Ultrasonic frequency mixing via local defect resonance for defect imaging in composites

A new approach to nonlinear frequency mixing based on local damage resonance is proposed, analysed and tested experimentally for flexural waves in composites. The method is free from stringent requirements on the mode types and frequencies for interacting waves. The resonance of damage enhances strongly its higher-order nonlinear response and boosts the efficiency of generation for numerous-order combination frequencies. The damage resonance combined with its strong nonlinearity also provides locality of nonlinear interaction even for continuous wave operation. The combination frequencies generated locally in the damaged area are the footprints of damage and used for its detection, location and visualization. A single C-scan yields a number of images of the defect corresponding to various nonlinearly generated frequencies. Various versions of the resonant frequency mixing are considered and applied to nonlinear imaging of defects in composite materials.