J. Davies

Bio: J. Davies is an academic researcher. The author has contributed to research in topics: Ultrasonic testing & Ultrasonic sensor. The author has an hindex of 1, co-authored 2 publications receiving 116 citations.

High-temperature (>500°c) wall thickness monitoring using dry-coupled ultrasonic waveguide transducers

Abstract: Conventional ultrasonic transducers cannot withstand high temperatures for two main reasons: the piezoelectric elements within them depolarize, and differential thermal expansion of the different materials within a transducer causes them to fail. In this paper, the design of a high-temperature ultrasonic thickness gauge that bypasses these problems is described. The system uses a waveguide to isolate the vulnerable transducer and piezoelectric elements from the high-temperature measurement zone. Use of thin and long waveguides of rectangular cross section allows large temperature gradients to be sustained over short distances without the need for additional cooling equipment. The main problems that had to be addressed were the transmission and reception of ultrasonic waves into and from the testpiece that the waveguides are coupled to, and optimization of the wave propagation along the waveguide itself. It was found that anti-plane shear loading performs best at transmitting and receiving from the surface of a component that is to be inspected. Therefore, a nondispersive guided wave mode in large-aspect-ratio rectangular strips was employed to transmit the anti-plane shear loading from the transducer to the measurement zone. Different joining methods to attach the waveguides to the component were investigated and experiments showed that clamping the waveguides to the component surface gave the best results. The thickness of different plate samples was consistently measured to within less than 0.1 mm. Performance at high temperatures was tested in a furnace at 730°C for 4 weeks without signal degradation. Thicknesses in the range of 3 to 25 mm could be monitored using Hanning windowed tonebursts with 2 MHz center frequency.

Ultrasonic Wall Thickness Monitoring at High Temperatures (>500 °C)

Abstract: Corrosion and erosion shorten the life of components that are used in the petrochemical industry. In order to mitigate the safety and financial risks posed by the degradation mechanisms, plant operators monitor wall thicknesses at regular inspection intervals. In high temperature locations inspections have to be carried out at plant shut downs because conventional ultrasonic sensors cannot withstand the high operating temperatures. The authors have developed a waveguide based high temperature thickness gauge for monitoring of wall thicknesses in high temperature areas. The waveguide allows the use of conventional transduction systems (max temp. 60 °C) at one end and guides ultrasonic waves into the high temperature region where the inspection is to be carried out. Slender stainless steel waveguides allow a temperature drop of ∼500–600 °C per 200 mm length to be sustained simply by natural convection cooling. This paper describes the technical challenges that had to be overcome (dispersion and source/receiver characteristics) in order to implement this “acoustic cable”. A range of experimental results of thickness measurements on components of different thickness, and furnace tests at different temperatures are presented. An accelerated corrosion test that demonstrates the effectiveness of the monitoring for corrosion is also presented.

Structural health monitoring: Closing the gap between research and industrial deployment:

Abstract: There has been a large volume of research on structural health monitoring since the 1970s but this research effort has yielded relatively few routine industrial applications. Structural health monitoring can include applications on very different structures with very different requirements; this article splits the subject into four broad categories: rotating machine condition monitoring, global monitoring of large structures (structural identification), large area monitoring where the area covered is part of a larger structure, and local monitoring. The capabilities and potential applications of techniques in each category are discussed. Condition monitoring of rotating machine components is very different to the other categories since it is not strictly concerned with structural health. However, it is often linked with structural health monitoring and is a relatively mature field with many routine applications, so useful lessons can be read across to mainstream structural health monitoring where there ar...

Guided wave tomography of pipes with high-order helical modes

Abstract: The transmission of guided ultrasonic waves across corrosion or erosion damage encodes information about the defect depth. Tomography maps the depth profile from multiple transmission experiments performed under different insonification angles by solving the so-called inverse problem; the accuracy of the depth estimation being dependent on the range of angles available for the inversion. Practical application of tomography to tubular structures, such as pipes and bends, requires the use of two ring arrays of ultrasonic transducers at the two ends of the pipe section to be inspected. However, such a configuration leads to an insufficient angular coverage when considering the signals that travel along the shortest temporal path between a pair of transducers. This paper introduces a general inversion method that extends the range of insonification angles by exploiting the information carried by the signals that wrap around the pipe multiple times before reaching the receive array, thus resulting in superior image resolution and increased depth estimation accuracy. In addition, to address typical thermal fluctuations encountered during continuous monitoring, a strategy that combines a temperature compensation scheme with the intrinsic thermal stability of electromagnetic acoustic transducers (EMATs) is developed and tested with full-scale experiments performed on a schedule of 40, 8″ diameter steel pipe instrumented with two ring arrays of EMAT transducers. It is shown that for an irregularly shaped defect the proposed inversion method yields maximum depth estimations that are as accurate as single point ultrasonic thickness gaging measurements and over a wide temperature range up to 175 °C. The results indicate that advanced inversion schemes in combination with EMAT transduction offer great potential for continuously monitoring the progression of corrosion or erosion damage in the oil and gas industry.

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

Abstract: Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.

High-temperature (>500 /spl deg C) ultrasonic transducers: an experimental comparison among three candidate piezoelectric materials

Abstract: High-temperature piezoelectric crystals, including YCa4O(BO3)3, LiNbO3, and AlN, have been studied for use in ultrasonic transducers under continuous operation for 55 h at 550°C. Additionally, thermal ratcheting tests were performed on the transducers by subjecting the crystals to heat treatments followed by ultrasonic performance testing at room temperature and 500°C. The changes resulting from the heat treatments were less than the statistical spread obtained in repeated experiments and were thus considered negligible. Finally, in situ measurements of the pulse-echo response of YCa4O(BO3)3 were performed at temperatures up to 950°C for the first time, showing stable characteristics up to these high temperatures.

High temperature thickness measurements of stainless steel and low carbon steel using electromagnetic acoustic transducers

Abstract: Thickness measurements were made on steel pipe at high temperatures using a non-contact watercooled EMAT (electromagnetic acoustic transducer) and also a laser-EMAT system where a portable Nd:YAG laser with a fibre optic cable was used to generate ultrasound on a sample, and a water-cooled coil-wound EMAT used to detect ultrasound. The set-up was designed so that the laser was fired through a hole in the centre of the EMAT, and a low pass 5 MHz filter employed to reduce the plasma noise. Back wall reflections were clearly visible at temperatures up to 900 °C on stainless and ferromagnetic low carbon steel, enabling wall thickness to be measured, taking into account thermal expansion of the sample. The water-cooled EMAT system can measure wall thickness on ferromagnetic low carbon steel at temperatures up to the Curie point; here, ultrasound generation is dependent on the magnetic state of the steel.