PSI - Issue 13

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

1653

3

a b Fig. 1. Typical diagram F –  for semi-cycle of loading ( a ); scheme of strain-gauge location on a specimen ( b ).

Stress intensity factor K op , which corresponds to the load F op and delimits the loading cycle to “open” and “close” parts, was determined by the point M as two tangents AM and DM intersection in accordance with O. Romaniv et. al. (1983), but not by the point С , above which a crack is fully opened. The CB arch corresponds to a transition from fully opened ( CD region) to fully closed ( AB region) crack. Crack edges during this part of semi-cycle of unloading close also although with a less rate (per force F unit), than for the part CD . Correspondingly material in the crack tip will cyclically deformed on the CB too. The optical microscope Neophot-21 was used for metallographic examinations and SEM EVO-40XVP – for microfractography. 3. Results and discussion The diagram da / dN – Δ K for the steel 20 is presented in Fig. 2. The clarification of the efficiency of the special technological environment supplied to the crack due to its possible influence on the fatigue crack growth was the following. At first, the crack was grown in air to a da / dN ~ 10 -8 m/cycle, which corresponded approximately to start of the Paris region on the diagram. After that the experiment was interrupted, crack closure was determined and water solution of the proposed chemical substance of a certain concentration put in a crack, and then continued cyclic loading of the specimen. The liquid easy filled the whole crack cavity after some cycles of loading, evidently due to the capillary effect. Liquid outflow from the crack tip during semi-cycle of unloading was an evidence of a total filling of a crack. The effect of the chemical substance of the different concentration in water on fatigue crack growth rate presented in Fig. 2 (dark symbols). It is established that the concentration of such substance as low as 2 g/l decreases crack growth rate essentially. The maximum effect was revealed for the middle of the Paris region (~ 30 MPa m 1/2 ), where fatigue crack growth rate dropped from ~ 2ꞏ10 -7 m/cycle by almost order of magnitude. This unusual effect cannot be explained by the cooling effect of the liquid on the metal in the crack tip, since the test was carried out at a low frequency of 1 Hz. It is known that some fractographic features of fatigue crack growth in air correspond to Δ K level of maximum crack retardation. Namely, striation mechanism is the specific fractographic feature of fatigue crack growth and for a certain relatively high Δ K every stria is created in every loading cycle. It means that a freshly created surface in the crack tip forms in every cycle and it is especially chemically active in a reaction with the active component of the proposed technological environment. Therefore an intensive creation of interaction products in the crack tip leads to the crack growth rate decrease as a result of such reaction. The microfractography of fatigue crack growth under Δ K ~ 33 MPa m 1/2 is presented in Fig. 3 a . A distance between striae was determined by the fractographic analysis; it was on average 0.32 µm. Hence, the so-called micro rate of crack growth was calculated with the assumption that the crack grows in each cycle at a distance equal to the striation step. It is ~ 3.2 10 -7 m/cycle which is close to the macro-crack growth rate represented by the fatigue crack growth curve da/dN – Δ K . Accordingly, the crack growth rate determined at the macro and micro levels are about the same. This means that starting from ΔK ~ 33 MPa m 1/2 , stria is formed in each loading cycle, that is, in each cycle, a freshly created surface is formed in the crack tip.