Fundamental S0 Research Articles

Analytical and experimental analysis of guided waves in an aluminum plate under bending load

Although guided waves offer great potential for monitoring various structures, interpreting signals from piezoelectric sensors remains a challenging task. One main reason is the significant influence of environmental conditions on the wave propagation. A lot of research has already been done on the influence of temperature effects and recently more attention has been shifted towards loads. While previous publications have mainly focused on uni- or bi-directional loads, this publication expands the developed models to include bending loads.After reviewing the analytical basis of acoustoelasticity, the derived equations are expanded to nonhomogeneous elastic bending loads using the partial wave method. The analysis is completed using recent results developed by C. Hakoda and C. J. Lissenden (2018) [1], that gave more physical insight in the propagation of guided waves in various frequency-bands.The focus of the experimental analysis is around the fundamental S0- and A0-Modes of Lamb waves. To validate the analytical results an aluminum plate is instrumented using piezoelectric transducers and loaded with varying bending loads. The experimental results are in good agreement with the analytical theory and demonstrate the influence of bending prestress on guided wave propagation. Based on these results an innovative measurement method for bending loads is developed, that is robust to small temperature changes.

Meta-structure enhanced second harmonic S0 waves for material microstructural changes monitoring

Cumulative second harmonic Lamb waves in nonlinear media feature increasing amplitudes with propagation distance, conducive to the monitoring of material microstructural changes in structures. The phenomenon can be readily generated by zero-order symmetric (S0) mode waves in the low-frequency range. However, in a practical piezoelectric-transducer-activated system, both S0 and A0 (zero-order antisymmetric) mode Lamb waves are inevitably excited, while only the former is responsible for cumulative effects. The generation efficiency of the cumulative second harmonics is then affected by the presence of the A0 waves. To tackle the problem, this study develops a metamaterial structure, referred to as a meta-structure, to tactically enhance the cumulative second harmonic S0 Lamb waves by converting the A0 mode components into S0 mode waves. Topology optimization is conducted to design the meta-structure, which is surface-mounted onto the structure under inspection, to achieve high-efficiency A0-to-S0 wave mode conversion. Through tuning the parameters and constraints of the optimization, the designed single-sided meta-structure breaks the structural symmetry in the thickness direction, while facilitating its practical implementation. Typical scenarios with different meta-structure materials are discussed. Numerical simulations demonstrate that the strain amplitudes of the fundamental S0 mode waves can be increased by 60% with the deployment of the meta-structure, alongside an enhancement of the second harmonic S0 mode waves at different sensing distances. Finally, the designed meta-structure is fabricated via 3D printing technique and tested experimentally on an aluminum plate subjected to thermal aging treatment for monitoring the heating-induced microstructural changes inside the structure. Experimental results confirm an increase in the wave amplitudes of the linear S0 mode waves with the assistance of the meta-structure. The developed system improves the sensitivity of nonlinear Lamb wave-based monitoring methods in characterizing material microstructural changes, which shows great promise for detecting incipient damage in practical structural health monitoring applications.

Equivalent elastic modulus measurement of cross-ply composite plates using Lamb waves

This paper proposes a simple and effective Lamb wave-based measurement method for the elastic moduli of cross-ply composite plates, considered equivalent to a single-layer orthotropic plate with nine independent elastic constants. The effects of theses constants on the phase velocities of the fundamental S0 and A0 modes are investigated. The phase velocity of the S0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson’s ratio in the low-dispersive frequency-thickness product regime, whereas the phase velocity of the A0 mode depends almost only on the equivalent shear elastic modulus. Moreover, the in-plane Poisson’s ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S0 mode. Therefore, the tensile and shear elastic moduli of the plates can be estimated using the S0 and A0 modes, and the mapping relations between the phase velocities of these modes and the elastic moduli are established. The proposed approach has been numerically and experimentally tested on cross-ply carbon fiber reinforced laminates, and validated by theoretical values and conventional tensile tests.

Dispersion curves and identification of elastic wave modes for fiber metal laminates

The current work is devoted to the problem of the validation of dispersion curves and numerical simulations of fundamental Lamb wave and shear horizontal elastic wave modes for fiber metal laminates (FML). In the present study, two types of FM laminates are considered. In the first case, the studied composite material consists of 7 layers: one layer is made of aluminum alloy AW-6060 while the rest of the layers are made of carbon/epoxy resin. In the second case, the FML is constructed of one layer of aluminum alloy and the rest six layers are made of glass/epoxy. In order to extract the dispersion curves for the fundamental S0, SH0, and A0 modes, as well as higher modes from the range 5 up to 1000 [kHz], the stiffness matrix method, is employed. In the first case the computations are performed for the following cross-ply stacking sequences: [Al, ±5°6], [Al, ±15°6], [Al, ±25°6], … , [Al, ±85°6]. Next, the FEM simulations of elastic wave propagation are performed in the case of plates, which are made of analyzed FML composites. In the first studied FML material, the fundamental symmetric mode S0 is generated while in the second case the fundamental antisymmetric mode A0 is excited. It occurred that in the case of aluminum alloy/carbon fibers the generation of the pure fundamental symmetric mode S0 is very difficult. In addition, the fundamental symmetric mode S0, the shear horizontal SH0, and the antisymmetric A0 wave modes are also present. In the case of aluminum alloy and carbon/epoxy, the pure antisymmetric mode A0 can be relatively easily generated. However, a significant dispersion phenomenon is visible. For the aluminum alloy and glass/epoxy, the obtained results are experimentally verified.

