Mode selectivity of SH guided waves by dual excitation and reception applied to mode conversion analysis

SH guided waves, generated by periodic permanent magnet arrays have been used previously in non-destructive evaluation of metal plates and pipes. When an SH guided wave interacts with a defect or change in sample thickness, the incident SH wave may undergo mode conversion. Analysis of mode conversion is complicated, due to the interference of several propagating modes in the received signal, that can often temporally overlap. This paper proposes a mode selection technique to help understand the interaction of SH guided waves with changes in sample thickness. Using an understanding of the propagation characteristics of the guided waves, SH guided waves are sequentially generated and detected on both surfaces of the plate, capturing four distinct waveforms. By superposition of the detected signals, symmetric modes can be clearly separated from antisymmetric modes in the processed, received signals. For this method to work well, the transducers used should have very similar responses and be precisely positioned on exactly opposite positions either side of the plate. Finite element simulations are also performed, mirroring the experimental measurements, and the results correlate well with the experimental observations made on an 8 mm thick plate with a region of simulated wall thinning machined into the sample

Machine learning-based corrosion-like defect estimation with shear-horizontal guided waves improved by mode separation

Shear Horizontal (SH) guided waves have been extensively used to estimate and detect defects in structures like plates and pipes. Depending on the frequency and plate thickness, more than one guided-wave mode propagates, which renders signal interpretation complicated due to mode mixing and complex behavior of each individual mode interacting with defects. This paper investigates the use of machine learning models to analyse the two lowest order SH guided modes, for quantitative size estimation and detection of corrosion-like defects in aluminium plates. The main contribution of the present work is to show that mode separation through machine learning improves the effectiveness of predictive models. Numerical simulations have been performed to generate time series for creating the estimators, while experimental data have been used to validate them. We show that a full mode separation scheme decreased the error rate of the final model by 30% and 67% in defect size estimation and detection respectively

Inspection of composite aerospace structures using capacitive imaging and guided waves

This thesis describes a possible new approach for the future of the NDT of aerospace materials by using both ultrasonic guided wave and capacitive imaging (CI) techniques. The two techniques complement each other and are selected depending upon the area inspected and the resolution required. Guided waves are used for long range defect detection, while capacitive imaging is used for localised characterisation. The guided waves are generated by means of electromagnetic acoustic transducers (EMATs). These devices employ a coil, for inducing eddy currents, and a magnet (or an array of them), for generating a static magnetic field. The interaction of these two quantities produces ultrasonic guided waves based on the Lorentz force mechanism, but needs an electrically conductive surface to operate. In this thesis the conductive surface is provided by using thin, self- adhesive, removable metallic patches for both insulating and conducting samples. Conversely, the CI technique employs a pair of electrodes to establish a quasi-static electric field within the sample, and requires the sample to have a low (basically zero) electrical conductivity for allowing the field to probe it. Both techniques are non-contact and non-invasive nature. Guided waves have been studied using periodic permanent magnet (PPM) EMATs, which here have been designed to generate shear horizontal waves, and predominantly the SH0 mode is used in the thesis. In the aerospace field, the materials used are composites, whose electrical conductivity is often too low for efficient EMAT use, even when they contain carbon fibres. There is a notable exception, where a copper mesh for lightning strike protection is integrated into composite, as direct use of an EMAT on these samples is possible. For the cases where removable metallic patches are used, analytical models were designed to predict the forces and the generated wave within the sample. The predictions show good agreement with experimental measurements for the propagation of SH guided waves within different samples such a carbon fibre and glass fibre composites. Consequently, the methodology has been used for the detection of several types of defect, such as impact damage, delamination and lightning strikes. Furthermore, the production of images via a SAFT algorithm allows preliminary evaluations of the severity of the defects detected. The CI technique has been investigated for various designs of CI probes using 2D and 3D finite element (FE) models in COMSOL. It is shown how conductivity affects performance, and the results from simulations of different probe designs has been compared to experiments in insulating materials, with good agreement. These results indicate that CI is a suitable NDT technique for samples such as glass fibre composites. Conventionally, the images from CI measurements are based on the amplitude of the received signal rather than phase, due to the higher signal-to-noise ratios that can be obtained with the amplitude measurement. In this work, an improved image processing method has been introduced. The method combines amplitude and phase information to form clearer images, and thus improving the evaluation of both sizing and location of defects. The use of both techniques has been illustrated for the case of damage within pultruded glass fibre composites. It is shown that guided waves using EMATs and a removable copper patch can be used to detect defects at extended ranges, and that these can be characterised further at higher resolution using a localised inspection, the CI technique

