PSI - Issue 64

Rui-Xin Jia et al. / Procedia Structural Integrity 64 (2024) 799–806 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

803

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also be seen that the amount of change in the first wave speed from 50kHZ to 75kHZ is smaller than that of the first wave speed from 75kHZ to 100kHZ.This trend is consistent with the group velocity change rule of the dispersion curve L (0, 1) in the low frequency band in Fig. 1. Because of the small amount of relative change in the first wave speed, the experimental results were further processed to establish the relationship between the acoustic time difference and the tension force, and the results of the linear fitting are shown in Fig.6. It can be seen that there is a linear relationship between the flight time of the first wave and the tension force of the rebar at different excitation frequencies. However, as the excitation frequency increases, its first-wave flight time increases significantly. It indicates that the guided wave first wave speed decreases with the increase of excitation frequency in 100 kHz or less.

Table. 2. Rebar material parameters

First wave speed m/s

Tensile force /kN

50kHZ

75kHZ

100kHZ 4835.590 4828.844 4825.220 4823.668 4821.084 4816.440 4813.349 4810.776

0

4963.305 4958.131 4954.310 4952.674 4949.951 4945.055 4941.796 4943.425

4888.647 4884.402 4882.283 4888.647 4880.165 4873.822 4870.130 4868.023

20 40 60 80

100 120 140

409.5

408.0

403.5 Flight time/ u s 405.0 406.5

50kHz 75kHz 100kHz

402.0

400.5

20 40 60 80 100 120 140

Tensile force/kN

Fig 5. First-wave acoustic time difference

The first wave speed used in this paper is not strictly the group speed of the guided wave, but the first wave speed can be seen as the speed of the first arriving energy. Now the theoretical calculation of 50kHz, 75kHz speed and test measured speed comparison results are shown in Table.3. It shows that the measured speeds are within a reasonable range compared with the theoretical speeds, which proves the accuracy of the experimental results.

Table. 3. Rebar material parameters Frequency

Theoretical speed

Experimental speed

Error 0.8% 2.6%

50kHz 75kHz

4918.5m/s 4760.3m/s

4963.1m/s 4888.6m/s

4.2. Influence of excitation power

In practical engineering in the long-distance signal excitation and reception of rebar, the excitation power required is different. Therefore, the law analysis of tension sensitivity under different excitation power is carried out.