C. Néron

Laser-Ultrasonics: From the Laboratory to the Shop Floor

Ultrasonics is a powerful technique for inspecting and characterizing industrial materials. It not only can detect bulk and surface flaws, but also obtain information on material microstructure, which determines engineering properties, such as elastic moduli and ultimate strength. However, traditional ultrasound requires liquid or contact coupling for its generation and detection, making it difficult or impossible to apply in many industrial situations. This occurs, in particular, on curved parts and on parts at elevated temperature, a situation widely found in industrial products and during the processing of industrial materials.Through a continuing effort that started more than 10 years ago, the Industrial Materials Institute of the National Research Council of Canada working in collaboration with UltraOptec Inc. has developed a technique called laser-ultrasonics, that circumvents the limitations of the conventional techniques. This novel technique is based on the generation and detection of ultrasound with lasers. The technology we have developed has been demonstrated to be applicable to real industrial conditions. In particular, a system was brought to a steel mill to measure on-line the wall thickness of tubes at 1000°C moving at 4 m/s. The capability of our technology to inspect advanced aircrafts made of composite materials was also demonstrated by inspecting a CF-18 in the hangar of a maintenance facility. UltraOptec Inc. is now in the process of commercializing this technology, in particular, for these two demonstrated industrial applications.

Performance of laser-ultrasonic F-SAFT imaging

The resolution and signal-to-noise ratio of laser-ultrasonics to detect small and buried defects can be greatly enhanced by using the synthetic aperture focusing technique (SAFT). Originally developed in the time domain, SAFT can also be implemented in the frequency domain (F-SAFT) using the angular spectrum approach for a significant reduction in processing time. In this paper, an F-SAFT based data processing method especially adapted to laser-ultrasonic data is presented. This method allows for further significant improvements towards laser-ultrasonic imaging of small defects. It includes temporal deconvolution of the waveform data, control for an optimal aperture and frequency bandwidth as well as spatial interpolation of the subsurface images. All the above operations are well adapted to the frequency domain calculations and embedded in the F-SAFT data processing. Also, the aperture control and spatial interpolation allow a reduction of sampling requirements to further decrease both inspection and processing times. The above improvements are illustrated using laser-ultrasonic data taken from an aluminum sample with flat-bottom holes.

Improved resolution and signal-to-noise ratio in laser-ultrasonics by SAFT processing.

Laser-ultrasonics is an emerging nondestructive technique using lasers for the generation and detection of ultrasound which presents numerous advantages for industrial inspection. In this paper, the problem of detection by laser-ultrasonics of small defects within a material is addressed. Experimental results obtained with laser-ultrasonics are processed using the Synthetic Aperture Focusing Technique (SAFT), yielding improved flaw detectability and spatial resolution. Experiments have been performed on an aluminum sample with a contoured back surface and two flat-bottom holes. Practical interest of coupling SAFT to laser-ultrasonics is also discussed.

Laser-Ultrasonic Inspection of the Composite Structure of an Aircraft in a Maintenance Hangar

Composite materials used in aerospace structures can be affected by a variety of defects, such as delaminations and disbonds, which may occur during fabrication or may be caused by impact during use. Such defects, which cannot usually be detected by simple visual inspection, may severely affect the mechanical integrity of components. Ultrasonics offers the best possibility for detection of flaws in composite components. However, ultrasonics as conventionally applied using piezoelectric transducers for generation and detection of the probing pulse has several limitations. Namely, the need for an acoustic coupling media or direct contact with the surface, and the requirement of near-normal incidence to the component’s surface. Laser-ultrasonics represents a practical means of avoiding the inherent difficulties with conventional ultrasonics [1–2].

