The effect of local defect resonance (LDR) on the nonlinear ultrasonic response of defects is studied and applied for enhancement of sensitivity of nonlinear ultrasonic NDT. Unlike the resonance of the whole specimen, the LDR naturally provides an efficient energy pumping from the wave directly to the defect. For simulated and realistic defects in various materials, the LDR-induced local increase in the vibration amplitude averages up to ~ (20-40 dB). As the local vibration amplitude increases, the LDR-“amplifier” exhibits transition to nonlinear regime with an efficient generation of nonlinear frequency components solely in the defects area. In addition, the concept of the defect as a nonlinear oscillator brings about different dynamic and frequency scenarios in its nonlinear behaviour characteristic of the parametric oscillations. The experiments confirm unconventional nonlinear dynamics of simulated and realistic defects subject to LDR. The threshold transition to the nonlinear modes is accompanied by instability of vibration with “jumps” of the amplitude measured. The nonlinear modes observed include sub- and superharmonic resonances with anomalously efficient generation of the higher harmonics and subharmonics. A modified version of the superharmonic resonance (combination frequency resonance) is used to enhance the efficiency of frequency mixing mode of nonlinear NDT. A strong localization of the resonance nonlinearity in the defect area is applied for high-contrast imaging of defects (delaminations, impacts, heat-induced damage) in composite materials.
Acoustic wave-defect interaction is a background of ultrasound activated techniques for imaging and NDT of materials and industrial components. The interaction, first, results in acoustic response of a defect, which provides attenuation and scattering of ultrasound used as an indicator of defects in conventional ultrasonic NDT. The derivative ultrasonic-induced effects include nonlinear, thermal, acousto-optic, etc. responses and are also used for NDT and defect imaging. These secondary effects are usually comparatively inefficient so that the corresponding NDT techniques require an elevated acoustic power and differ from conventional ultrasonic NDT set-ups for their specific instrumentation particularly adapted to high-power ultrasonics.
In this paper, a consistent way to enhance acoustic, optical and thermal defect responses is suggested by using selective ultrasonic activation of defects based on the concept of local defect resonance (LDR). A straightforward phenomenology and the finite element simulation are developed to evaluate the fundamental LDR frequencies of flat bottomed holes (FBH), also applicable to laminar defects in rolled sheet metals and delaminations in composites. The LDR provides a selective excitation of a defect resulting in a strong local vibration amplitude and an enhancement of the defects’ thermal and optical responses measurable even for a few mW acoustic input. This proposes LDR application as an efficient and sensitive mode for thermosonic and optical defect-selective imaging in NDT.
To experimentally reveal LDR, an ultrasonic excitation by a wide-band piezoelectric transducer is combined with a laser vibrometer scan of the specimen surface. For both simulated and realistic defects, the LDR-induced local resonance “amplification” of the vibration amplitude averages up to 20-40 dB. A strong increase in vibration amplitude at LDR enables to reliably detect and visualize the defect as soon as the driving ultrasonic frequency is matched to its LDR frequency. This also provides a high frequency selectivity of the LDR-based imaging, i.e. an opportunity of detecting a specific defect among a multitude of different defects. Multiple case studies demonstrate a strong increase in sensitivity for both optical (laser vibrometry and shearography) and thermosonic frequency-selective imaging of defects (FBH, delaminations, impacts, heat-induced damage) in a variety of materials and components.