A broad overview of the field of laser‐ultrasonics is presented. This overview draws from developments at the Industrial Materials Institute of the National Research Council of Canada as well as elsewhere. The principles of generation and detection are presented, stressing a few key characteristics of laser‐ultrasonics: the material is actually the emitting transducer and transduction is made by light, thus eliminating any contact. These features carry both advantages and limitations that are explained. Another feature, which has been an impediment, is actually the complexity of the “laser‐ultrasonic transducer”, but in spite of this complexity, it can be made very reliable for use in severe industrial environments. It also can be very cost effective for a number of applications. Three applications that are now used in industry are presented: the inspection of polymer matrix composites used in aerospace, the measurement of thin layers in microelectronics and the thickness gauging of hot steel tubing in production. Technological aspects, such as interferometer design, detection lasers and others are also discussed. As an overall conclusion, laser‐ultrasonics that was for a long time a laboratory curiosity has definitely now made its transition to industry. Nevertheless, developments should continue to improve performance, to make it well adapted to specific inspection or characterization tasks and more affordable.

Laser‐Ultrasonics: From the Laboratory to Industry

The prototype implementation of a permanently installable high temperature crack monitoring system is described. The system uses an array of waveguides to excite shear horizontal (SH) waves on the surface of the component that is to be inspected. The waveguide is made of poorly heat conducting stainless steel so that it can sustain a large temperature gradient. This ensures that conventional piezoelectric transducers can be used at one end of the waveguide to excite ultrasonic waves while the other end is in contact with the component to be monitored which is situated in a harsh environment at elevated temperature (up to 600 °C). Different crack monitoring strategies using SH waves are discussed. Simulation and experimental results for the array prototype on notched plates are presented. Furthermore, experimental results on samples containing fatigue cracks are shown and measurement results that were collected from a notched plate in a furnace at 550 °C are also presented.

High temperature ultrasonic crack monitoring using SH waves

Non destructive test systems are increasingly applied in the industrial context for their strong potentialities in improving and standardizing quality control. Especially in the intermediate manufacturing stages, early detection of defects on semi-finished products allow their direction towards later production processes according to their quality, with consequent considerable savings in time, energy, materials and work. However, the raw data coming from non destructive test systems are not always immediately suitable for sophisticated defect detection algorithms, due to noise and disturbances which are unavoidable, especially in harsh operating conditions, such as the ones which are typical of the steelmaking cycle. The paper describes some pre-processing operations which are required in order to exploit the data coming from a non destructive test system. Such a system is based on the joint exploitation of Laser and Electro-Magnetic Acoustic Transducer technologies and is applied to the detection of surface and sub-surface cracks in cold and hot steel slabs.

Pre-processing of data coming from a laser-EMAT system for non-destructive testing of steel slabs

Image-based sizing of surface-breaking cracks by SH-wave array ultrasonic testing.

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Laser-Ultrasonics—Principles and Industrial Applications

A new method to obtain a detailed image of surface‐breaking tight cracks by using laser‐ultrasonics is proposed. The surface opposite the cracking is scanned by the generation and detection lasers. Detected signals are processed by the Fourier‐domain Synthetic Aperture Focusing Technique (F‐SAFT). The point of this study is to use the laser‐generated shear waves, which interacts very effectively with the crack roots. A detailed image of the stress corrosion cracks, having depths of typically 0.5 mm and widths of less than 0.05 mm, is successfully reconstructed.

Visualization of Surface‐Breaking Tight Cracks by Laser‐Ultrasonic F‐SAFT

Ultrasonic diffraction pattern at a crack tip and crack depth sizing based on the diffraction are studied by using laser‐generated bulk waves Directivity patterns oriented essentially along the normal for the diffracted longitudinal wave and essentially at 45 (deg) for the diffracted shear wave for any incident angles is obtained A laser‐ultrasonic system combined with time‐of‐flight (TOFD) analysis is produced and demonstrates accurate crack sizing on an artificial slot having a variable depth from 0 mm to 10 mm An improvement of signal‐to‐noise ratio by split spectrum processing (SSP) is also suggested

A. Blouin

Papers

Visualization of Surface‐Breaking Tight Cracks by Laser‐Ultrasonic F‐SAFT

Laser Ultrasonic Study of Crack Tip Diffraction