Investigation of laser pulse fatigue effect on unpainted and painted CFRP structures

Laser ultrasonic based nondestructive evaluation (NDE) techniques are being increasingly used in aerospace industries. Generally, the service lifetime for an aircraft could be more than 25years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy in the long-term repetitive maintenance inspection. In this paper, the effect of repeat laser pulse scanning on the mechanical characteristics of unpainted and painted CFRP specimens (USN175BX Carbon UD prepreg, Bisphenol A epoxy resin) is investigated to verify the reliability regarding the use of laser ultrasonic scanning based NDE techniques on the inspection of the CFRP structure. A high-speed laser ultrasonic scanning system is setup to perform repeat scanning of 1300 times on both CFRP specimens at the five laser pulse energy levels using the 532-nm and 1064-nm Q-switched continuous wave lasers. Elastic modulus assessment based on the ultrasonic Lamb wave pitch–catch method is used and the surface condition of the scanned area is investigated by a microscope. As a result, the laser pulse fluences that is shown in this paper are suitable for the long-term repetitive maintenance inspection in unpainted and painted CFRP structure even if it demonstrates an embrittlement phenomenon similar to the modulus measurement resolution in the unpainted CFRP specimen. In addition, the laser pulse fluence for maximum signal-to-noise ratio without any damage is investigated in both unpainted and painted CFRP specimens. As a result, both 102.45mJ/cm2 in unpainted CFRP specimen and 51mJ/cm2 in painted CFRP specimen can be the laser pulse energy for the maximum signal-to-noise ratio without any damage.