Impact of Foreign Object Damage to Leading Edge on Fatigue Capability of CMC Turbine Vanes

Abstract

Abstract Ceramic matrix composites (CMCs) are expected to make turbine components lighter because they have lower density than nickel-based super alloys which have been used for turbine components. Also they are expected to improve the efficiency of gas turbine engines by realizing higher turbine inlet temperature (TIT), because they have higher temperature capability. One of the technical issues of CMCs is that they have relatively low impact resistance comparing with nickel-based alloy. In the previous work, it was estimated that the foreign object damage (FOD) on CMC turbine vanes under the actual engine condition is less than 0.36-mm-depth. The concern of such damaged CMC turbine vanes is a decrease in a fatigue capability due to the dent particular to the leading edge (L/E). The fatigue strength reduction can be caused by the stress concentration due to the dented shape and the oxidation due to the coating spallation. There are various works about the impact resistance or the fatigue capability of CMCs, but there are little works which assess the fatigue capability of CMCs which was damaged by foreign object. The objective of this work was to clarify the impact of FOD to L/E on the fatigue capability of SiC/SiC CMC turbine vane. The tension-tension low cycle fatigue (LCF) tests and high cycle fatigue (HCF) tests were conducted using four types of test pieces at elevated temperature in the steam environment. The first one was smooth test piece with anti-oxidation coating, the second one was the test piece which had notch with coating, the third one was the test piece which had notch without coating at notched area and the fourth one was the test piece which was damaged by dropped weight test simulating FOD. The result showed that impact of stress concentration due to notch shape on fatigue capability is small. It was also cleared that uncoated condition did not have impact on the LCF capability but had impact on the HCF capability. The HCF strength reduction of TP without coating to the TP with coating was about 35% at 107 cycles. More fiber breakage was observed at the fracture surface of the TP without coating tested in HCF condition, on the other hand more fiber pullout was observed at that of other TPs. The results suggest that the HCF strength reduction was caused by oxidation when the bare CMC was exposed by FOD. From this work, it was concluded that the loss of coating due to FOD impacted on HCF strength of CMC turbine vane, but it can be accepted because turbine vane is normally designed so that it can endure the stress concentration at L/E due to FOD (typically kt = 3).