Research On Characteristics Of Steel Strand Ultrasonic Guided Wave Signal

Steel strand is used as one of the primary component in prestressed structures, and its health status affects the safety of these structures. The characteristics of magnetostrictive guided wave signals detected in strand are studied in detailed in this thesis which is supported by National High Technology Research and Development Program of China.Firstly, according to the structure of strands which are composed of seven steel wires (six helical wires twisted around a straight wire), the reflection characteristics of guided waves from notches in steel wires are studied. The reflection coefficient as a function of the defect axial extent and cross-section is obtained respectively with finite element models, which agree well with experimental results. Then a multi-frequency testing approach is present for the defect sizing.Secondly, in the laboratory, strands with fractured wires in two different statuses are detected by guided waves and the characteristics of the testing signals are studied. Signals acquired from a strand specimen with three fractured surfaces in a wire indicate that fractured surfaces in the wire can reduce the forward transmission energy of guided waves in a strand, but waves can continue propagating simultaneously in the fractured wire and other wires because of the contact stresses between the wires. This result is also convinced by signals detected in the second specimen which has three fractured surfaces in different wires. Compared with signals obtained in the two specimens, it is concluded that the contact situation between wires of a strand has significant effect on defect signals.Thirdly, artificial defects are designed and machined in two strand specimens to simulate the pit corrosion and large area corrosion respectively. Moreover, another specimen is also corroded by sulfate. The characteristics of signals detected in these corroded specimens are studied. results show that the waveform of defect signals is closely related to the corrosion length,which is basically consistent with the simulation results of the steel wire.At last, the empirical mode decomposition (EMD) method is employed to process the strand guided wave signals in order to detect the small defect. The testing signals are decomposed into series of the intrinsic mode functions (IMF). The small defect signal is enhanced by analyzing the different order of the IMF. It is easy to find the small defect signal in the first or the second IMF. This result indicates that the EMD approach can suppress noise effectively and improve the ability of guided wave testing of the small defect.