The nondestructive assessment of the damage that occurs in components during service plays a key role for condition monitoring and residual life estimation of in-service components/structures. Ultrasound has been widely utilized for this; however most of these conventional methods using ultrasonic characteristics in the linear elastic region are only sensitive to gross defects but much less sensitive to micro-damage. Recently, the nonlinear ultrasonic technique, which uses nonlinear ultrasonic behavior such as higher-harmonic generation, subharmonic generation, nonlinear resonance, or mixed frequency response, has been studied as a positive method for overcoming this limitation. In this paper, overall progress in this technique is reviewed with the brief introduction of basic principle in the application of each nonlinear ultrasonic phenomenon.

Nonlinear ultrasonic techniques for nondestructive assessment of micro damage in material: A review

Ultrasonic testing of materials: Josef and Herbert Krautkramer Springer-Verlag Publishing Company, Berlin, Heidelberg, New York (1975) 669 pp, $63.00

Review of progress, QNDE

Soviet Physics JETP

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites.

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Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components.

A self-modulation mode of nonlinear acoustic vibrations of cracked defects has been analyzed and observed experimentally. Spectral patterns of the self-modulation are attributed to the sum-frequency type resonance in a multi-degree-of-freedom system and include a subharmonic spectrum as a particular case. Both the subharmonic and self-modulation modes demonstrate a high localization around the defect and provide opportunities for early detection and recognition of damaged areas.

Nonlinear self-modulation and subharmonic acoustic spectroscopy for damage detection and location

It is experimentally shown that, to provide maximum acoustic wave-defect interaction, the concept of a local defect resonance should be applied. The model of a resonant defect is used for the selection of the wave frequency to enhance the excitation of the defect in nonlinear acoustics and ultrasonic thermography. An increase in nonlinear response of the defect at its local resonance exceeds substantially the one at natural frequencies of the specimen. The strong wave-defect interaction is confirmed by resonance induced rise of local temperature of the defect in the frequency band of its local resonance.

A local defect resonance to enhance acoustic wave-defect interaction in ultrasonic nondestructive evaluation

Unlike conventional resonant ultrasonic spectroscopy aimed at determining elastic constants and related parameters of solids, resonant ultrasound spectroscopy of defects (RUSOD) addresses an opportunity to detect, visualize, and classify mechanical defects in materials. The approach is based on the resonant ultrasonic wave-defect interaction due to local defect resonance. RUSOD is shown to be defect- and frequency selective imaging technique capable of distinguishing between different defects by variation of ultrasonic frequency.

Resonant ultrasound spectroscopy of defects: Case study of flat-bottomed holes

It is shown that low-frequency elastic vibrations of near-surface planar defects cause high-frequency ultrasonic radiation in surrounding air. The frequency conversion mechanism is concerned with contact nonlinearity of the defect vibrations and provides efficient generation of air-coupled higher-order ultraharmonics, ultrasubharmonics, and combination frequencies. The nonlinear air-coupled ultrasonic emission is applied for location and high-resolution imaging of damage-induced defects in a variety of solid materials.

Nonlinear air-coupled emission: The signature to reveal and image microdamage in solid materials

In this paper, the effect of local defect resonance (LDR) on the nonlinear ultrasonic responses of defects is studied and applied for enhancement of sensitivity of nonlinear NDE. Unlike the resonance of the whole specimen, the LDR provides an efficient energy pumping from the wave directly to the defect and causes an efficient generation of the higher harmonics and wave mixing even at moderate input signals. At higher levels of excitation, a combined effect of LDR and nonlinearity results in qualitatively new “nonclassical” features characteristic of the nonlinear and parametric resonances. The resonant nonlinear defects demonstrate threshold dynamics of instable vibrations, hysteresis, super- and subharmonic resonances. Under nonlinear LDR conditions nearly total input energy can be converted into higher harmonic or subharmonic vibrations of the defect. This proposes nonlinear LDR application as an extremely efficient and sensitive mode for nonlinear imaging and NDE.

Igor Solodov

Papers

Nonlinear self-modulation and subharmonic acoustic spectroscopy for damage detection and location

A local defect resonance to enhance acoustic wave-defect interaction in ultrasonic nondestructive evaluation

Resonant ultrasound spectroscopy of defects: Case study of flat-bottomed holes

Nonlinear air-coupled emission: The signature to reveal and image microdamage in solid materials

Resonant Acoustic Nonlinearity of Defects for Highly-Efficient Nonlinear NDE