Marc Choquet

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.

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].

Absolute optical absorption spectra in graphite epoxy by Fourier transform infrared photoacoustic spectroscopy

Optical absorption is obviously of prime interest in the efficiency of laser generation of ultrasound in graphite-epoxy laminates. However, no quantitative spectrum of optical absorption in this composite material has yet been published in the literature. Transmission techniques are inefficient, and other techniques, like attenuated total reflectance or diffusive reflectance, do not give absolute values. The Fourier transform photoacoustic spectroscopy technique seems to be a good alternative that can analyze adequately and quantitatively a graphite-epoxy laminate. We used three different methods to compute the absolute optical absorption from the photoacoustic signal. The three methods are: the saturation of the real part of the photoacoustic spectrum, the comparison of the spectra obtained with two different mirror velocities, and the calibration of the photoacoustic cell with a transmission measurement. The spectra obtained in the IR band of 2.5 to 25 μm are presented, and the problems and limitations of each method are discussed. The results permit a better understanding of the absorption process in the composite laminate, and in this way, will help us enhance the efficiency of laser generation of ultrasound in graphite epoxy.

REMOTE THICKNESS MEASUREMENT OF OIL SLICKS ON WATER BY LASER-ULTRASONICS

ABSTRACT At the National Research Council of Canada Industrial Materials Institute, research is in progress on the application of laser-ultrasonics to remote measurement of the thickness of oil on ...

AIRBORNE OIL SPILL SENSOR TESTING: PROGRESS AND RECENT DEVELOPMENTS

ABSTRACT It is now possible to measure the thickness of an oil slick on water by remote sensing. A laboratory sensor has been developed to provide this absolute oil slick thickness measurement. A j...

Laser Ultrasonic System for On‐Line Steel Tube Gauging

A laser‐ultrasonic system has been installed on a seamless tubing production line of The Timken Company and is being used to measure on‐line the wall thickness of tubes during processing. The seamless process consists essentially in forcing a mandrel through a hot cylindrical billet in rotation and typically results in fairly large wall thickness variations that should be minimized and controlled to respect specifications. The system includes a Q‐switched Nd‐YAG laser for generation of ultrasound by ablation, a long pulse very stable Nd‐YAG laser for detection coupled to a confocal Fabry‐Perot interferometer, a pyrometer to measure tube temperature and two laser Doppler velocimeters to measure the coordinates of the probing location at the tube surface. The laser, data acquisition and processing units are housed in a cabin off line and connected to a front coupling head located over the passing tube by optical fibers. The system has been integrated into the plant computer network and provides in real time...

Laser ultrasonic system for on‐line steel tube gauging and process control

A laser ultrasonic system has been installed on a seamless tubing production line at The Timken Company and is being used to measure on‐line the wall thickness of tubes during processing. The seamless process consists essentially in forcing a mandrel through a hot cylindrical billet in rotation and results in wall thickness variations that should be minimized and controlled to respect specifications. The system includes a Q‐switched Nd‐YAG laser for the generation of ultrasound by ablation, a long pulse very stable Nd‐YAG laser for detection coupled to a confocal Fabry–Perot interferometer. The lasers, data acquisition, and processing units are housed in a cabin off‐line and connected to a front coupling head located over the passing tube by optical fibers. The system also includes a fiber‐coupled pyrometer to measure tube temperature profile and two fiber‐coupled optical velocimeters to measure the coordinates at the probing location on the surface of the passing, rotating hot tube. During the presentati...

Detection of Flaws in Materials by Laser-ultrasonics

Laser-ultrasonics is a novel technology that uses lasers for the generation and detection of ultrasound and presents many advantages compared to conventional piezoelectric based ultrasonics. In this presentation, the developments performed in this area at the Industrial Materials Institute of the National Research Council of Canada will be reviewed. Laser-ultrasonics has been used to detect flaws in a variety polymer-matrix composite materials and has been commercialized for this application. The detection of small flaws, in particular in metallic materials, by laser-ultrasonics has been addressed by using a numerical reconstruction algorithm based on the Synthetic Aperture Focusing Technique.

Laser ultrasonics: a new tool for the industry

In this paper we explore laser induced breakdown spectroscopy (LIBS) at relatively low energies in the range 10 - 350 tJ. We present measurements ofthe threshold laser energy needed for LIBS and the scaling of plasma size and crater size with energy. The effects of the laser pulse length and gating of the detector on the LIB spectra are studied and we also assess the use ofmicrojoule LIBS for the identification ofAl alloys.

Improved Performance of Laser-Ultrasonic F-SAFT Imaging

The spatial resolution of laser-ultrasonics depends upon the spot sizes of the generation and detection lasers and may be inadequate for detecting small and buried flaws. The use of a broad laser spot at the surface of the specimen to produce an ultrasonic beam with little divergence gives a spatial resolution limited by the spot size. In the opposite case, focusing the laser beam to a small laser spot yields a strongly diverging acoustic wave, leading also to poor spatial resolution. As in conventional ultrasonics, spatial resolution can be greatly enhanced by the use of the Synthetic Aperture Focusing Technique (SAFT) [1,2]. In addition, the coherent summation performed by SAFT yields an improved signal-to-noise ratio (SNR). Originally developed in the time domain, SAFT can be advantageously implemented in the frequency domain (F-SAFT). F-SAFT is based on the angular spectrum approach [2–4], which allows a significant reduction in processing time as compared to time-domain SAFT.