Experimental measurements of vibrations of artificial sub-surface cracks and evaluation of identification potential for the electro-mechanical impedance method

As standard method for structural health monitoring (SHM) the electro-mechanical impedance method evaluates the frequency response of a piezoelectric transducer, which is attached to a mechanical structure of interest. The piezoelectric element is excited by a harmonic voltage signal, which causes typically harmonic oscillations on both element and structure. The measured impedance of the piezoelectric element reflects thus the structural response. Consequently, changes of the impedance indicate structural changes, i.e., damage. This contribution investigates linear and possibly non-linear vibrations provoked by contact acoustic non-linearity of a sub-surface crack, a damage typical for composite delamination, in a harmonically excited structure. The considered structure is an aluminum beam with a sub-surface crack, which is introduced artificially according to a specific manufacturing process developed at the author’s research group and already presented at SPIE Smart Structures+NDE 2018. Numerical studies presented at IWSHM 2017 and SPIE Smart Structures+NDE 2018 showed that the considered damage causes non-linear response to harmonic excitation. The proposed work continuous this research by experimental measurements of the vibration response of the considered beam with sub-surface crack to harmonic excitation by a piezoelectric transducer. Laser-scanning vibrometer measurements along the entire beam and in particular at the crack location identify linear and non-linear vibrations, allow its mode shape visualization and to assign the structure as most probable source for non-linearity. Furthermore, a piezoelectric transducer that simultaneously records the transfer frequency response function passing through the sub-surface crack shows high potential for vibration-based SHM methods like the electro-mechanical impedance method to assess the non-linear response of this damage type for identification.