Mixed harmonic generation of surface guided waves by collinear wave mixing for debond identification in FRP-strengthened beams

Abstract

The high incidence of debonding as a failure mode in FRP-strengthened structures highlights the importance of regular inspections of the bond layer for any discontinuity. In contrary to most previous studies, which focus on the linear features of waves like amplitude and velocity, the present study utilises a novel collinear wave mixing technique for the generation of mixed harmonics of surface guided waves for debond characterisation. Experimental study is conducted in which a particular surface guided mode, sensitive to bond layer, is excited using wedge transducers. A three-dimensional finite element model is developed to understand the nonlinear interaction of surface guided waves with debonding, and the results obtained are compared with those obtained from the experimental results in both time and frequency domains. The numerical results are found to be in good agreement with the experimental ones. Later, extensive parametric studies are conducted on the validated model by changing the debond size. A damage index is proposed that rely on the band frequency amplitude of fundamental and higher order mixed harmonics paving the way for baseline-free debond quantification. The damage index (DI) exhibited a consistently increasing trend with debond size. Finally, a sensitivity analysis is conducted by increasing the debond size and comparing the relative parameter values of linear features (scattered amplitude and change in time of arrival) with the proposed damage index. The analysis results revealed that the proposed DI is more sensitive to the debond size than the conventional linear parameters that also requires pristine condition data. Thus, the study offers valuable insights into the collinear wave mixing phenomenon in FRP strengthened RC structures and proposes an efficient and practical technique for debond detection in the field.