Listening for Nonlinear Defects: A New Methodology for Nonlinear NDE

It is experimentally shown that low-frequency elastic vibrations of near-surface planar defects cause high-frequency ultrasonic radiation in surrounding air. The frequency conversion mechanism is concerned with contact nonlinearity of the defect vibrations and provides efficient generation of air-coupled higher-order ultra-harmonics, combination frequencies, ultra-subhamonics, and ultra-frequency pairs. The nonlinear air-coupled ultrasonic emission is applied for high-resolution imaging of damage-induced defects in a variety of engineering materials.