Jean F. Bussière

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 monitoring of phase transformations in steels

The ferrite-austenite transformations in A36 and in IF steels were monitored using laser-ultrasonics. Sudden variations of ultrasonic attenuation were observed at transformation temperatures. Dilatometry and standard metallographic observations support an interpretation of these attenuation variations in terms of nucleation and growth of the new phase. Laser-ultrasonics is a new technique to monitor microstructural changes that take place during phase transformations. This information is obtained in real-time and may greatly facilitate laboratory studies on phase transformations. Using ultrasonic scattering models, and with reliable values of the austenite elastic constants as a function of temperature, laser-ultrasonics could provide a quantitative evaluation of grain sizes during phase transformations.

Analysis of thin rod flexural acoustic wave gravimetric sensors

An analysis of thin rod flexural acoustic wave gravimetric sensors is presented. The diameter of the thin rod is much less than a wavelength. The lowest order flexural acoustics mode, F11, is of interest. For small added masses, the calculated mass sensitivity is −1/(2ρa), where ρ is the density and a is the radius of the thin rod. Measurements of velocity dispersion of the F11 mode in 21‐μm‐diam gold wires are shown to agree well with the theoretical calculation. Devices based on thin rods are shown to potentially have the same advantages as plate‐mode gravimetric sensors.

Ultrasonic velocity measurements during phase transformations in steels using laser ultrasonics

This article presents accurate laser-ultrasonic measurements of longitudinal velocity in the 500–1000 °C temperature range in carbon steel samples for different conditions known to affect phase transformations such as cooling rate, carbon concentration, and rolling. Measurements were performed during continuous heating and cooling at rates varying between 0.1 and 20 °C/s. Carbon concentrations ranged from 0.0% to 0.72%. Hot-rolled and cold-rolled samples were measured. For the hot-rolled samples, a reproducible hysteresis was observed in the dependence of the ultrasonic velocity versus temperature of samples having a significant carbon concentration. This hysteresis is attributed to the combined effects of the phase transformation and of the ferromagnetic–paramagnetic transition. In particular, the rate of change of velocity with temperature during heating suddenly diminishes at the Curie temperature, and the velocity behavior during cooling shows clearly the start and end of phase transformations, even allowing discrimination between ferrite and pearlite nucleations. For the cold-rolled samples, significant drops in ultrasonic velocity were observed at the transformation temperatures when the samples were heated for the first time. However, the magnitude of these drops decreased for subsequent heating. These drops are attributed to irreversible crystallographic orientation changes caused by phase transformations. This effect was modeled using the Kurdjumov–Sach transformation relationship. The model calculation agrees well with the experimental data.

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.

Laser-Ultrasonic Determination of Elastic Constants at Ambient and Elevated Temperatures

It is well known that the elastic constants of an isotropic solid (the bulk and shear moduli or the Young’s modulus and the Poisson’s ratio) can be determined ultrasonically when both longitudinal and shear wave velocities are measured. At high temperature, traditional ultrasonic techniques are difficult to apply because they require a coupling medium operating in the same temperature range. However, some results have been previously reported using momentary contact1–2, but the application remains difficult, especially for shear wave coupling, and above 1000°C. Obviously, an ultrasonic technique where ultrasound is generated and detected without contact can avoid such problems.

Rapid Microstructure Assessment in Rolled Steel Products using Laser-Ultrasonics

Metallography is probably the preferred metallurgists’ method to characterize microstructures. Unfortunately, metallographic techniques are time-consuming and destructive. For these reasons, quite often, only a few micrographs are used to characterize large specimens and uniformity is assumed. This assumption may lead, in some cases, to inaccurate interpretations of mechanical properties in terms of microstructures. A nondestructive tool is therefore required to rapidly characterize microstructures.

Laser-ultrasonic developments towards industrial applications

Two applications of laser-ultrasonics in the steel industry are described: thickness gauging at elevated temperature of seamless pipes and grain size evaluation by measurement of ultrasonic attenuation. A laser-ultrasonics generating/detecting system for use in the steel industry is also discussed.<<ETX>>

Laser Generation of Annular Converging Surface Acoustic Waves

Converging surface acoustic waves have been generated and detected using a noncontacting technique. A YAG laser together with an axicon lens produces a focussed optical ring which excites both converging and diverging SAW on the test sample. An optical interferometer is used to measure the vertical displacement of the SAW. Variations in the surface acoustic waveforms due to different optical power, beam width and shape, as well as the anisotropic and dispersive properties of the samples are analyzed. The focussing effect and the resolution of the converging surface waves are illustrated. Applications of this technique, using a full or partial optical e xcitation ring, for characterization as well as nondestructive testing of materials are discussed.