Laser Ultrasound Flexible System using Mid-Wave Infrared Generation Laser for Non-Contact Inspection of Composite Structures made of CFRP

Ultrasonic testing (UT) is commonly used in the industry for the investigation of carbon fiber reinforced polymer (CFRP). It is wide-spread and very efficient but, major problems arise when the shape of the element to be investigated is complex (peak, valley, small radius of curvature...). To overcome these problems laser ultrasonic testing (LUT) can be used and the recent developments show promising results.
Compared to classical UT, LUT offers the advantage that no contact nor couplant are needed and points difficult to access can be illuminated by laser beams at distance. LUT usually combines two principles: the first one is the generation of an ultrasonic wave by a laser. For generation, CO2 lasers emitting at 10 μm wavelength are usually used. However, the optical fibers for 10 μm wavelength are not capable to cope with laser ultrasonic system needs. Therefore, infrared systems use a jointed articulated beam delivery system which limits the flexibility of the arm. To circumvent this limitation, an all-fibered laser ultrasonic system can be used. For this purpose, visible wavelengths are becoming more common. However, visible generation is more likely to damage the sample under investigation.
Halfway between these two technologies, OPO lasers are commercially available in the range of 3 to 3.5 μm wavelength. They already have been considered in earlier experiments with CFRP which show that it is an advantageous wavelength compared to visible/near infrared or far infrared generation. Mid-infrared (MIR) generation is expected to combine both advantages of visible and far-infrared. Solutions now exist for transporting MIR light of OPO lasers into fiber, thus avoiding articulated beam delivery system. Also MIR can be used at higher power than visible light without damaging the surface of the sample under investigation. For these reasons, we have started developing
a fully-fiber coupled laser ultrasonics head with MIR OPO generation to put on a robot arm for the non-destructive inspection of CFRP components. In this conference, we will present the first step of this development, and results obtained on a reference plate with the OPO and are compared to those obtained with the green (532 nm) and far infrared (10.6 μm) generations.