Fatigue cracking identification using nonlinear Lamb waves for FRP-reinforced steel plates

Bonding carbon fibre reinforcement polymer (CFRP) laminates to steel structures has attracted a lot of attention for extending the service life of the steel structure. To ensure the functionality of the strengthened structures, an appropriate monitoring method is required. Nonlinear Lamb-wave based method for fatigue crack detection of CFRP-reinforced steel plates was presented in this study. Numerical and experimental studies were carried out for a plate with the CFRP laminates, where a fatigue crack was generated by the servo-hydraulic testing machine to achieve contact acoustic nonlinearity (CAN). The symmetric wave mode S1 was used as the excitation and when it came across the fatigue crack in specimens, S2 mode would be induced. After processing the received signals, the damage-induced Lamb wave nonlinearities were identified with the generation of second harmonic and it was found that the nonlinearities in the CFRP-reinforced steel plate were weaker when the sensing path is further from the fatigue crack. With the correlation coefficient of benchmark and signal with damage at double frequency in the time domain, an imaging method was introduced to locate the fatigue crack in CFRP-reinforced steel plates.