Quantitative evaluation of ultrasonic techniques for the detection and monitoring of corrosion in pipes

Corrosion of the oil and gas pipeline infrastructure has the potential to lead to catastrophic consequences, including loss of life. By performing continual diagnostic non-destructive evaluation (NDE) assessments of the pipeline infrastructure, corrosion can be tracked, mitigated and controlled. However, the sheer scale of the modern pipeline networks around the world requires new innovative techniques to efficiently and cost-effectively monitor and screen for corrosion. Currently there are two well-established ultrasonic techniques for monitoring and screening corrosion. Spot measurements extract the pipe wall thickness at a single point location. This method can extract an accurate value of wall thickness, but the measurement is highly localised. In contrast, long range guided waves (LRGWs) screen for damage by exciting a pulse which travels axially down a pipe and will reflect off any large areas of corrosion. This method has a large coverage area, but is much less sensitive to corrosion than the spot measurement techniques. The first part of this thesis explores the sensitivity of a circumferential guided wave technique, which is suggested to mitigate against some of the disadvantages of existing methodologies. This method propagates a guided wave around the circumference of a pipe, and uses either the reflected pulse from the defect or the through transmitted pulse, to detect the presence of a defect. The viability of this method was assessed by modelling the ultrasonic response to a range of different defects and using probabilistic techniques. The S0, SH1 and SH0 modes were studied and it was revealed that in reflection the technique is able to detect narrow and deep defects, whereas in transmission wide defects are detectable. Wavelength normalised results are also presented. The second part of this thesis investigates the interaction between spot measurement (shear bulk waves) and hemispherical pits. This is with a view to further understanding the mechanics behind the interaction between shear waves and pitting corrosion, and to assessing current transducer types to the tracking and monitoring of individual pits. It was found that an anisotropic relationship exists between the pit’s location relative to the shearing direction of the bulk wave. This is due to either the inherent anisotropy of the transducer or the mode conversion of the shear wave.Open Acces

Defense Technical Information Center thesaurus

This DTIC Thesaurus provides a basic multidisciplinary subject term vocabulary used by DTIC to index and retrieve scientific and technical information from its various data bases and to aid DTIC`s users in their information storage and retrieval operations. It includes an alphabetical posting term display, a hierarchy display, and a Keywork Out of Context (KWOC) display

Pure SH1 Guided-Wave Generation Method with Dual Periodic-Permanent-Magnet Electromagnetic Acoustic Transducers for Plates Inspection

High frequency guided-waves offer a trade-off between the high sensitivity of local bulk ultrasonic thickness measurements and the large area scanning of lower frequency guided-waves, so it has been a growing interest for corrosion inspection with the dispersive SH1 mode. However, according to the dispersive curve, it is hard to generate the pure SH1 mode since the non-dispersive SH0 mode will be excited simultaneously. Thus, this paper investigates a transducer design method to generate a pure SH1 guided-wave, where the dual periodic-permanent-magnet electromagnetic acoustic transducers (PPM EMATs) are placed on exactly opposite positions either side of the plate symmetrically. The suppression effect for SH0 and the enhancement effect for SH1 of the dual PPM EMATs are mainly discussed by theoretical analysis and simulation analysis, and the influence of positioning errors of PPM EMATs placed on opposite sides of the plate on its performances are analyzed. Employing the proposed dual PPM EMATs, some experiments are performed to verify the reliability of finite element simulation. The results indicate that the dual PPM EMATs can suppress the SH0 mode and generate the pure SH1 mode effectively. Moreover, the longitudinal and lateral positioning errors can affect the dual PPM EMATs performances significantly

Abstracts on Radio Direction Finding (1899 - 1995)

The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography). Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM. The contents of these files are: 1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format]; 2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format]; 3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion