Fatigue testing of CFRP in the Very High Cycle Fatigue (VHCF) regime at ultrasonic frequencies

Abstract

Carbon fiber reinforced polymers (CFRP) are more and more used for high performance applications such as aircraft structures. These structural components are subjected with more than 108 cycles during their lifetime. At present the fatigue behavior of CFRP and the corresponding failure mechanisms are not sufficiently investigated in the VHCF regime. To obtain comprehensive knowledge about the fatigue behavior as well as the failure mechanisms of CFRP in the Very High Cycle Fatigue (VHCF) regime, a new Ultrasonic Testing Facility (UTF) for cyclic three point bending of CFRP at 20 kHz has been developed at the Institute of Materials Science and Engineering (WKK), University of Kaiserslautern. This UTF with combined online nondestructive testing via laser vibrometry and IR-thermography enables VHCF experiments up to 109 cycles in twelve days. All experiments are carried out at a stress ratio between 0.29 and 0.49. To keep the specimen temperature below the glass transition temperature of the CFRP the cyclic bending load is divided in pulse pause sequences. The specimen material used in this investigation is a commercially available carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). For the accurate calibration of the loads as well as for the visualization of the strain distribution and the oscillation mode, measurements with 3D-scanning-laser-vibrometry have been carried out. Furthermore light optical micrographs plus SEM investigations have been performed upon completion and in defined interceptions of the experiments. Based on the results of the microscopic investigations the failure mechanisms of CF-PPS for the VHCF are described.