Nonlinear NDT: A Route to Conventional Ultrasonic Testing

The bottleneck problem of nonlinear NDT is a low efficiency of conversion from fundamental frequency to nonlinear frequency components. In this paper, it is proposed to use a combination of nonlinearity with Local Defect Resonance (LDR) to enhance substantially the input-output conversion. Since LDR is an efficient resonance “amplifier” of the local vibrations, it manifests a profound nonlinearity even at moderate ultrasonic excitation level. As the driving frequency matches the LDR-frequency band, a strong enhancement of the higher harmonic amplitudes generated locally in the defect area is observed. Besides a strong higher harmonic response, a high quality factor of LDR can also be used for efficient frequency mixing nonlinear NDT.

         The “conventional” nonlinear effects, like higher harmonic generation and wave mixing are not the only dynamic scenario of nonlinear phenomena for resonant defects. At higher level of excitation, a combined effect of LDR and nonlinearity results in qualitatively new features characteristic of nonlinear and parametric resonances. Under resonance conditions nearly total input energy at fundamental frequency can be converted into higher harmonic or subharmonic vibrations of the defects. Both super- and subharmonic LDR are strongly localised in the defect area that provides a background for highly-sensitive defect-selective imaging. LDR mode of nonlinear NDT requires much lower acoustic power to activate nonlinearity of defects that makes it possible to avoid high-power ultrasonic instrumentation and switch to conventional ultrasonic equipment instead.