Backside delamination detection in composites through local defect resonance induced nonlinear source behavior

Abstract

Shallow damages can be detected by searching for local defect resonance behavior in the fundamental vibrational response. However, this is not possible for defects which are deeper than half the thickness of the coupon due to the limited bending stiffness difference between the defect and the sound material.In this study, it is shown that delaminations under harmonic excitation at the local defect resonance frequency behave as sources of higher harmonic vibration components. These higher harmonic components are detectable when examining the delaminated component from the side where the delamination is close to the surface, but more importantly, also from the other side where the delamination is at the backside. As such, monitoring of these harmonics allows for the detection of backside delaminations which cannot be found using linear LDR techniques. First, a finite element simulation with nonlinear contact conditions is performed as a proof of concept. Experimental validation is performed for a delaminated CFRP coupon using low-power piezoelectric actuation and scanning laser Doppler vibrometer response measurements.