Controlling Bragg gaps induced by electric boundary conditions in phononic piezoelectric plates

A Phononic Crystal (PC), constituted of a homogeneous piezoelectric plate covered by a 1D periodic arrangement of thin metallic electrodes on both surfaces, is studied. The application of Electric Boundary Conditions (EBCs) on the electrodes enables the propagation control of the ultrasonic guided waves in the PC. The band structure is investigated for different EBCs: the electrodes are either at a floating potential or they are alternately short-circuited and at a floating potential. In the latter case, a Bragg gap appears for the fundamental S0 guided Lamb mode. These results are verified experimentally and compared to finite element calculations. A physical interpretation is also given, relying on the symmetry of the electric potential fields associated with these guided modes.

High-Frequency Guided Wave Scattering by a Partly Through-Thickness Hole Based on 3D Theory**Supported by the National Natural Science Foundation of China under Grant Nos 11474195, 11274226 and 61171145.

We present a theoretical investigation of the scattering of high frequency S0 Lamb mode from a circular blind hole defect in a plate based on the 3D theory. The S0 wave is incident at the frequency above the A1 mode cut-off frequency, in which the popular approximate plate theories are inapplicable. Due to the non-symmetric blind hole defect, the scattered fields will contain higher order converted modes in addition to the fundamental S0 and A0 modes. The far-field scattering amplitudes of various propagating Lamb modes for different hole sizes are inspected. The results are compared with those of lower frequencies and some different phenomena are found. Two-dimensional Fourier transform (2DFT) results of transient scattered Lamb and SH wave signals agree well with the analytical dispersion curves, which check the validity of the solutions from another point of view.

The High-frequency Scattering of the S0 Lamb Mode by a Circular Blind Hole in a Plate

The scattering problem of an incident high-frequency S0 Lamb wave in a plate with a circular blind hole is investigated. The study is not limited to the low-frequency range of this wave, in which the popular approximate plate theories are inapplicable. A 3D analytical approach is used where the wave fields are expanded in all possible Lamb modes, including propagating and evanescent modes. Due to the non-symmetric blind hole defect, the scattered fields will contain higher order converted modes in addition to the fundamental S0 and A0 modes. The far field scattering amplitudes of various propagating Lamb modes for different hole sizes are inspected. The results are compared with those of low frequencies and some different phenomena are found.

Peculiarities of the surface and guided waves propagation in heterogeneous composites with nematic coatings

AbstractUsing the effective low frequency model of nematic medium we study the phase speeds and attenuations of the quasi‐Rayleigh and Love waves in composite made of elastic core with the peripheral coating, as well as the fundamental S0 and A0 Lamb modes in heterogeneous plates. The analytical and numerical analysis revealed two types of unusual resonance effects disturbing wave propagation. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved step scan FTIR spectroscopy and DFT investigation on triplet formation in Peridinin-Chlorophyll-a-Protein from Amphidinium carterae at low temperature

We have used time-resolved step-scan FTIR spectroscopy to investigate the process of triplet formation in the peridinin–chlorophyll-a–protein (PCP) at 100 K. Results have led to the identification of possible marker bands for peridinin (Per) and 3Per. Band assignment has been carried out performing a DFT investigation on Per, 3Per and on model molecules. The main spectral feature is a couple of bands at 1746 (negative) and 1719 (positive) cm–1. The assignment of these two bands to the lactonic C═O of Per in (respectively) its fundamental S0 and triplet T1 state is based on: (1) comparison with FTIR spectra of isolated Per in literature; (2) good agreement with DFT calculations for Per and 3Per; (3) the downshift of the band upon triplet formation as predicted by DFT calculations in Per and similar lactonic systems; (4) the kinetic of band disappearance, in agreement with literature data. Another possible band for Per in its fundamental state has been identified at 1523 cm–1.

Time-resolved step scan FTIR spectroscopy and DFT investigation on triplet formation in peridinin–chlorophyll-a–protein from Amphidinium carterae at low temperature

We have used time-resolved step-scan FTIR spectroscopy to investigate the process of triplet formation in the peridinin–chlorophyll-a–protein (PCP) at 100 K. Results have led to the identification of possible marker bands for peridinin (Per) and3Per. Band assignment has been carried out performing a DFT investigation on Per,3Per and on model molecules. The main spectral feature is a couple of bands at 1746 (negative) and 1719 (positive) cm–1. The assignment of these two bands to the lactonic C═O of Per in (respectively) its fundamental S0 and tripletT1state is based on: (1) comparison with FTIR spectra of isolated Per in literature; (2) good agreement with DFT calculations for Per and3Per; (3) the downshift of the band upon triplet formation as predicted by DFT calculations in Per and similar lactonic systems; (4) the kinetic of band disappearance, in agreement with literature data. Another possible band for Per in its fundamental state has been identified at 1523 cm–1.The results are in overall agreement with a recent step-scan FTIR study on PCP at 298 K (Alexandre et al.,Biophys. J. 93 (2007), 2118–2128) even though some small discrepancies emerge, probably related to the different temperature at which experiments were carried out. Further DFT calculations have been performed to rationalise these spectral discrepancies. From a methodological point of view, the work demonstrates the potential of a combined step-scan FTIR/DFT approach in the study of photophysical processes in proteins. Furthermore, it has been shown that, when small differential IR signals are concerned, nagging photothermal effects can interfere and can possibly lead to misleading interpretations.