Detecting broken-wire flaws at multiple locations in the same wire of prestressing strands using guided waves

Broken wires often occur at multiple locations in the same wire of a strand due to the recovery length, which is defined as the length of the wire taking up its full share of the axial load from the break point. The detection of broken-wire flaws at multiple locations along the same wire is investigated using guided waves below 400kHz. Herein, a sample with three broken-wire flaws in the same wire is analyzed using magnetostrictive guided waves. Our data show that three flaws are found using the low-frequency guided waves (50kHz) but only one flaw is found using the high-frequency guided waves (320kHz). By analyzing the reflection and transmission coefficients at the three different flaws, we observe that the energy exchange decreases as the frequency increases along the same propagating distance. Hence, the recovery length for elastic waves, the length of the wire taking up its full share of elastic-wave energy from the break point, is observed. The recovery length for elastic waves in prestressing strands increases with the frequency. To detect prestressing strands using magnetostrictive guided waves, several one-broken-wire flaws at different locations can be distinguished from in different wires or the same wire by employing both low-frequency waves and high-frequency waves. Nevertheless, we cannot identify in which wire the flaws are located because the magnetostrictive sensor analyzes the whole strand.