Ultrasonic Wave Propagation in Progressively Loaded Multi-Wire Strands

In recent years methods based on guided ultrasonic waves gained increasing attention for the nondestructive evaluation and the health monitoring of multi-wire strands used in civil structures as prestressing tendons and stay cables. The study of wave propagation properties in such components has been challenging due to the load-dependent inter-wire contact and the helical geometry of the peripheral wires. The present paper addresses an experimental investigation on the ultrasonic wave propagation in seven-wire strands loaded at different stress levels. Wafer piezoelectric sensors are employed in a through transmission configuration for the generation and detection of stress waves. The response of the lowest-order longitudinal mode is studied at different levels of load. Those ultrasonic features, associated with the transmitted ultrasonic energy, sensitive to the variation of applied load are identified and discussed as possible means of a load monitoring.     

Fiber-to-field angle dependence of proton nuclear magnetic relaxation in collagen

Longitudinal and transverse proton relaxation times were measured on pig tendon. For T₁, dispersion curves and more accurate measurements at 20 MHz are presented. Values of T₂ were obtained from CPMG pulse sequences, at 20 MHz. The dependence of relaxation times against the fiber-to-field angle was particularly investigated. Longitudinal relaxation rate was found to be almost orientation independent, and presented quadrupolar peaks between 1 and 4 MHz. On the contrary, transverse relaxation, that was well fitted by the sum of four exponentials, was highly orientation dependent. Deconvolution showed that the exponentials decaying most quickly are most orientation dependent. For those two fractions, a cross-relaxation model allowed explaining the fiber-to-field angle dependence, and the specially low rate corresponding to the magic angle of 55°. Finally, each decaying mode was assigned to a fraction of protons localized in the macromolecular structure and characterized by particular dynamics.