Stress measurement and defect detection in steel strands by guided stress waves

Health monitoring of steel strands is the subject of much research in the nondestructive evaluation and civil engineering communities. This paper deals with a guided stress wave method for stress monitoring and defect detection in seven-wire strands. A simplified acoustoelastic formulation of the Pochhammer-Chree vibrations in cylindrical waveguides is derived in the framework of the partial wave representation for guided waves. Magnetostrictive transducers are used to excite and detect the waves in the experiments. Results from acoustoelastic measurements on single wires and on strands are presented, showing the feasibility of the method for stress measurement, although an anomalous behavior of the strands at low stress levels remains the subject of current investigation. Improvements to the inherently low sensitivity of acoustoelastic stress measurements are suggested by adding the effect of strand elongation. The role of the strand anchorages is also examined in the context of wave attenuation. Finally, the suitability of the guided wave method for the detection of indentations and broken wires in the strands is demonstrated, including the possibility of inspecting the critical anchored regions.