Amato, Silvio (2021) Inspection of composite aerospace structures using capacitive imaging and guided waves. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis (PhD)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TL Motor vehicles. Aeronautics. Astronautics
Library of Congress Subject Headings (LCSH):	Aeronautics -- Materials -- Testing, Aeronautics -- Materials -- Nondestructive testing, Ultrasonic testing, Electromagnetic testing, Aerospace engineering
Official Date:	June 2021
Dates:	Date Event June 2021 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Hutchins, David A. ; Dixon, Steve M.
Sponsors:	Horizon 2020 (Programme)
Format of File:	pdf
Extent:	xxv, 183 leaves : illustrations (some colour)
Language:	eng

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