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.
ABSTRACT
Acoustic wave interaction with non-bonded contacts was found to be inherently non-linear due to asymmetrical stiffness distribution across the interface. A diode-type non-linearity results in local static elastic fields inside the contact, which are shown to be a source of transient longitudinal and shear DC-acoustic pulses polarized oppositely to a biasing contact stress. A parametric modulation of contact stiffness leads to acoustic instability effects, multiple sub-harmonics, and amplitude "self-modulation", and provides chaotic noise-like non-linear acoustic excitations in solids. For realistic cracked flaws, contact acoustic non-linearity exhibits amplitude hysteresis and storage caused by acoustic wave impact on the defect. Maximum storage time observed comes to several hours for read-in time less than half a minute. A long-term non-linear memory is believed to be due to a slow relaxation of thermally induced micro-strain within the crack area.
