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

Review of progress, QNDE

Modelling of nonlinear crack–wave interactions for damage detection based on ultrasound—A review

Review of air-coupled ultrasonic materials characterization.

The purpose of this work is threefold: to investigate material requirements to produce impedance matching layers for air-coupled piezoelectric transducers, to identify materials that meet these requirements, and to propose the best solution to produce air-coupled piezoelectric transducers for the low megahertz frequency range. Toward this end, design criteria for the matching layers and possible configurations are reviewed. Among the several factors that affect the efficiency of the matching layer, the importance of attenuation is pointed out. A standard characterization procedure is applied to a wide collection of candidate materials to produce matching layers. In particular, some types of filtration membranes are studied. From these results, the best materials are identified, and the better matching configuration is proposed. Four pairs of air-coupled piezoelectric transducers also are produced to illustrate the performance of the proposed solution. The lowest two-way insertion loss figure is -24 dB obtained at 0.45 MHz. This increases for higher frequency transducers up to -42 dB at 1.8 MHz and -50 at 2.25 MHz. Typical bandwidth is about 15-20%.

Acoustic impedance matching of piezoelectric transducers to the air

CAN: an example of nonclassical acoustic nonlinearity in solids

Acoustic non-linearity for defect selective imaging.

Air-coupled ultrasound inspection of various materials.

Most of current acoustic imaging systems are “linear” systems, i.e. they image a linear sound field of the frequency identical to that of the acoustic wave injected into the medium. As a result, the image obtained normally provides information on some linear elastic parameters or flaws responsible for sound transmission and reflection inside a sample. Imaging of local elastic nonlinear properties has always been a challenging task with possible promising outcome for seismology, medical diagnostics, material characterisation and NDE applications.

Nonlinear Acoustic Imaging: Fundamentals, Methodology, and NDE-Applications

Quality assurance of safety relevant structures requires fast and reliable techniques of testing. Our contribution shows that nonlinear vibrometry fulfills these demands since it highlights defect areas selectively. This new method works by launching a sinusoidal wave into the inspected sample and measuring the resulting local spectra on the surface by using a scanning laser vibrometer. We present experimental results obtained on various kinds of materials and defects where we applied this method successfully for damage selective imaging.

N. Krohn

Papers

CAN: an example of nonclassical acoustic nonlinearity in solids

Acoustic non-linearity for defect selective imaging.

Air-coupled ultrasound inspection of various materials.

Nonlinear Acoustic Imaging: Fundamentals, Methodology, and NDE-Applications

Nonlinear vibrometry for quality assurance