Long acoustic imaging probes

Theoretical considerations and experimental measurements on two long acoustic imaging probes are presented. One is a cladded glass rod and the other is a glass rod with a graded acoustic velocity profile across the rod diameter. The acoustic velocity profiles have been measured using a 225-MHz line focus beam scanning acoustic microscope. Spherical acoustic lenses have been fabricated at the ends of the rods. Focused and unfocused ultrasonic measurements obtained with these long buffer rods at ambient and elevated temperatures are demonstrated.<<ETX>>

Characterization of cladded glass fibers using acoustic microscopy

Spatial distribution profiles of leaky surface acoustic wave velocity (VLSAW ) and attenuation across the diameters of cladded glass fibers are presented. The profiles are obtained by using a novel V(x,z) analysis with a reflection scanning acoustic microscope operated at 775 MHz, and are compared with optical refractive index profiles. Optical fibers with different dopants and dopant concentrations have been investigated. The role of acoustic property profiles in the design of optical and acoustic fibers is outlined.

Acoustic graded‐index lenses

Experimental investigations of the focusing behavior of rods having acoustically graded‐index profile across the rod diameter are presented for the first time. The ray acoustics approach is used for the theoretical interpretation. Acoustic velocity profiles have been measured using a 225 MHz line focus beam scanning acoustic microscope. The focusing behavior is visualized with a Schlieren system.

Fabrication and characterization of continuously cast clad metallic buffer rods

The fabrication, characterization, and ultrasonic measurements of clad metallic ultrasonic buffer rods are presented. The core and the cladding consist of tin/lead alloy and pure tin, respectively, produced by a vertical continuous casting technique. The acoustic velocity profiles across the rod diameter are measured using a 225‐MHz line focus beam scanning acoustic microscope. A schlieren visualization system confirms the concentration of acoustic energy in the core. Spherical acoustic lenses have been fabricated at the ends of the rods. Focused and unfocused ultrasonic measurements at frequencies between 2 to 10 MHz and signals with excellent signal to noise ratios have been obtained.

Doppler Frequency‐Shift Compensated Photorefractive Interferometer for Ultrasound Detection on Objects in Motion

Two‐wave mixing based interferometry has been demonstrated to be a powerful technique for non‐contact, broadband and speckle insensitive measurements of the small surface displacements produced by ultrasonic waves propagating in an object. When the object is in rapid motion along the line‐of‐sight of the probing laser or when the laser beam is rapidly scanned on a wavy surface, the two‐wave mixing photorefractive interferometer loses sensitivity to the point it could become useless. To circumvent the Doppler frequency‐shift produced by this relative motion, we propose a dynamic compensation scheme. We report a particularly simple scheme to implement this concept by monitoring the low‐frequency output signal of a balanced two‐wave mixing demodulator whose output is proportional to the frequency difference between the pump and signal beams, and feeding this signal back to the acousto‐optic shifter. With this new concept, the two‐wave mixing interferometer can operate on objects in rapid motion while maintaining its sensitivity to low frequency ultrasound.

SAFT data processing applied to laser-ultrasonic inspection

By relying on optics for providing the transduction of ultrasound, laser-ultrasonics brings practical solutions to a variety of nondestructive evaluation problems that cannot be solved by using conventional ultrasonic techniques based on piezoelectric transduction [1,2]. Laser-ultrasonics uses two lasers, one with a short pulse for the generation of ultrasound and another one, long pulse or continuous, coupled to an optical interferometer for detection. Laser-ultrasonics allows for testing at a large standoff distance, inspection of moving parts on production lines and inspection in hostile environments, such as the one encountered in the steel industry. The technique features also a large detection bandwidth, which is important for numerous applications, particularly involving material characterization. Another feature of laser-ultrasonics, particularly useful for inspecting parts of complex shapes, is the generation of an acoustic wave propagating normally to the surface, independently of the shape of the part and of the incidence angle of the optical generation beam. This characteristic feature occurs either when the ablation mechanism is used for generation or when light from the generation laser penetrates sufficiently deep below the surface. This last condition occurs usually with many polymer-based materials and on materials with painted surfaces.