Abstract: Stress loss in steel strands will threaten the safety of whole prestressed structures. Therefore, it is important to achieve in-service prestress measurement nondestructively of steel strand in engineering. Prestress measurement method in steel strand based on ultrasonic guided wave technique is analyzed theoretically and verified in the experiment. According to wave theory and acoustoelastic theory, longitudinal and shear wave velocities in steel strand under different applied prestress levels are obtained, the relationship between group velocity of the lowest longitudinal guided mode L(0,1) and applied stress in steel strand is analyzed and constructed. In experiment, L(0,1) mode of ultrasonic guided waves is excited and received by a pair of self-developed magnetostrictive sensors in a seven-wire, 17.80 mm nominal diameter steel strand under different applied prestress levels. Group velocities of L(0,1) mode under different stress state conditions are measured in the steel strand and the actual relation curve between experimental group velocities of the mode and applied stresses in the steel strand is obtained. Results show that group velocity of L(0,1) mode will decrease approximately linearly with the increase of applied stress when the stress is above 0.5 GPa. Experimental results are well coincident with predicted assumption. The conclusion provides a certain experimental foundation for application of ultrasonic guided waves technique to measurement of applied prestress in steel strand.

Key words: Acoustoelastic effect, Longitudinal mode, Prestress, Steel strand, Ultrasonic guided waves