface is stable but at a higher Δ K crack growth rate increases, as a result the retardation effect of the environment reduces up to it full elimination. Significantly positive effect was obtained under a concentration 20 g/l of the chemically active substance in water. After its supply into a crack for several minutes, the crack growth rate began to drop sharply until the complete crack arrest. Further long-term cyclic loading did not reveal crack increment. Thus it is possible to assert about the crack arrest under such conditions of cyclic loading.

a b Fig. 3. Striation micromechanism of fatigue crack growth in the steel 20 ( f = 1 Hz, R = 0) under Δ K ~ 33 MPa m 1/2 ( a ); view of fatigue crack on the specimen outside surface ( b ). Crack closure was periodically measured during the fatigue tests. Some experimental results are presented in Fig. 4 and in Table 1. They indicate a sharp increase of crack closure (a decrease of U coefficient) in a case when the crack treated with solution. It leads, in its turn, to a sharp decrease of Δ K eff , which causes crack retardation up to its arrest. Creation of artificial crack closure by the proposed method consists in a filling the crack cavity with solid products as a result of interaction fracture surfaces of fatigue crack (crack surfaces metal) with the technological environment. Such products prevent crack closure in semi-cycle of unloading. Metallographic view of the beam specimen illustrating the crack filled with interaction products is presented in Fig. 3 b.