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<p>Most methods for measuring changes in the group velocity of the first-order longitudinal mode of ultrasonic guided waves (UGWs) are effective for assessing stress variations (SVs) in long, unbonded steel strands. In practical applications, the grout surrounding the steel strands complicates the extraction of the first-order longitudinal mode from the mixed propagation modes of UGWs and reduces the signal-to-noise ratio of the recorded waveforms. This situation poses challenges to the applicability of the previously mentioned methods that are based on the group velocities of the longitudinal modes. Utilizing the peak time shift (PTS) indices extracted from the phase velocity changes of the mixed multiple longitudinal propagation modes of UGWs, this manuscript presents a measurement method designed to identify SVs in both unbonded and semi-bonded steel strands. The primary challenge of this study lies in demonstrating the existence of a linear relationship between the SVs in the strands and the changes in phase velocity for any order of flexural or mixed propagation mode. Because the phase velocity cannot be measured directly in mixed propagation modes, this study first theoretically demonstrates that observing the aforementioned linear relationship is equivalent to observing the linear relationship between the SVs and the PTS index changes. Then the latter linear relationship is experimentally observed in both cases of unbonded and semi-bonded steel strands, indicating the existence of the former linear relationship. Subsequently, the optimal excitation frequencies for unbonded and semi-bonded steel strands are experimentally determined to be 180&#xa0;kHz and 280&#xa0;kHz, respectively. The proposed method is applicable for assessing stress levels that exceed 570&#xa0;MPa in steel strands. When the SV surpasses 260&#xa0;MPa, the maximum relative errors in the identified SV are approximately 20%. This error consistently decreases as the SV increases. Furthermore, a comprehensive PTS-based measurement system is thoroughly introduced. The aforementioned findings provide a foundation for subsequent investigations into measuring SVs in bonded steel strands.</p>

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Measurement of stress variation in unbonded and semi-bonded prestressing steel strands using the peak time shift of guided ultrasonic waves

  • 期刊论文

  • 发表日期：

    2025年5月5日

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  • Sheng Shen,
  • Fan Yang,
  • Huihuang Wang,
  • Zhifu Yu,
  • Jianzhong Zhang,
  • Shuzhong Tan,
  • Yi Yu,
  • Xiaohua Zhang

摘要

Most methods for measuring changes in the group velocity of the first-order longitudinal mode of ultrasonic guided waves (UGWs) are effective for assessing stress variations (SVs) in long, unbonded steel strands. In practical applications, the grout surrounding the steel strands complicates the extraction of the first-order longitudinal mode from the mixed propagation modes of UGWs and reduces the signal-to-noise ratio of the recorded waveforms. This situation poses challenges to the applicability of the previously mentioned methods that are based on the group velocities of the longitudinal modes. Utilizing the peak time shift (PTS) indices extracted from the phase velocity changes of the mixed multiple longitudinal propagation modes of UGWs, this manuscript presents a measurement method designed to identify SVs in both unbonded and semi-bonded steel strands. The primary challenge of this study lies in demonstrating the existence of a linear relationship between the SVs in the strands and the changes in phase velocity for any order of flexural or mixed propagation mode. Because the phase velocity cannot be measured directly in mixed propagation modes, this study first theoretically demonstrates that observing the aforementioned linear relationship is equivalent to observing the linear relationship between the SVs and the PTS index changes. Then the latter linear relationship is experimentally observed in both cases of unbonded and semi-bonded steel strands, indicating the existence of the former linear relationship. Subsequently, the optimal excitation frequencies for unbonded and semi-bonded steel strands are experimentally determined to be 180 kHz and 280 kHz, respectively. The proposed method is applicable for assessing stress levels that exceed 570 MPa in steel strands. When the SV surpasses 260 MPa, the maximum relative errors in the identified SV are approximately 20%. This error consistently decreases as the SV increases. Furthermore, a comprehensive PTS-based measurement system is thoroughly introduced. The aforementioned findings provide a foundation for subsequent investigations into measuring SVs in bonded steel strands.