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

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control Vol. 58; no. 1; pp. 156 - 167
• Main Authors: Cegla, F B, Cawley, P, Allin, J, Davies, J
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01-01-2011; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustical measurements and instrumentation; Acoustics; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Loading; Physics; Piezoelectricity; Receiving; Shear; Steel; Strips; Surface waves; Temperature gradient; Temperature measurement; Thermal expansion; Transducers; Transduction; acoustical devices for the generation and reproduction of sound; Wave propagation; Waveguides; Acoustic measurement; Coupled waveguide; Thermal insulation; Thermomechanical properties; Dry; Antiplane strain; High temperature; Wall thickness; Experimental study; Aspect ratio; Waveguide; Optimization; Wave dispersion; Temperature gradient; Thickness measurement; Plate; Wave transmission; Thermal expansion; Bypass; Ultrasonic control; Size effect; Rectangular cross section; Ultrasonic transducer
• ISSN: 0885-3010; 1525-8955
• DOI: 10.1109/TUFFC.2011.1782

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.