title: Nonlinear modulation technique for NDE with air-coupled ultrasound.
abstract: The present study is aimed at expanding flexibility and application area of nonlinear acoustic modulation (NAM-) technique by combining the benefits of noncontact ultrasound excitation (remote locating and imaging of defects) with sensitivity of nonlinear methods in a new air-coupled NAM-version. A pair of focused air-coupled transducers was used to generate and receive (high-frequency) longitudinal or flexural waves in plate-like samples. Low-frequency (LF-) vibrations were excited with a shaker or a loudspeaker. Temporal and spectral analysis of the output signal revealed an extremely efficient nonlinear amplitude modulation and multiple frequency side-bands for sound transmission and flexural wave propagation through cracked defects. On the contrary, a negligible modulation was observed for large and medium scale inclusions and material inhomogeneities (linear defects). A new subharmonic mode of the NAM was observed at high excitation levels. It was also shown for the first time that nonlinear vibrations of cracks resulted in radiation of a very high-order harmonics (well above 100) of the driving excitation in air that enabled imaging of cracks remotely by registration their highly nonlinear "acoustic emission" with air-coupled transducers.
authors.affiliation: Department of Physics, Moscow State University, Moscow 119899, Russia.
authors.forename: E M
authors.initials: EM
authors.lastname: Ballad
authors.forename: S Yu
authors.initials: SY
authors.lastname: Vezirov
authors.forename: K
authors.initials: K
authors.lastname: Pfleiderer
authors.forename: I Yu
authors.initials: IY
authors.lastname: Solodov
authors.forename: G
authors.initials: G
authors.lastname: Busse