Issue 38

G. S. Serovaev et alii, Frattura ed Integrità Strutturale, 38 (2016) 392-398; DOI: 10.3221/IGF-ESIS.38.48 395 Figure 2 : Models of delamination. R ESULTS he frequency range from 0-20 kHz was analyzed. The size of delamination was changed from 0 to 0,05 m with a step of 0.002 m. The change of eigenfrequencies, depending on the size of delamination in the range of 0-20 kHz, is shown in Fig. 3 and 4 (the upper plots show the results for the free mode model of delamination, the lower – for constrained model). The vertical axis represents the defect size, the horizontal axis corresponds to the frequency of oscillations in Hertz. The reaction of the natural frequencies to increase in the size of the defect for the free mode model is not observed up to 4 kHz. At higher frequencies a sudden drop in the natural frequencies for the defect of the large size is observed. Figure 3 : Shifts of natural frequencies in the range of 0-10 kHz. In the range of frequencies from 10-20 kHz a similar abrupt pattern of natural frequency shifts is observed (Fig. 4). Restriction applied to the component of displacement in the delamination zone and thereby precluding mutual penetration of the adjacent volumes in this area significantly affects the results. This model is characterized by a more gradual and smaller change in magnitude of natural frequencies with increasing the size of the defect. For both models a clear tendency towards greater sensitivity of high frequency vibrations to the defects of a smaller size is observed, which is explained by the fact that the reaction of frequencies to the appearance of a defect depends not only on its size but also on its location. If a defect is located in the area of small or zero strains of mode shape, the frequency will remain unchanged. The mode shapes of high frequencies have a large number of zones with non-zero strains (bends), providing greater sensitivity of these frequencies to the defect. That's what makes these frequencies more appropriate for detection of small sized defects. T