Monitoring of breathing cracks of RC beams based on nonlinear vibro-acoustic modulation technique

For challenges of early microcrack monitoring of reinforced concrete (RC) structures, a nonlinear vibro-acoustic modulation (VAM) based study was carried out, aiming at investigating the influence law between the excitation signal form and the nonlinear effect of sensing signals through both finite element analysis (FEA) and experimental validation. The mechanism of the nonlinearity in sensing signals coming from microcrack surface vibration collisions due to the breathing effect was found which is the theoretical basis of the VAM-based damage detection. The results show that the nonlinear modulation effect is the highest at the resonant frequency of the structure. The side frequency (SF) amplitude of VAM-based sensing signal in the RC beams with microcracks (damage degree between 0% and 70%) is obvious, but when the cracks continue developing, the effective contact area decreases sharply during the crack opening and closing process due to the nonlinear contact between interfacial crack surfaces. As the width of microcrack is altered from 0 nm to 100 nm, the normalised nonlinear coefficient β is reduced from 0.4 to 0.06 in the approximate form of power function, which can be used to characterise the damage development of the early cracks in RC structures.