DC effects, sub-harmonics, stochasticity and "memory" for contact acoustic non-linearity

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.