Probing the limits of full-field linear local defect resonance identification for deep defect detection

Local Defect Resonance (LDR) is exploited for non-destructive testing (NDT) by using ultrasonic vibrations to get a localized resonant activation of defected zones. The LDR technique relies on the local stiffness difference between the defect and the sound material. Analyzing the structure's displacement field at this localized resonance frequency reveals the defect's location and provides information about the defect's characteristics, i.e. geometry, size and depth. In this study, the opportunities and limitations of linear LDR for NDT of materials are investigated in a parametric way. Both finite element simulations and experiments (using scanning laser Doppler vibrometry) are performed for aluminum alloy and carbon fiber reinforced polymer coupons with flat bottom holes and delaminations ranging in both depth and diameter. The resonance frequencies as well as the associated defect-to-background ratios are parametrically evaluated. For shallow defects, a clear LDR is observed caused by the strong local stiffness reduction at the defect. On the contrary, deep defects are associated with a limited stiffness decrease that results in the absence of LDR behavior. The local stiffness reduction at damages is further exploited using a weighted band power calculation. It is shown that using this technique, deep defects can be detected for which no LDR behavior was observed.