Stress Measurement in Seven-Wire Strands using Higher Order Guided Ultrasonic Wave Modes

This paper investigates the use of higher order longitudinal guided modes for stress measurement within individual wires of a steel strand. The effect of stress on the phase velocity of higher order modes is studied using an approximate theory, which does not require the solution of dispersion curves. To validate the proposed approach, a prestressing bed was designed to apply a tensile load to a strand up to 25% ultimate tensile strength while recording guided wave signals. Guided waves were excited within individual wires of a strand, and the stress sensitivity of their phase velocity was used for stress measurement. Stress measurement was performed with higher order modes using the approximate theory with parameters for a steel of similar carbon content (Hecla 17), as well as with calibrated parameters. Using the Hecla 17 parameters, roughly 15% mismatch in stress was observed, whereas roughly 5% error was observed using calibrated parameters. Stress measurement was also performed using the fundamental mode, in order to compare the accuracy of higher order modes with the mode used previously in the literature. The greater stability of higher order modes across mode and frequency yielded significantly increased stress measurement accuracy, using both Hecla 17 and calibrated parameters.