High Pressure Gas-Coupled Ultrasonic Inspection for Discontinuities

A liquid couplant is usually necessary between the transducer and the specimen for ultrasonic crack detection and thickness gaging. This work demonstrates the possibility of using pressurized gas as the couplant for noncontact inspection. Historically, obstacles to gas coupling have been the high signal transmission losses at gas solid interfaces and dynamic range limitations of front end receiver circuits. Both problems are due to the large mismatch in specific impedance at the gas-solid interface. In this article, we describe how to overcome such obstacles at megahertz frequencies with a coupling gas (nitrogen) pressurized to as high as 7 MPa (1000 psi); a receiver preamplifier with a large dynamic range (over 60 dB); a high power (8 kW) pulser, a wide band, well damped, 2.25 MHz piezoelectric ceramic transducer; and a fast, efficient transmit/receive switch (diplexer). With this approach, we were able to generate and detect compact longitudinal and shear wave signals inside flat plates of ferritic pipeline steel 13 mm (0.5 in.) thick. Normally incident longitudinal waves measured the plate thickness, and shear waves, refracted at 45 degrees, detected thin vertical notches 2.5 mm (0.1 in.) deep. While the gas was static for most of the measurements, a preliminary test in a flow loop facility was promising. These experiments point to a feasible adaptation for inspection of natural gas pipelines.