The effect of frequency on the propagation of L (0, 1) guided waves in the prestressed strand anchorage segment

Abstract

In prestressed anchorage systems, steel strands are typically encased within protective materials which restricts guided wave excitation from the external free segment. Effective assessment of internal strand integrity requires wave propagation through the anchorage segment. This study investigated the effect of frequency on the propagation characteristics of L (0, 1) guided waves in prestressed strand anchorage segments. Semi-analytical finite element models were developed for both the free and tensile segment of the steel strand to analyze the displacement distribution of the L (0, 1) guided wave within the frequency range of 80–320 kHz.. The results show that the energy of the L (0, 1) guided waves is more concentrated at the cross-section center in the outer helical steel wires with the increasing of the frequency. These phenomena indicates that the higher frequency L (0, 1) guided waves are less affected by the anchor constraint boundary conditions. To further investigate wave behavior in the anchorage segment, a finite element simulation model was established to investigate the transient propagation characteristics of L(0,1) guided waves through the anchorage. The simulation results show that the strong interface reflection echoes are induced when the L (0, 1) guided waves propagate through the anchorage segment from the free segment. In addition, due to energy leakage from the steel wire into the anchorage, a trailing leakage wave packet is generated after the guided wave passes through the anchorage. However, as the frequency of the L (0, 1) guided wave increases from 80 kHz to 320 kHz, both the reflection and leakage phenomena in the anchorage segment gradually diminish. Finally, a guided wave through-anchorage detection experiment was conducted, and the results demonstrated that the reflection coefficient reaches 0.57 at 80 kHz and decreases rapidly with increasing frequency to 0.12 at 200 kHz and then stabilizes. In contrast, the transmission coefficient shows a non-monotonic trend due to interference between leakage and transmitted waves, increasing from 0.37 to 0.45 in the 80–120 kHz range, decreasing to 0.17 between 120–240 kHz, and then recovering to 0.43 in the 240–320 kHz range, which aligns well with the simulation results. This study provides a basis for selecting guided wave modes and frequencies for detecting steel strand defects within anchorage systems.