Abstract
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.
Highlights
1.1 Impact damage in fiber-reinforced plasticsImpact damage is a short-term point load perpendicular to the part surface
On the basis of these discoveries, a completely new testing approach based on ultrasonic thermography was developed at the Institut für Kunststofftechnik (IKT), which has already been successfully turned into a first prototype
The thermogram is taken at each individual pixel and a discrete Fourier transform (DFT) is evaluated at the sweep duration of the known excitation period
Summary
Impact damage is a short-term point load perpendicular to the part surface. Fiber-reinforced plastics (FRP) are most susceptible to this load direction as this is not a principle fiber orientation. Often cited causes are tool drop, hail or bird strike. One of the most prevalent occurrences in aerospace is the so-called “ramp crash” with ground service vehicles (Kanki and Brasil, 2009). Depending on the impact energy matrix cracks, delaminations or even fiber cracks and penetration of the impactor may occur. Even small energies may produce delaminations that are (barely) visible from the outside (BVID). The stress distribution at impact load leads to increasing damage with increasing depth and is presented in
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