Ultrasonic monitoring and evaluation of very high cycle fatigue of carbon fiber reinforced plastics

Lightweight materials as carbon fiber reinforced plastics (CFRP) have increasingly gained significance as lightweight material for industrial applications such as e.g. aerospace structures and automotive body parts. This entails the request for nondestructive testing (NDT) techniques for quality assurance of CFRP components during production and in operation covering materials characterization and defect and damage detection as well as monitoring and evaluation of ageing phenomena (fatigue) and failure prediction. CFRP components in service are subjected to variable loads, which may amount up to 1011 load cycles in a typical lifespan of more than 20 years, i.e. the fatigue of CFRP materials in the very high cycle regime (more than 109 loading cycles) is of special interest. In a collaborative project with the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern, Germany, a three point bending ultrasonic fatigue testing system in combi nation with online monitoring of the fatigue processes was developed. Its operation frequency of 20 kHz allows to investigate very high cycle fatigue in a reasonable time. The ultrasonic excitation is simultaneously used as input for ultrasonic monitoring techniques. During cyclic loading, ultrasonic vibrations of small CFRP plates are recorded by a laser vibrometer, while the sound irradiated from the plate into the air is recorded via a microphone. The time signals are evaluated by signal processing procedures such as Fast and Short Time Fourier Transformation (FFT and STFT) as well as Hilbert Huang Transformation (HHT). Furthermore, ultrasonic NDT techniques were used to characterize the samples offline in their pristine state as well as during loading pauses along the fatiguing process and after failure. The results are discussed and related to typical failure scenarios of CFRP components