Monitoring steel strands via ultrasonic measurements

Steel strands are widely used in civil structures as pre- stressing tendons and stay-cables. The structural criticality of strands has led to an increasing interest in developing methods able to monitor applied loads as well as detect potential defects. In this paper ultrasonic guided waves, generated and detected via magnetostrictive sensors, are exploited to address this need. The sensors were developed in collaboration with the US Federal Highway Administration NDE Validation Center. An acousto elastic formulation of the Pochhammer-Chree longitudinal vibrations in cylindrical waveguides is proposed to predict the change of ultrasonic velocity as a function of applied stress. Results from acousto elastic experiments performed on seven- wire strands and on single wires are presented and compared to the theoretical predictions. Ways to enhance the inhernetly-low sensitivity of the acousto elastic measurements are proposed and investigated. The different behavior exhibited by the strand when compared to the single wire suggests the need for widening the theory governing the acousto elastic phenomenon in multi-wire members. The role of the strand anchorages is examined in the context of wave attenuation. The suitability of the guided wave method for the detection of defects is demonstrated including the possibility of inspecting the critical anchored regions.