LASER-BASED IMAGING: Laser "pings" parts for ultrasonic images

Using laser pulses to generate ultrasonic waves in materials, researchers at Hughes Research Laboratories (Malibu, CA) have produced ultrasound internal images of materials and parts. Termed Compensated Laser Ultrasonic Evaluation (CLUE), the technique offers remote, real-time, nondestructive inspection for production processes, even under harsh industrial conditions.

Many present inspection methods, such as for welds, involve destructive testing, which can be costly and generates unusable scrap. Many nondestructive techniques are expensive, such as x-ray systems, or, like conventional ultrasound, are not amenable to manufacturing environments having high heat, noise, and airborne contamination.

In ultrasonic inspection, one transducer generates high-frequency sound waves that probe a component or a weld joint, while another senses the resultant vibrations. With CLUE, laser beams replace the ultrasound transducers, which normally require intimate contact, a liquid couplant, or immersion of the piece to induce and sense its vibrations. A number of laser wavelengths are suitable. Hughes is currently using a Q-switched Nd:YAG laser producing 400 mJ/pulse at 10 Hz to "ping" the part, generating ultrasonic waves. A CW diode-pumped 200-mW frequency-doubled Nd:YAG laser remotely senses the surface vibrations induced by the ultrasound waves as they pass through the material. These signals are processed to determine voids or other internal discontinuities. The lasers permit ultrasonic inspection right on the production line such as at a welding station.

Because lasers can scan over irregularly shaped surfaces, robots could be used to position the CLUE system as part of a flexible manufacturing line. The inspection system could also provide real-time feedback in the manufacturing process for automatic control and process optimization.

Gil Dunning, CLUE project manager, says, "Laser-generated ultrasound has been a topic of intense study since the 1980s, typically performed under ideal laboratory conditions." Commercial applications in industrial environments have been difficult. "The most useful ultrasonic response of a surface typically occurs at high frequencies (1-20 MHz), with small surface displacements (1-10 nm). Detection of these minute surface vibrations, on the order of atomic lattice spacings, can be overwhelmed by gross workpiece vibrations, the surface roughness, and by probing through turbulent atmospheres and plasmas—typical factory-floor conditions," says Dunning. The CLUE system’s double-pumped phase-conjugate mirror in concert with heterodyne detection compensates for these limits, making it possible, Dunning notes, "to perform laboratory precision measurements in an industrial environment."

Currently in its development, the CLUE system can acquire and store laser ultrasound data on lap-weld joints for eventual classification. Hughes has implemented a neural network processing algorithm into the operating system which, once having "learned" from the acquired data, will provide the operator with necessary weld-joint integrity information to adjust the weld as it is formed. Next year feedback-processing control should be demonstrated for material thickness monitoring.

Other nondestructive-testing applications with CLUE include corrosion detection, remote temperature determination, and metallurgical measurements such as hardness, grain, and phase transitions. Composites and adhesives can be inspected for debonding, semiconductors checked for thickness, and thin-film growth processes monitored. And Hughes researchers have experimentally attained 0.001-in.-thickness resolution on parts moving at 400 ft/min—performance compatible with manufacturing on-line inspection requirements and feedback-processing control.