Larry P. Zawada

Frequency dependence of high-cycle fatigue behavior of CVI C/SiC at room temperature

Effects of loading frequency on high-cycle fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite were investigated. Tension–tension fatigue tests were conducted at three frequencies, 4, 40 and 375 Hz. Fatigue run out was set to 107 cycles. Applied stress versus cycles to failure (S–N) relationships were developed for these three frequencies. At 4 and 40 Hz, fatigue run out was achieved at a stress level of 375 MPa. At 375 Hz, stress level for run out was 350 MPa. Frequency dependence was observed between the two lower frequencies (4 and 40 Hz) and the higher frequency (375 Hz), but not between two lower frequencies (4 and 40 Hz). This manifested as a reduction in cycles to failure at 375 Hz compared to 4 and 40 Hz at a given stress level. Specimen surface temperature increased due to internal heat generation from sliding friction between constituents of the composite under cyclic loading. This increase was directly related to frequency and/or applied cyclic stress level. There was no clear indication that frequency greatly impacted either the stress-strain response or the overall appearance of fracture surfaces. However, a closer examination of specimens cycled at the highest frequency (375 Hz) showed evidence of the localized oxidation at fiber surfaces that might have attributed to the reduction in fatigue life at this frequency.

Simultaneous Fatigue and Combustion Exposure of a SiC/SiC Ceramic Matrix Composite

A melt-infiltrated (MI) woven ceramic matrix composite consisting of a silicon carbide matrix reinforced by boron nitride coated Hi-Nicalon type STM SiC fiber, Hi-Nic-S/BN/SiC, was tested under tension-tension fatigue loading in combination with combustion conditions representative of those experienced by hot-section components such as turbine blades and vanes in modern gas turbine engines. The burner rig fatigue data and fracture surfaces were analyzed for the effects of oxidation on life, failure, and damage mechanisms. These test results were then compared with those obtained from similar fatigue tests performed in a standard furnace under laboratory air environment. Fatigue life in the combustion condition was lower by an order of magnitude in comparison to the isothermal furnace results across the range of applied stress, and so demonstrates the importance of representative combined environment testing in conjunction with fundamental load testing. The observed difference in fatigue performance is attributed to the thermal gradient stress and increased rate of oxidation due to a high moisture level in the combustion rig test condition. The former was verified using finite element analysis and the latter from microscopic analysis of the fracture surfaces.

Evaluation Of Ceramic Matrix Composite Exhaust Nozzle Divergent Seals

During the last six years substantial research efforts have been devoted towards evaluating ceramic matrix composite (CMC) materials for aerospace gas turbine engines. A majority of these technical activities have been directed towards the insertion of CMCs into the exhaust nozzles of aerospace military turbine engines that utilize an afterburner. The effort discussed here is studying an advanced self-sealing ceramic matrix composite called SEPCARBINOX A500. Carbon fibers are utilized as the reinforcement. The matrix is applied by Chemical Vapor Infiltration (CVI), and consists of a novel technology that utilizes sequential layers of Silicon Carbide (SiC) and specific sequences of Si, C, and B. The specific application involves the F100 gas turbine engine exhaust nozzle divergent seals. This CMC has been subjected to extensive material evaluations, subelement testing in simulated exhaust nozzle environment conditions, and over two years of ground testing. The lack of any degradation in the ground tested hardware run to 1.5X the design life prompted the start of a feld service evaluation (FSE). Starting in July, 2005, a total of 8 CMC divergent seals began flying at an operation base on two F-16 aircraft. In February, 2006, 20 additional CMC divergent seals began flying on F-15 aircraft at a second operational base. Discussion will address the unique CMC material and how it is performing in flight. A CMC seal has been removed from the FSE program and evaluated for retained tensile strength and microstructural stability. The seal showed no evidence of wear, the micrographs of the microstructure showed no signs of degradation, and no decrease in strength was measured.

Elevated Temperature Tensile Behavior of Nextel™ 720 Fibers

The tensile behavior of Nextel 720 fibers at elevated temperature was compared with room temperature results for both bare and monazite-coated fibers. While coated and uncoated fibers have nearly identical tensile strengths and Weibull moduli at room temperature, differences in response were seen at elevated temperature. Coated fibers tested at 1200°C were found to have a 40% drop in strength. Uncoated fibers at high temperature exhibited 55% less strength than at room temperature. However, the tensile strength distribution for uncoated fibers tested at high temperature exhibited two distinct populations, indicating two different failure mechanisms. One population showed a 50% drop while the other showed a 64% drop. The coating was thus found to have a protective effect in terms of short-duration high-temperature exposure. Further, the effect of soaking on strength was investigated by thermally soaking coated and uncoated fibers in air at 1200°C for 100 hours prior to tensile testing at elevated temperature. In this case, the long duration of thermal exposure appeared to eliminate the beneficial effects of the coating. Soaked fibers, both coated and uncoated, were found to have nearly identical strengths at 1200°C—a reduction of about 60%.Copyright