Ultrasonic fatigue and microstructural characterization of thermoplastic and thermoset CFRP in the very high cycle fatigue regimeWednesday (28.09.2016) 11:15 - 11:30 Part of:
Carbon fiber reinforced polymers (CFRP) are increasingly used for high performance applications such as aircraft structures which are often subjected to more than 10E8 loading cycles during their operation time. To utilize the full mechanical performance of CFRP for lightweight applications, the very high cycle fatigue (VHCF) behavior has to be well understood. To realize investigations in the VHCF regime in an economic reasonable time period a novel ultrasonic fatigue testing facility called “Ultrafast-WKK-Kaiserslautern” for CFRP was developed. This facility works with cyclic three-point bending at a frequency of 20 kHz and enables experiments up to 109 loading cycles in twelve days without unacceptable heating of the polymer due to pulse and pause sequences. The VHCF behavior of a carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) was analyzed systematically up to 10E9 cycles for the first time using this testing facility. Lifetime-oriented investigations showed a significant decrease of the bearable stress amplitudes in the range between 10E6 and 10E9 cycles. Based on light optical and SEM investigations the fatigue damage mechanisms in the VHCF regime from first fiber-matrix debonding up to meta- and finally macro-delaminations were characterized in detail. The fatigue damage development in the specimen’s volume was investigated using computed tomographic scans as well as X-ray refraction measurements. A free edge effect was not detectable. In comparison to the investigated CF-PPS the VHCF behavior of a carbon fiber satin fabric reinforced epoxy resin (CF-EP) was also investigated up 10E9 loading cycles. A significantly different VHCF behavior was observed so far. No fiber-matrix debonding or transversal cracks were present on the specimen edges, but a sudden specimen failure along with carbon fiber breakage. With regard to the monotonic properties the bearable stress amplitudes for CF-EP were higher compared to CF-PPS.