Laser-Ultrasonics: From the Laboratory to Industry

Abstract

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.

A New Diagnosing Method of the Internal Structure of the Titanium Fan of a Jet Engine with Laser Ultrasonics

“…The technique also features a large detection bandwidth, which is important for numerous applications, particularly small-defect detection and material characterization. The ultrasonic method combined with a laser is a non-contact and nondestructive method, able to be applied to the surfaces of complicated form [4][5][6]. Depending on the laser beam and its interaction with the material being examined, the ultrasonic waves can be generated in a wide range of frequencies up to the hypersonic range.…”

Non destructive inspection of aerospace composites by a fiber-coupled laser ultrasonics system

“…Compared to classical UT, Laser UT (LUT) offers the advantage that no contact or couplant is needed and difficult to access points can be accessed by laser beams at distance. LUT usually combines two principles: the first one is the generation of an ultrasound wave at distance by a laser [2] , the second is the detection of ultrasound at distance by an interferometric system. For the generation, a pulsed laser beam insulates briefly the material on its surface which generates the ultrasound through thermoelastic nondestructive effect or through the destructive ablative effect.…”

“…In any cases, a laser beam is sent on the surface to be probed and the ultrasound wave travelling to the surface will perturb the phase of the laser probe beam. These variations can be measured through a passive interferometer, like the confocal Fabry Perot (FP) [2] , or through adaptive interferometers based on the two-wave mixing (TWM) in photorefractive crystals [8] .…”

“…For industrial application on composites, the second one has to be avoided and the laser and its parameters must be correctly chosen to work in thermoelastic regime. The second principle is the detection of ultrasound at distance by an interferometric system with a large optical etendue allowing working with potentially scattering surfaces [2] . Various principles have been presented in literature.…”

“…Laser ultrasonics [1] [2] is an interesting technique for the inspection of composite structures with complex shapes. The wider use of composite materials in the aerospace industries induces intensive research on adapted nondestructive inspection techniques.…”