Load measurement and health monitoring in cable stays via guided wave magnetostrictive ultrasonics

Loaded cables made of wires, strands or bars are employed in civil structures as load carrying members in cable stayed and suspension bridges. Steel strands are also employed in prestressed structures for the pretensioning or posttensioning of concrete. Accurate and real time knowledge of the force acting on these stressing systems can contribute immensely to ensuring the safety of a bridge structure. Monitoring cable loads in service can also allow for detection of excessive wind and traffic overloads or, in the worst scenario, problems such as accidental broken wires and corrosion. The overall objective of this paper is to further the understanding of an ultrasonic method that is able to monitor live loads in multiwire steel strands as well as detect possible discontinuities such as indentations and broken wires. The characterization of wave propagation in steel strands is achieved through a broadband, laser ultrasonic setup and time frequency wavelet transform processing. Those vibrating frequencies propagating with minimal losses are identified as they are suitable for long range testing of the strands. In addition, the wave transmission spectra are found to be sensitive to the load level. Sensors based on the magnetostrictive effect are used for the wave generation and detection. The acoustoelastic effect is also considered for live load monitoring. Signal processing based on the discrete wavelet transform is used to enhance the discontinuity detection sensitivity and acquisition speed.