The aim of the present work is to develop a system of smart devices that could be permanently attached on the surface of the composite structure and monitor the interaction of low-frequency Lamb waves with defects. A linear array of transmitters would generate a relatively uniform wavefront allowing the inspection of large areas with a limited number of sensors. The asymmetric A0 Lamb mode is generated in carbon fibre reinforced plastic (CFRP) quasi-isotropic laminates using an array of thin piezoceramic transmitters operating in-phase. In this paper, the effect of damage size on the propagation of Lamb waves is presented. Experimental verification is also presented in multidirectional CFRP composite panels. Critical size impact damage is detected. Finally, the technique is applied to a stiffened panel. Damage on the skin or flange is detectable while damage on the web or cap is not detected with the current experimental set up.

Piezoelectric transducer arrangement for the inspection of large composite structures

This study concentrates on the reflection of Lamb waves at the free end of a plate. The conversion phenomena are examined in detail over a large frequency range and the energy conversion coefficients are obtained by three different ways: theoretically, numerically (finite element method) and experimentally. The experimental energy determination is obtained from the measurement of the plate normal displacements, by mean of a laser interferometer. All results are in relatively good agreement and the energy balance between incident and reflected waves is discussed in each case.

Lamb wave reflection at the free edge of a plate.

This paper demonstrates the effectiveness of certain structural health monitoring (SHM) strategies based on propagation of guided elastic waves within thin-walled shell structures. This is realized based on results of numerical and experimental investigations obtained by the use of the spectral finite element method (SFEM) together with laser scanning vibrometry (LSV) and subsequent application of two different integral-based indices for damage quantification, these being the integral mean value (IMV) and the root mean square (RMS). Numerical tests by SFEM were carried out for a square plate with through-hole damage, a cracked fuselage section with two stiffeners and a cracked wing section skin, all made out of aluminium. Experimental measurements by LSV included tests on a square aluminium plate, composite plate and a composite stabilizer of a PZL W-3A helicopter, all with simulated damage. During numerical and experimental investigations damage sensitivity of both IMV and RMS indices was tested by considering various displacement components, signal timescales or weighting factors. A certain practical method for quick construction of RMS damage maps based on experimental measurements by LSV was also presented.

https://iopscience.iop.org/article/10.1088/0964-1726/21/3/035024/pdf

Damage detection strategies based on propagation of guided elastic waves

Laser ultrasonics is used to optically excite and detect acoustic waves at the interface between a liquid and a solid or coated solid. Several case studies show that this technique is feasible to investigate experimentally the theoretically predicted fundamental properties of different aspects of interface waves at liquid–solid interfaces and to characterize the elastic properties of soft solids. The theoretical prediction that the leaky Rayleigh (LR)-type root of the characteristic determinant becomes forbidden when the shear velocity of the solid lies below the bulk velocity of the liquid was experimentally confirmed. The depth profiling and nondestructive testing potential of Scholte waves was experimentally illustrated and explained by the properties of the wave displacement profile.

On the character of acoustic waves at the interface between hard and soft solids and liquids.

The resonance for the elastic plate with a free edge is studied from the point of view of complex resonance. The variations of the real part and of the imaginary part of the complex resonance frequency as a function of the Poisson ratio ν are determined numerically. The results confirm the real resonance frequency theoretically predicted in I. Roitberg et al., Q. J. Mech. Appl. Math. 51, 1–13 (1998) for a zero Poisson ratio ν1=0, and a real resonance frequency that corresponds to a Lame mode is discovered for a Poisson ratio ν2=0.2248. It is shown that both real resonance frequencies may exist, at these two particular values of ν, because of the decoupling between the propagating Lamb mode and the set of evanescent Lamb modes.

Revisiting the edge resonance for Lamb waves in a semi-infinite plate

This paper deals with the reflection of a two-dimensional harmonic Lamb wave at the beveled end of a plate. The existence of a resonant edge mode is described by a numerical model. It is proved that the edge mode is the resonance of different complex modes. The behavior of this mode as a function of the bevel angle is studied. Its amplitude decreases and its resonance frequency shifts as the bevel angle decreases from 90 to 85 deg. An unexpected strong variation of the repartition of the reflected energy is linked to this phenomenon.

Numerical description of the edge mode at the beveled extremity of a plate.

The resonance scattering theory (RST) and the singularity expansion method (SEM) are both based on the complex-frequency poles of the scattering amplitude in the scattering of acoustic, elastic, or electromagnetic waves from elastic or impenetrable objects, or from cavities. These poles, situated off the real frequency axis at locations with negative imaginary parts, are found to yield, at the real frequencies of the experiments, prominent resonances for acoustic and elastic-wave scattering from elastic objects as discussed in our earlier review (Uberall et al, Appl Mech Rev43 (10), 1990, 235). However, as the authors demonstrated before (Uberall et al, J Acoust Soc Am61, 1977, 711), the origin of these resonances lies in the phase matching of circumferential or surface waves generated on the target objects during the scattering; hence a study of the resonances will lead to an understanding of, and information on these surface waves. This has been the topic of a large number of studies in recent years, and the results are summarized in the present review for immersed elastic target objects of plane, spherical, and cylindrical geometry, including both elastic-type and fluid-borne surface waves. For multilayered elastic structures, we also describe possible layer-resonance identifications based on acoustic and elastic-wave scattering experiments.

Acoustic Scattering Resonances: Relation to External and Internal Surface Waves

The Scholte wave propagating at a liquid/solid interface is studied. A device associating an interdigital transducer and a thin piezoelectric film generates such a wave on dielectric substrates; appropriate sticking allows propagation on conducting mediums. The location of the Scholte wave was experimentally checked, its velocity was measured, its weak attenuation was verified, how it decays was widely examined in the liquid medium perpendicular to the interface.

Jean Duclos

Papers

Lamb wave reflection at the free edge of a plate.

Numerical description of the edge mode at the beveled extremity of a plate.

Acoustic Scattering Resonances: Relation to External and Internal Surface Waves

Scholte wave characterization and its decay for various materials

Experimental study and signal analysis in the Lamb wave conversion at a bevelled edge