Effect of Laser Pulse Fatigue on the Mechanical Characteristics of a CFRP Plate

Abstract

A laser ultrasonic based nondestructive evaluation (NDE) technique has been widely used in aerospace industries for inspecting parts and structures made of composite materials. The thermoelastic regime is used for the ultrasonic generation, so no plasma is formed on the surface of composite structure. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy of a laser ultrasonic generator in the long-term periodic maintenance inspection. In this paper, the effect of laser pulse fatigue on the mechanical characteristics of a carbon-fiber-reinforced polymer (CFRP) plate (USN175BX Carbon UD preprag) with the stacking sequence of [0/45/-45/90]s is investigated to verify the reliability of the use of a laser ultrasonic based NDE technique on the CFRP plate specimen inspection. A high-speed laser ultrasonic scanning system (400mm/s at the intervals of 0.4 mm) was setup to perform repeat scanning of 1300 times on a CFRP plate specimen with the scanning area of 70 mm x 60 mm. These repeat scanning times were set in consideration of the periodic maintenance inspection scheduled to be 1 time/week x 52 weeks/year x 25 years. A 532nm Q-switched continuous wave laser (QL) was used and set at the laser pulse energy levels of 0.6 mJ and 1.2 mJ. Lamb wave assessment based on pitch-catch method was proposed in this paper to monitor the mechanical characteristics of a composite specimen. In each completion of 100 times repeat scanning, the Young’s modulus of the scanning area was evaluated based on the group velocity of S0 Lamb wave mode. In addition, the surface condition of the scanning area was investigated by using a microscope.