Christian Padioleau

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.

On-line measurement of texture, thickness and plastic strain ratio using laser-ultrasound resonance spectroscopy

Laser-ultrasound resonance spectroscopy, a non-contact ultrasonic technique, was used to determine reliably and rapidly the crystallographic texture, the average plastic strain ratio, and the thickness of sheet metal on the production line. As with laser-ultrasonics, a short laser pulse is used to generate a wide-band pulse of ultrasound and a laser interferometer is used for its detection. In this paper, a large number of echoes are collected and analyzed together using Fourier techniques to measure the natural resonance frequencies in the thickness of the sheet. One longitudinal and two shear resonance frequencies were measured together with their harmonics. From these frequencies, two crystallographic orientation distribution coefficients, W(400) and W(420), are obtained, as well as a highly accurate measurement of the sheet thickness that is corrected for changes in ultrasonic velocity caused by texture variations. Using these coefficients, the average and in-plane twofold and fourfold variations of the plastic strain ratio, respectively r delta(2)r, and delta(4)r, can be evaluated. These parameters are indications of the formability of metals sheets, which is of industrial interest. Measurements on 1 mm thick, low carbon steel sheets have shown the following measurement accuracies: r to within +/-0.08, delta(2)r, and delta(4)r to within +/-0.1, and thickness to better than +/-1 microm. On-line tests at LTV Steel Company showed that the sensitivity of the apparatus is sufficient to detect systematic variations in texture along the length of similar production coils and that the on-line repeatability for r was of order +/-0.02.

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].

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 ...

Surface inspection of hard to reach industrial parts using low-coherence interferometry

Optical inspection tools based on low-coherence interferometry and specialized for hard to reach industrial parts are presented. A common path configuration using optical fiber components is described. Small diameter probes originally developed for biomedical applications have been specialized for industrial inspection. Probes that can be used with a Cartesian surface scanning system or a cylindrical scanning system are presented. The probes include a reference that makes absolute accuracy measurements easier. Characterization of the internal surface of a worn plasma torch electrode has been realized using that technique. Surface profiling of the barrel of a gun was also performed.

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...

Differential Confocal Fabry‐Perot for the Optical Detection of Ultrasound

The best detection limit of an optical system for the detection of ultrasound is obtained for a system limited by the shot noise. However, typical laser oscillators are not free of intensity and phase fluctuations, and such laser noise exceeds the shot‐noise level above a given laser light power. Moreover, since typical industrial surfaces are optically rough and absorbing, laser amplifiers are frequently used to increase the light power collected back from the surface. Such amplifiers add both intensity and phase fluctuations. While intensity fluctuations can be eliminated by a simple differential scheme when using a confocal Fabry‐Perot, no simple solution has been given to reduce the phase noise. In this paper, differential schemes that reduce both laser intensity and phase noises are proposed for the transmission and reflection configurations and experimental results on the noise reduction reached are presented. The advantages of this differential approach to image variable reflectivity surface parts ...

Single-frequency pulsed laser oscillator and system for laser-ultrasonics

We present a new pulsed laser oscillator and system for the optical detection of ultrasound in materials. A single-frequency laser oscillator based on a pulse pumped Nd:YAG rod inside a ring cavity is proposed. The laser delivers single-frequency pulses of 35 W power. Pulse to pulse stability of the laser is obtained with a classical Pound–Drever–Hall method. Power of about 1 kW can be obtained when the second rod of a dual-rod pumping chamber is used as an amplifier. The performance of the system is then investigated with a GaAs photorefractive crystal-based two-wave mixing phase demodulator. In particular, the intensity noise of the laser can be made small enough to allow the detection limit to be set by the shot-noise of the laser. The coherence length of the laser is about 20 m, which makes it a versatile laser-ultrasonic inspection system operated with a two-wave mixing-based phase demodulator. A complete compact and affordable system is obtained when the second rod of the pumping chamber is used to operate a short pulse laser for the generation of ultrasound. Tests of this laser-ultrasonic system on metallic samples are presented.

Ex vivo imaging of early dental caries within the interproximal space

Optical coherence tomography (OCT) is emerging as a technology that can potentially be used for the detection and monitoring of early dental enamel caries since it can provide high-resolution depth imaging of early lesions. To date, most caries detection optical technologies are well suited for examining caries at facial, lingual, incisal and occlusal surfaces. The approximal surfaces between adjacent teeth are difficult to examine due to lack of visual access and limited space for these new caries detection tools. Using a catheter-style probe developed at the NRC-Industrial Materials Institute, the probe was inserted into the interproximal space to examine the approximal surfaces with OCT imaging at 1310 nm. The probe was rotated continuously and translated axially to generate depth images in a spiral fashion. The probe was used in a mock tooth arch model consisting of extracted human teeth mounted with dental rope wax in their anatomically correct positions. With this ex vivo model, the probe provided images of the approximal surfaces revealing morphological structural details, regions of calculus, and especially regions of early dental caries (white spot lesions). Results were compared with those obtained from OCT imaging of individual samples where the approximal surfaces of extracted teeth are accessible on a lab-bench. Issues regarding access, regions of interest, and factors to be considered in an in vivo setting will be discussed. Future studies are aimed at using the probe in vivo with patient volunteers.

Measurement of texture in steel by laser-ultrasonic surface waves

A laser-ultrasonic approach based on the propagation of surface skimming longitudinal wave (P-wave) and Rayleigh wave (SAW) is investigated for the measurement of texture in moderately thick steel plates. The angular dependence of the P-wave and SAW velocities presents different patterns, both of amplitude of a few % attributed to texture. Moreover, the difference of the P-wave and SAW profiles is shown to be very robust against small path length changes or other detrimental effects. For measurements on large plates, a solution based on the rotation of a generation line using an axicon lens is tested. Texture variations across the width of steel plates and the influence of residual stress are discussed.