Low-cost ultrasonic NDT method yields real-time results

Ultrasonic nondestructive inspection methods are based on the principle of wave propagation through a solid medium. A transmitter emits a sound wave, above the range audible to humans, that excites the structure, while the same (or a second) transducer picks up the resulting structural waves. In a typical setup, a C-scan, transducers are positioned on both sides of a part in "through transmission" mode. A robot or other automated system moves the transducers over the part, in line with each other and perpendicular to the part surface to create a two-dimensional map of flaws. C-scans, however, take time. Scanning a large part can consume many hours, and scans of parts with contours or varying thickness require additional programming to account for shape changes. Imperium Inc. (Silver Spring, Md.) now offers a variation — an easily understood, real-time ultrasound camera system. Its trademarked and patented digital acoustic video, or DAV, is incorporated into a small, portable, handheld device that delivers high-resolution video images, at video rates, says company CEO and cofounder Robert Lasser: "It provides real images at a lower cost than other systems, and is simpler than conventional ultrasound — an operator can be trained in two days."

The DAV excites a large-area, unfocused transducer, which generates a collimated plane wave. As the resulting wave strikes the target and scatters, an acoustic lens collects the scattered energy and focuses it onto an ultrasound-sensitive detector array in either through-transmission mode or "pulse echo" mode, in which the transducer is at an angle to the part surface. The camera is placed in intimate contact with the part via an acoustic coupling gel or compliance pads or membranes; the camera housing is water-filled to minimize signal attenuation.

The technology has been tested by the U.S. Air Force Air Mobility Battlelab (Ft. Dix, N.J.) and by a commercial aircraft OEM. In the latter case, a 26-inch by 7-inch (660 mm by 178 mm) monolithic carbon/epoxy test panel, varying in thickness from 0.375 inch to 1.5 inches (9.5 mm to 38.1 mm), was constructed with known defects. The Imperium I500 Acoustocam device, in pulse echo configuration, enabled the examiner to view the sample at various depths in real time, at 30 frames per second, to 0.02 inch/0.5 mm resolution, says Lasser. Including setup, the scan was completed within 15 minutes.