Ultrasonic Defect Detection of Structural Plates Using Quasi-Rayleigh Waves

Abstract

This article discusses the application of so-called ultrasonic quasi-Rayleigh waves to detect surface defects of mechanical constructions, namely plate structures. The application of quasi-Rayleigh waves allows the extension of the scope of detection using conventional ultrasonic methods that are based on bulk waves. This extension means larger distances as well as higher sensitivity of the detection of surface defects such as fatigue or corrosion cracks. An advantage of this method is the transfer of wave energy from one side of a plate to another, which helps to overcome one-sided obstacles (such as cross-pieces, reinforcement elements). The article describes characteristic properties of quasi-Rayleigh waves that are important for the proper (in terms of frequency in particular) design of the excitation of waves towards the structure. FEM simulation results then provide information regarding the sensitivity of the wave response to the presence and sizes of surface defects (perpendicular slots) in an isotropic material with the properties of steel. The theoretical knowledge is set against experimental measurements obtained with the use of a steel plate with cross-pieces welded to it.KEYWORDS: ultrasonic, quasi-Rayleigh waves, dispersion curves, finite element method, angle beam wedges, steel plate IntroductionUltrasonic methods used to detect defects of mechanical constructions have been widely used and proven for several decades. They are used in the detection of faults in critical structural points, e.g. welds or rails. The assessment of presence of a certain defect is based on the measurement of response to the excitation of longitudinal and traverse (shear) bulk waves. The main drawback of those methods is a relatively high rate of attenuation with distance and interaction with surface elements, which cause the reflection of waves and thus make the assessment of wave response more complicated. This is why their use is limited to the examination of local, relatively small structural parts.Recently, one area of focus for development has been the use of guided waves that are able to propagate over longer distances with relative low wave energy losses. Central to the research is the capability of such waves to respond to the occurrence of a particular fault and then transmit the indication to a wave sensing point. The main impediment in the development and application of those methods is the fact that guided waves are able to exist only in materials with certain cross-sectional shapes (beams, panels, bars). One type of guided waves is quasi-Rayleigh waves as a group of Rayleigh-Lamb waves. Quasi-Rayleigh waves are able to propagate in flat structural members (Fig.1), whether straight or rounded.Plate elements have many uses in practice, including aircraft structures, pressure vessels, load-carrying members, etc. Quasi-Rayleigh waves lend themselves to the detection of surface defects in such parts, even over extended distances. Their great benefit is the ability of guided wave energy transmission betw...

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