Mid-infrared pulsed laser ultrasonic testing for carbon fiber reinforced plastics

•The mid-IR laser source was developed based on an optical parametric oscillator.•The mid-IR laser generated a significantly larger ultrasonic amplitude in CFRPs.•The ultrasonic amplitude increased with the increase of the laser fluence.•The surface epoxy matrix of CFRPs is important in laser ultrasonic generation. Laser ultrasonic testing (LUT) can realize contactless and instantaneous non-destructive testing, but its signal-to-noise ratio must be improved in order to measure carbon fiber reinforced plastics (CFRPs). We have developed a mid-infrared (mid-IR) laser source optimal for generating ultrasonic waves in CFRPs by using a wavelength conversion device based on an optical parametric oscillator. This paper reports a comparison of the ultrasonic generation behavior between the mid-IR laser and the Nd:YAG laser. The mid-IR laser generated a significantly larger ultrasonic amplitude in CFRP laminates than a conventional Nd:YAG laser. In addition, our study revealed that the surface epoxy matrix of CFRPs plays an important role in laser ultrasonic generation.