Identification of Contact Acoustic Nonlinearities of Subsurface Cracks Located at Free-Edges

Structural health monitoring (SHM) is the continuous onboard monitoring of a structure’s condition during operation by integrated systems of sensors. Standard methods of SHM are vibrationbased methods like the electromechanical impedance (EMI) method. This method uses a piezoelectric transducer that is attached to a mechanical structure of interest. The transducer is supplied by a harmonic voltage that results in a typically harmonic oscillation of both transducer and structure and allows to monitor the combined dynamic response by measuring the supplied current, and thus, the impedance. Changes of the impedance reflect changes of the structure and can be used to conclude on a potential damage. Today, linear response is typically evaluated by the EMI. Critical damages like delamination in fiber reinforced polymer components could also provoke nonlinear response that might allow more conclusions on the damage. This contribution demonstrates the existence of contact acoustic nonlinearity in an aluminum beam with a free-edge-delamination-like subsurface crack at the end. The investigation is based on transfer frequency response functions between an exciting transducer and out-of-plane surface velocities of the beam measured by laser Doppler scanning vibrometry. For identification of the nonlinear response an identification method is used and verified that is readily published. Furthermore, the diffusion of the nonlinear behavior shall be investigated to conclude on its possible measurement by a transducer that is located at some distance to such a damage.