Ultrasonic fatigue and microstructural characterization of carbon fiber fabric reinforced polyphenylene sulfide in the very high cycle fatigue regime

Abstract

Carbon fiber reinforced polymers (CFRP) are increasingly used for high performance applications, especially for aircraft structures. These structural components are subjected higher than 108 loading cycles during their operation time of more than 20 years. To utilize the full mechanical performance of CFRP for lightweight applications, the very high cycle fatigue (VHCF) behavior has to be well understood. Therefore the VHCF behavior of a carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) was analyzed systematically up to 109 loading cycles. To realize these investigations in an economic reasonable time period a novel ultrasonic fatigue testing facility for CFRP was used. This facility works with cyclic three-point bending at a frequency of 20 kHz. Lifetime-oriented investigations at stress ratios between R = 0.21 and 0.51 showed an exponential decrease of the bearable stress amplitudes in the range between 106 and 109 cycles. Interrupted constant amplitude tests were performed for the characterization of the fatigue damage development. Based on light optical as well as SEM investigations the fatigue damage mechanisms in the VHCF regime were characterized in detail. The surface crack density was determined for several load amplitudes and in different fatigue states by analyzing merged light optical micrographs. Furthermore the stiffness degradation was measured ex-situ during interruptions of the constant amplitude tests, which shows a good correlation with the surface crack density of ultrasonically fatigued CF-PPS.