Structural health monitoring by ultrasonic guided waves

Guided ultrasonic waves provide a highly efficient method for the non-destructive evaluation (NDE) and the structural health monitoring (SHM) of solids with finite cross-sectional dimensions (waveguides). Compared to the widely used ultrasonic bulk waves, guided waves provide larger monitoring ranges and the complete coverage of the waveguide cross-section. Compared to global vibrations, guided waves offer increased sensitivity to smaller defects due to the smaller wavelengths involved. These advantages can be fully exploited only once the complexities of guided wave propagation (multimode, dispersion, frequency-dependent attenuation) are unveiled and managed for the given test structure. This doctoral dissertation is aimed at developing a Semi-Analytical Finite Element (SAFE) method for modeling wave propagation in waveguides of arbitrary cross-section. The method requires the finite element discretization of the cross- section of the waveguide, and assumes harmonic motion along the wave propagation direction. The general SAFE technique was extended to account for viscoelastic material damping by allowing for complex stiffness matrices for the material. The dispersive solutions are obtained in terms of phase velocity, group velocity (for undamped media), energy velocity (for damped media), attenuation, and cross-sectional mode shapes. Once the dispersive properties are computed, the wave motion can be interpreted and the forced response can be predicted. The proposed SAFE formulation was applied to enhance the use of ultrasonic guided waves in a number of applications and to interpret the corresponding experimental results. The following three applications were considered : defect detection in adhesively-bonded joints found in the wing skin-to-spar assemblies of Unmanned Aerial Vehicles; defect detection in railroad tracks; load monitoring and defect detection in seven-wire steel strands used in cables and prestressed concrete structure