Far-sided defect recognition of FRP sandwich structures based on local defect resonance

Abstract

This work provides a novel methodology to non-destructively detect far-sided defect of sandwich structures by local defect resonance-based acoustic activation. Sandwich panel specimens with 10 mm thick balsa wood core and 25 mm thick Nomex honeycomb core are investigated, and artificial facesheet-core disbond with a size of 30 mm × 30 mm is introduced to the specimens. Finite element modeling is constructed to investigate resonance frequency of defect region. Experimental apparatus is constructed to detect far-sided facesheet-core disbond defect, where vibration activation and measurement is performed at near-sided facesheet. It is shown that if the far-sided defect is activated at local defect resonance frequency, distinguishable nonlinear vibration characteristic could be observed from the near-sided facesheet, and the 3rd order superharmonic is especially significant. A stepped frequency sweep excitation approach is further developed for recognition of far-sided defect, which better fits automated defect detection process as local defect resonance frequency is not required to be preliminarily determined.