Direct compensation of lift-off oscillation effect in magnetic impedance–based damage detection

Owing to its magneto-mechanical coupling characteristics, a magnetic transducer such as the electro-magnetic acoustic transducer can excite a metallic structure by means of the Lorenz force, and its electrical impedance is actually directly related to the mechanical impedance of the structure. Therefore, the change in electrical impedance measured can be used as an indicator of damage occurrence. As such, this type of magnetic transducer can be used in impedance-based structural damage detection, which features a non-contact advantage. Nevertheless, one key issue is that the coupling between the magnetic transducer and the structure monitored is strongly influenced by the lift-off distance (i.e. the distance from the transducer to the structure) which oscillates as the structure is oftentimes subject to oscillation/movement due to environment disturbance. In this research, we propose a new data analysis approach of transformed impedance that is immune to the lift-off distance oscillation during measurements. This data analysis approach takes advantage of the lift-off distance information embedded in the impedance measurement and is capable of removing the lift-off oscillation effect without explicitly measuring it. By doing so, the damage signature can be highlighted directly. Numerical simulations and experimental validations are carried out to demonstrate the effectiveness.