Monitoring load levels in multi-wire strands by nonlinear ultrasonic waves

Monitoring load levels in multi-wire steel strands is relevant to ensuring the proper structural performance of post-tensioned concrete structures, suspension bridges, and cable-stayed bridges. This article investigates the use of ultrasonic nonlinearity as a means to determine the level of load applied to the strands. Since an axial load on a multi-wire strand generates proportional contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of axial load. This article shows that the higher harmonic generation of ultrasonic guided waves propagating in individual wires of the strand indeed changes monotonically with the applied load, with smaller higher harmonic amplitudes with increasing load levels. This trend is consistent with known studies on higher harmonic generation from ultrasonic plane waves incident on a contact interface under a changing contact pressure. The article presents experimental studies on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free strands.