Nonlinear Acoustic Response of Damage Applied for Diagnostic Imaging

Abstract

Nonlinear acoustic material response, which is inherently related to the frequency changes of elastic waves, has long been identified as a sensitive instrument for material characterization and nondestructive evaluation (NDE). However, a bottleneck problem on the way of applications of nonlinear NDE is found to be a low efficiency of conversion from fundamental frequency to nonlinear frequency components. Fortunately, the situation changes for the better in the case of localized damaged areas, whose nonlinear acoustic response can be enhanced dramatically. The two major factors that contribute to the nonlinearity of cracked defects are concerned with specific type of acoustic nonlinearity of the damage fragments and a local mechanical resonance of the damaged area. The combination of these features turns the defect into a nonlinear oscillator that manifests peculiar nonlinear dynamics of local vibrations with efficient generation of higher harmonics and combination frequencies even at moderate excitation level. A frequency match to the damage resonance results in qualitatively new features characteristic of nonlinear and parametric resonances that provide an instable growth of local nonlinear vibrations. The resonant vibrations are strongly confined in the defect area that brings about an opportunity for high-resolution defect-selective imaging and proposes nonlinear resonant application as an extremely efficient means for nonlinear NDE and diagnostic imaging of damage. Due to the high efficiency of nonlinear frequency conversion, the resonant response of damage requires substantially lower input power to energize the defects that enables to avoid high-power instrumentation and even to realize for the first time noncontact nonlinear imaging via airborne sonic activation.