PSI - Issue 16

Roman Chepil et al. / Procedia Structural Integrity 16 (2019) 211–217 Roman Chepil, Olena Stankevych, Orest Ostash1, Bohdan Klym / Structural Integrity Procedia 00 (2019) 000 – 000

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5

approximates the projection WT ( t ) of the single pulse on the 2D time projection (Fig. 3d); (5) the calculation of the energy E WT of the single AE event by relationship

t

2

2 ( ) . t E WT t dt   WT a

(6)

1

The fracture type were identified by the energy criterion: E WT  0.01 – ductile fracture (plastic deformation); 0.01< E WT  0.1 – ductile-brittle fracture (microcracking); E WT > 0.1 – brittle fracture (macrocrack growth).

Fig. 3. Schemes for determining local AE pulse signal parameters using its CWT.

The regularities of early stages of fatigue failure (Fig. 4a), in particular, the microdamage accumulation, network of microcracks formation and macrocrack initiation (Ostash et al. (1999)) have been confirmed. Signals recorded in the period 500  800 s (Fig. 4b, section I) were characterized by parameter E WT within 0.017  0.033. This indicates that the formation of a plastic zone and microcracking near the notch root occurs. The zone of intensive microplastic deformation extends in 200  300 μm from the notch root, which corresponds to the size d * = 250 μm of the fatigue process zone for this material. The spurt of AE signals during the period 1500  2000 s (Fig. 4b, Section II) corresponded to the intensification of microplastic deformations and the formation of the microcracks network, which was confirmed by visual observations (Fig. 4a) and the magnitude of the parameter E WT within 0.01  0.056. The subsequent intensification of AE signals in the period 3900  4200 s (Fig. 4b, Section III) indicated the further growth of microcracks ( E WT = 0.06  0.086), but none of them exceeded the size of 300  m. At section IV (5800  6200 s) the macrocrack was formed and began to grow (Fig. 4a). As a result, the AE signals with the energy parameter E WT = 0.1  0.18 were generated, thus a brittle fracture was realized.