Determination of optimal probing frequency for enhancing nonlinear vibro-acoustic modulation behaviors of delaminated CFRP based on local defect resonance

The magnifying effect of resonance vibration on vibro-acoustic modulation (VAM) is important for the study of nonlinear ultrasonic behaviors and yet to be completely investigated. In this paper, the determination of optimal probing frequency for enhancing nonlinear VAM behaviors of delaminated carbon fiber reinforced plastics (CFRP) based on local defect resonance (LDR) is studied through a twofold approach, i.e., a 3D finite element model and a non-contact experimental setup based on laser scanning Doppler vibrometer (LSDV). A searching procedure based on scaling subtraction method (SSM) is proposed to better determine the LDR of a delaminated CFRP and the magnifying effect of probing frequency in a considerable bandwidth including LDR on the VAM behavior is examined. Damage contour maps based on modulated sidebands are established to identify the delamination existence and the maximum response amplitude (MRA) is proposed to quantify the enhancement of nonlinear VAM response. Results are presented and show that the LDR can improve the delamination detection compared to arbitrary frequency and global resonance frequency. The highest sensitivity of delamination detection is achieved when the probing frequency is selected as the sum or difference of the LDR and pumping frequency.