A mobile nondestructive testing (NDT) system for fast detection of impact damage in fiber-reinforced plastics (FRP)

Abstract

Impact damage in fiber-reinforced plastics, such as carbon-fiber-reinforced plastics (CFRP) and glass-fiber-reinforced plastics (GFRP), involves high challenges to nondestructive testing (NDT). The anisotropic material structure significantly complicates the interpretation of results in conventional testing. Resonant frequency sweep thermography (RFST) based on local defect resonance combined with well-known ultrasonic thermography enables the fast and simple detection of relevant impact damages. RFST utilizes frequency sweep excitation in the low- and mid-kilohertz range to activate defect resonances with low acoustical power of a few megawatts. Resonances of defects amplify the acoustic vibration amplitude by more than 1 order of magnitude and lead to a significant enhancement of the corresponding thermal signal. This is based on both crack friction and/or visco-elastic heating and can be detected at the part surface by an infrared camera. The defect detection threshold depends on excitation power and the distance between the defect and the ultrasonic source. For this new NDT approach, a first prototype system in the form of a tripod with an integrated infrared (IR) camera and ultrasonic excitation was developed. It stands out due to its simple handling and flexible applications. Augmented reality assists the inspector to interpret the results and mark the defect by projecting the evaluated test result onto the part surface. In this article, the first results from a series of impact damages in CFRP of varying impact energies and crack sizes are presented.