There is an industrial drive for the improved detection of sub-mm sized sur¬face breaking defects using non-destructive evaluation (NDE) methods [1]. Electromagnetic acoustic transducers (EMATs) are a non-contact NDE technique that utilise the generation and detection of Ultrasound using primarily Lorentz force mechanisms [2]. They are relatively safe and inexpensive, however, they suffer from low generation efficiency. The precise industrial drive for this work is improved ultrasonic crack detection of surface defects hidden by a thin metallic paint coating. The majority of standard ultrasonic techniques are not applicable as they require direct contact to the sample surface. Laser techniques, while non-contact, are still impeded by the coating, and eddy current techniques are difficult to implement due to interference from the metallic coating. EMATs are applicable, however their low generation efficiency limits the minimum defect that can be detected. This work presents improved resolution surface wave EMATs using geometric focusing for the detection of sub-mm sized surface breaking defects. Three main design types have been presented: a pseudo-pulse-echo focused meander-line EMAT, a pitch-catch focused racetrack EMAT and a pitch-catch focused linear EMAT. The first two designs have been fully characterised, finding the relations between coil geometry, focal point location and size, and the optimum operation frequencies [3, 4, 5]. Both designs have been used to size the lengths of a set of drilled calibration defects to accuracies of 10.5 and 10.4 mm respectively, and the pitch-catch design has been used to create a calibration curve for defect depth measurements. In addition, both designs have been used to map a pair of real surface breaking cracks in an aluminium billet sample to sub-mm resolution. The pitch-catch design has been used to detect a set of mm-size real thermal fatigue cracks in steel through a 40 - 60 ktm thick metallic paint coating. A four-coil EMAT design based on the pitch-catch focused racetrack EMAT has been built and demonstrated to detect surface breaking defects regardless of their surface orientation. Finally, the meander-line, racetrack, and linear coil design types have been compared based on their signal strength and their performance at lift-off from a sample surface. The meander-line designs have the strongest signal to noise ratios (SNR), with over 40 dB found when in contact with the sample, but the largest SNR loss with increased lift-off, reducing to 0 dB by 0.3 mm lift-off. The linear designs have the weakest SNRs, under 30 dB when in direct contact, but the smallest SNR loss with increased lift-off, dropping to 0 dB by around 1 mm, depending on the frequency of operation. This makes the linear coil designs optimal for situations requiring higher lift-off. Lower frequency designs are shown to perform better with increased lift-off regardless of the coil design, however, lower frequencies have less spatial resolution capabilities. A proposed linear-meander-line phased EMAT design is presented to generated 1 MHz signals but with the improved lift-off capabilities of the linear designs. This proves that surface wave EMATs can be optimised for surface wave detection of sub-mm defects through a metallic paint coating. While pseudo-pulse-echo focused meander-line EMATs are already in exsitence, there was previously no published work on their capabilities and full charaterisation. The other focused designs presented here are new designs in the field.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:767148 |
| Date | January 2019 |
| Creators | Thring, Claire B. |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/114456/ |
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