PSI - Issue 57

Qingyang Wei et al. / Procedia Structural Integrity 57 (2024) 262–270 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. Time-domain acceleration responses of node 1, (a) x -direction; (b) y -direction; (c) z -direction.

The time-domain acceleration responses in Fig. 4 are transformed into the frequency domain as shown in Fig. 5 by using a common Fast Fourier Transform program. As shown in Fig. 5 (a), the peak frequencies of each mode mainly appear in the frequency range above 300 Hz. In Fig. 5 (b) and Fig. 5 (c), many frequencies appear below 300 Hz, which reflect the global vibration information of the entire bridge. These frequencies below 300 Hz are mainly generated by secondary vibrations in the other two directions induced by the impact. Although the secondary vibrations are present in the x -direction, they are masked because their intensity is small compared to the main vibration component in the x -direction. The main vibration component in the x -direction is caused by the local vibration of the vertical rib induced by the impact. This local vibration is approximatively equivalent to the local mode of the entire bridge, and the frequency of the local vertical rib is also the high-order frequency of the entire bridge. Therefore, these peaks should vary with the appearance of cracks.

Fig. 5. Frequency-domain acceleration responses of node 1, (a) x -direction; (b) y -direction; (c) z -direction.

Figure 6 shows the frequency spectra of acceleration responses with different crack depths. In Fig. 6 (a), the peaks in various cases can be corresponding identified, and the frequencies show a left shift with the increase of crack depth. However, when the crack depth reaches 100 mm, the position and amplitude of the peak values completely show a different distribution pattern, which also means that the dynamic property of the vertical rib has been largely changed. In Fig. 6 (b) and (c), almost all of the peaks stay still with increasing crack depth, suggesting that the global frequencies of the bridge are not sensitive. Only the peak around 380 Hz shows a left shift, indicating this peak is also the high order frequency of the entire bridge about the vertical rib.