Application of Wigner-Ville transform to evaluate tensile forces in seven-wire prestressing strands

Seven-wire steel strands are widely used in various types of prestressed concrete structures. When a strand is subjected to a tensile force, the traveling time of the stress wave will be affected due to the elongation of the strand together with the changes in wave velocities. In this paper, the Wigner-Ville transform technique was used to analyze the measured stress waves in order to identify the arrival time of each frequency component. A numerical calculation considering the elastic waveguide theory and the acoustoelastic effect was conducted. A commonly used 12.7-mm (1/2 in.) diameter seven-wire prestressing strand (Grade 270) was tested. Tensile force up to 142 kN (32 kips) was applied to the strand. The results indicate that evaluation of the tensile force in the strand can be accomplished by measuring the traveling time of a single-frequency component of the propagating stress wave. The Wigner-Ville transform technique can efficiently identify the arrival time of each frequency component of the stress-wave signals with reasonable accuracy. The numerical and experimental results correlate well with each other. Results of this study present a technique that can provide an efficient nondestructive measurement of stress retension levels in long post-tensioned steel strands.