Egidijus Žukauskas

Validation of Dispersion Curve Reconstruction Techniques for the A0 and S0 Modes of Lamb Waves

The properties of ultrasonic Lamb waves, such as relatively small attenuation and high sensitivity to structural changes of the object being investigated, allow performing of non-destructive testing of various elongated structures like pipes, cables, etc. Due to the dispersion effect of Lamb waves, a waveform of the received informative signal is usually distorted, elongated and overlapping in the time domain. Therefore, in order to investigate objects using the ultrasonic Lamb waves and to reconstruct the dispersion curves, it is necessary to know the relationship between frequency, phase and group velocities and thickness of the plate. The zero-crossing technique for measurement of phase velocity of Lamb waves (the A0 and S0 modes) has been investigated using modelled dispersed signals and experimental signals obtained for an aluminium plate having thickness of 2 mm. A comparison between two reconstruction methods of Lamb wave phase velocity dispersion curves, namely, the two-dimensional fast Fourier transform (2D-FFT) and zero-crossing technique, along with the theoretical (analytical) dispersion curves is presented. The results indicate that the proposed zero-crossing method is suitable for use in reconstruction of dispersion curves in the regions affected by strong dispersion, especially for the A0 mode.

Investigation of accurate imaging of the defects in composite materials using ultrasonic air-coupled technique

In this paper air-coupled ultrasonic investigation of delamination type defects in GLARE3-3/2 composite material is presented. An air-coupled ultrasonic measurement technique has a great potential for investigation of those materials in manufacturing and maintenance. An important advantage of this technique is that investigations can be performed without couplants and therefore various materials that can absorb liquids (honeycombs, porous) can be tested. The objective of the work was to investigate the interaction of the ultrasonic wave with delamination type defect in GLARE3-3/2 composite material and to determine influence of interaction effects on accuracy of the defect size measurement. Numerical and experimental investigation was carried-out in the through transmission mode. Results of numerical investigation and experimental verification of interaction of the ultrasonic wave are presented. Estimation of accuracy of the measurement of the delamination defect sizes is also presented.

Analysis of Ultrasonic Guided Waves Propagation in Complex Composite Structures

The object of the investigation is a honeycomb structure of composite sandwich made of glass-epoxy laminating layers and a honeycomb core of epoxy impregnated paper. Large composite tanks possessing cylindrical shape are produced using the winding process. Therefore, the final products have uneven thickness and fibre orientation lamina layers, and also an unevenly impregnated honeycomb layer. The aim of this research is to develop an economically attractive embedded ultrasonic measurement technique for on-field diagnostics of complex composite structures used for production of large liquid storage tanks. Development of the relatively cheap and easy to operate embedded diagnostic/monitoring technique is important aiming to assure safety of liquid storage tanks (monitoring structural integrity against overpressure, etc.) and detection of accidental defects that may appear during transportation, installation and exploitation of those structures. Typical defects that are aimed to be detected are relatively large delaminations/disbonds (area having diameter of 150–200 mm) of skin layers caused by low energy impacts that cannot be detected visually and show severe influence on the structural strength and safety of liquid tanks. This work presents results of numerical modeling and experimental research in low frequency (50 kHz) ultrasonic guided waves (UGW) propagation in large honeycomb composite structures. Finite element (FE) simulation of UGW propagation has been made aiming to reduce quantity of ultrasonic transducers and optimize their placement on composite structure. The possibilities to place ultrasonic transducers and receivers on both sides (internal or external) of composite structures and to use such a proposed technique for detection of delamination/debonding areas caused by low energy impacts or alternating semi static loads (e.g. filling and draining of liquid from storage tank) and monitoring of partial self-healing of relatively rigid composite structures were proved by experimental testing.

Energo-Mechanical Evaluation of Damage Growth and Fracture Initiation in Aviation Composite Structures

ABSTRACT The aim of the present paper is to (1) highlight the results of laboratory damage detection and monitoring in the aviation composite materials, during a mechanical testing constituted of multiple loadings, and (2) obtain a detailed understanding of damage evolution of composite specimens with regard to impact energy. Woven 12-ply glass fiber and 16-ply carbon fiber–reinforced epoxy composites (GFRP 92 125/L285/287 and CFRP 98 131/L285/287) were used as less studied subjects in research. This study explored the resistance to cracking and delamination of glass and carbon fiber laminates with the same resin system under low-load conditions. GRAPHICAL ABSTRACT

Propagation of Ultrasonic Shear Horizontal Waves in Rectangular Waveguides

The aim of this paper is to investigate the propagation of ultrasonic shear horizontal guided waves along waveguides with a rectangular cross-section and with a finite constant and variable width and to determine the peculiarities of propagation of those waves. The dispersion curves of guided waves in finite-width waveguides were modeled by using a semi-analytical finite element (SAFE) technique. The propagation of pulsed shear horizontal ultrasonic guided waves was investigated numerically by using 3D finite element modeling. It was found that in the case of finite-width waveguides, the SH0 shear horizontal wave splits into a family of SH-type dispersive modes propagating with different phase velocities. It was also found that the number of propagating modes depends on the width-to-thickness ratio. The first time spatial distributions of pulsed displacements across the waveguide were determined for waveguides of different widths. Investigation of the waveguides with a rectangular cross-section and varying lateral dimensions was performed. It was found that by properly selecting the geometry of the transient zone of waveguides with a rectangular cross-section, it is possible to improve the performance of such waveguides, e.g. to increase the amplitude of the transmitted pulse type signal without significant distortions of the waveforms.

Numerical Investigation of Propagation of Ultrasonic Waves in the Waveguides with Mode Conversion

Ultrasonic investigation techniques are widely used in materials characterisation and non-destructive testing applications. In special cases of applications, such as investigation of properties of melted polymers, metals and hot liquids, measurements must be performed in a wide temperature range. However conventional piezoelectric transducers cannot withstand higher temperatures than the Curie temperature. Therefore in order to protect conventional ultrasonic transducers from influence of a high temperature and to avoid depolarization, measurements must be performed using special waveguides with a low thermal conductivity between the object under investigation and the ultrasonic transducer. For measurements of the material properties, such as viscoelastic properties of materials, additional shear wave transducers must be used. In this work approach how to excite both, longitudinal and shear waves using special waveguides with mode conversion, using pair of conventional ultrasonic longitudinal wave transducers is presented. In this work numerical investigation of propagation of longitudinal and shear ultrasonic waves in the waveguides with mode conversion using finite element method and CIVA software was carried out. Modelling of propagation of simultaneously generated longitudinal and shear waves using pair of longitudinal ultrasonic transducers was performed. Influence of temperature gradient to the required incidence angle of the longitudinal wave was evaluated.