Artem Eremin

Forced wave propagation and energy distribution in anisotropic laminate composites.

Elastodynamic response of anisotropic laminate composite structures subjected to a force loading is evaluated based on the integral representations in terms of Greens matrices. Explicit and asymptotic expressions for guided waves generated by a given source are then obtained from those integrals by means of series expansions and the residue technique. Unlike to conventional modal expansions, such representations keep information about the source, giving an opportunity for a quantitative near- and far-field analysis of generated waves. An effective computer implementation is achieved by the use of fast and stable algorithms for the Green matrix, pole, and residue calculations. The potential of the model is demonstrated by examples of anisotropy manifestation in the directivity of radiated waves. The effect of main energy outflow in the direction of either upper- or inner-ply orientation depending on the source size and frequency is discussed.

Lamb wave excitation and propagation in elastic plates with surface obstacles: proper choice of central frequencies

Experimental and theoretical investigations of Lamb wave excitation and sensing using piezo patch transducers and the laser vibrometer technique have been performed, aiming at the development of adequate mathematical and computer models for the interpretation of sensing data and for the choice of optimal parameters for structural health monitoring. The proposed models are validated by experimental results. Furthermore, a methodology is presented which allows for the determination of central frequencies at which maximal values of the structural response spectrum can be expected in the case of wave propagation monitoring with laser vibrometry.

Resonance blocking and passing effects in two-dimensional elastic waveguides with obstacles

Resonance localization of wave energy in two-dimensional (2D) waveguides with obstacles, known as a trapped mode effect, results in blocking of wave propagation. This effect is closely connected with the allocation of natural resonance poles in the complex frequency plane, which are in fact the spectral points of the related boundary value problem. With several obstacles the number of poles increases in parallel with the number of defects. The location of the poles in the complex frequency plane depends on the defects relative position, but the gaps of transmission coefficient plots generally remain in the same frequency ranges as for every single obstacle separately. This property gives a possibility to extend gap bands by a properly selected combination of various scatterers. On the other hand, a resonance wave passing in narrow bands associated with the poles is also observed. Thus, while a resonance response of a single obstacle works as a blocker, the waveguide with several obstacles becomes opened in narrow vicinities of nearly real spectral poles, just as it is known for one-dimensional (1D) waveguides with a finite number of periodic scatterers. In the present paper the blocking and passing effects are analyzed based on a semi-analytical model for wave propagation in a 2D elastic layer with cracks or rigid inclusions.

Group velocity of cylindrical guided waves in anisotropic laminate composites

An explicit expression for the group velocity of wave packets, propagating in a laminate anisotropic composite plate in prescribed directions, is proposed. It is based on the cylindrical guided wave asymptotics derived from the path integral representation for wave fields generated in the composites by given localized sources. The expression derived is theoretically confirmed by the comparison with a known representation for the group velocity vector of a plane guided wave. Then it is experimentally validated against laser vibrometer measurements of guided wave packets generated by a piezoelectric wafer active sensor in a composite plate.

Frequency dependent directivity of guided waves excited by circular transducers in anisotropic composite plates

Lamb wave propagation in fiber-reinforced composite plates is featured by a pronounced directivity of wave energy transfer along the fibers from a point surface source. In the case of non-point (sized) source, the main lobe of radiation diagram may turn with frequency up to the orthogonal to the fibers direction. This effect has been theoretically studied and physically explained in the context of semi-analytical integral-equation based mathematical model. The present paper gives its experimental verification.

Localization of inhomogeneities in an elastic plate using the time reversal method

Theoretical and practical aspects of applying time reversal of elastic waves to localize a source of oscillations or a defect are considered in problems of active ultrasonic monitoring of thin-walled metal structures. Backward reradiation of a time-reversed signal is implemented using a computer model based on a semianalytical integral approach. The proposed algorithm is verified experimentally on aluminum samples excited by piezoelectric wafer active sensors. The results corroborate the possibility of reliably determining of the position and size of the load application region and a local inhomogeneity with a relatively small number of signal measurement points on the sample surface.

Interaction of elastic guided waves with localized three-dimensional thickness changes in plate-like metallic structures

The paper deals with theoretical and experimental investigation into the peculiarities of elastic guided wave interaction with flat-bottom holes simulating pitting corrosion in a metallic plate-like structure. Of particular interest is the resonance scattering. Numerical evaluation of the complex diffraction resonance frequencies relies on the general equations of three-dimensional linear elastodynamics solved with finite element method. The experimental data obtained with contactless laser Doppler vibrometry verify the predicted values of the resonance frequencies, and are further employed for time-reversal base-line free reconstruction of the damage location.

Guided Elastic Waves in CFRP Plates with Random Material Properties

Structural health monitoring (SHM) of modern composite materials, e.g. carbon fiber reinforced plastics (CFRP), is a challenging task. Their complex microstructure has a strong influence on the propagation of ultrasonic guided waves. The classical deterministic finite element (FEM) does not take this fact into account. In the current work, an approach for the simulation of waves in structures with random material properties is presented. An important element in the analysis of uncertain systems is the representation of the stochastic properties in such a manner that they can be implemented in a finite element formulation of the given problem. This is then called stochastic finite elmenent method (SFEM) and can be seen as an extension of the classical FEM. The application of the SFEM to two-dimensional (2D) elastic wave propagation problem is presented. The Youngs modulus is modeled as a 2D random field in two ways. In the first case it is constructed with the Karhunen-Loeve Expansion (KLE) and in the second case, with the integration point method.

Propagation and Attenuation of Elastic Guided Waves in Laminate Fiber-Reinforced Composite Plates

Elastic guided wave phenomenon in modern fiber-reinforced laminates is a complex mechanical process. Along with the amplitude and dispersion directivity of source-induced wave fields conditioned by the microscopic material anisotropy, the effects originating from the microstructure of fibrous composites play a non-neglectable role. Among such features are the wave attenuation due to the polymer matrix viscosity and the continuous mode conversion phenomenon originating from the severe difference between matrix/fiber mechanical properties. Possessing remarkable intensity, these features should be accounted for in ultrasonic non-destructive testing and structural health monitoring systems for the reliable operation. In this work, we investigate their influence on guided wave propagation in unidirectional laminates experimentally and numerically. In the computational model, viscosity driven attenuation is addressed through the complex stiffness matrix, and semi-analytical integral approach is employed for parametric analysis of source-induced guided wave dispersion properties and transient propagation. To handle the continuous mode conversion effect, the concept of spatially varying material properties and the finite element method are used. Experimental measurements are performed for piezoelectrically excited guided waves with scanning laser Doppler vibrometry technique.

Ultrasonic Guided Wave Characterization and Inspection of Laminate Fiber-Reinforced Composite Plates

Computer simulation of wave processes in composite structures is a challenging task due to complicate waveguide properties induced by their anisotropy and lamination. In the present contribution, recent advances in analytically based guided wave (GW) simulation within 3D anisotropic elasticity are present. The wave fields generated in anisotropic laminate structures by surface or buried sources are explicitly expressed via integral and asymptotic expressions in terms of Green’s matrix of the structure considered and the source load. On this basis, a set of low-cost computer models for a reliable quantitative near- and far-field analysis has been developed and experimentally validated. Their abilities are illustrated with several examples: (i) the evaluation of structural frequency response and radiation pattern diagrams for GWs generated by piezoelectric wafer active sensors (PWAS) ; (ii) the reconstruction of effective elastic moduli of fiber-reinforced composite plates based on laser vibrometer measurements; (iii) PWAS frequency tuning with accounting for the wave energy supplied by the source and the radiation directivity caused by the plate’s anisotropy.