Structural health monitoring (SHM) involves the implementation of damage identification
methods in engineering structures to ensure structural safety and integrity. The paramount
importance of SHM has been recognised in the literature. Among different damage
identification methods, guided wave approach has emerged as a revolutionary technique.
Guided wave-based damage identification has been the subject of intensive research in the past
two decades. Meanwhile, applications of fibre reinforced polymer (FRP) composites for
strengthening and retrofitting concrete structures have been growing dramatically. FRP
composites offer high specific stiffness and high specific strength, good resistance to corrosion
and tailorable mechanical properties. On the other hand, there are grave concerns about longterm
performance and durability of FRP applications in concrete structures. Therefore, reliable
damage identification techniques need to be implemented to inspect and monitor FRPretrofitted
concrete structures.
This thesis aims to explore applications of Rayleigh wave for SHM in FRP-retrofitted
concrete structures. A three-dimensional (3D) finite element (FE) model has been developed
to simulate Rayleigh wave propagation and scattering. Numerical simulation results of
Rayleigh wave propagation in the intact model (without debonding at FRP/concrete interface)
are verified with analytical solutions. Propagation of Rayleigh wave in the FRP-retrofitted
concrete structures and scattering of Rayleigh waves at debonding between FRP and concrete
are validated with experimental measurements. Very good agreement is observed between the
FE results and experimental measurements. The experimentally and analytically validated FE
model is then used in numerical case studies to investigate the scattering characteristic. The scattering directivity pattern (SDP) of Rayleigh wave is studied for different debonding size
to wavelength ratios and in both backward and forward scattering directions. The suitability
of using bonded mass to simulate debonding in the FRP-retrofitted concrete structures is also
investigated. Besides, a damage localisation method is introduced based on the time-of-flight
(ToF) of the scattered Rayleigh wave. Numerical case studies, involving different locations
and sizes of debonding, are presented to validate the proposed debonding localisation method.
Nonlinear ultrasonics is a novel and attractive concept with the potential of baseline-free
damage detection. In this thesis, nonlinear Rayleigh wave induced at debondings in FRPretrofitted
concrete structures, is studied in detail. Numerical results of nonlinear Rayleigh
wave are validated with experimental measurements. The study considers both second and
third harmonics of Rayleigh wave. A very good agreement is observed between numerical and
experimental results of nonlinear Rayleigh wave. Directivity patterns of second and third
harmonics for different debonding size to the wavelength ratios, and in both backward and
forward scattering directions, are presented. Moreover, a damage image reconstruction
algorithm is developed based on the second harmonic of Rayleigh wave. This method provides
a graphical representation for debonding detection and localisation in FRP-retrofitted concrete
structures. Experimental case studies are used to demonstrate the performance of the proposed
technique. It is shown that the proposed imaging method is capable of detecting the debonding
in the FRP-retrofitted concrete structures.
Overall, this PhD study proves that Rayleigh wave is a powerful and reliable means of
damage detection and localisation in FRP-retrofitted concrete structures.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 201
