NASA Scientific and Technical Aerospace Reports

20040112043 NASA Glenn Research Center, Cleveland, OH, USA
Probabilistic Assessment of a CMC Turbine Vane
Murthy, Pappu L. N.; Brewer, Dave; Mital, Subodh K.; [2004]; 20 pp.; In English; ASME Turbo Expo 2004, 14-17 Jun. 2004,
Vienna, Austria
Contract(s)/Grant(s): 22-714-30-09; No Copyright; Avail: CASI; A03, Hardcopy
In order to demonstrate the advanced CMC technology under development within the Ultra Efficient Engine Technology
(UEET) program, it has been planned to fabricate, test and analyze an all CMC turbine vane made of a SiC/SiC composite
material. The objective was to utilize a 5-II Satin Weave SiC/CVI SiC/ and MI SiC matrix material that was developed
in-house under the Enabling Propulsion Materials (EPM) program, to design and fabricate a stator vane that can endure
successfully 1000 hours of engine service conditions operation. The design requirements for the vane are to be able to
withstand a maximum of 2400 F within the substrate and the hot surface temperature of 2700 F with the aid of an in-house
developed Environmental/Thermal Barrier Coating (EBC/TBC) system. The vane will be tested in a High Pressure Burner Rig
at NASA Glenn Research Center facility. This rig is capable of simulating the engine service environment. The present paper
focuses on a probabilistic assessment of the vane. The material stress/strain relationship shows a bilinear behavior with a
distinct knee corresponding to what is often termed as first matrix cracking strength. This is a critical life limiting consideration
for these materials. The vane is therefore designed such that the maximum stresses are within this limit so that the structure
is never subjected to loads beyond the first matrix cracking strength. Any violation of this design requirement is considered
as failure. Probabilistic analysis is performed in order to determine the probability of failure based on this assumption. In the
analysis, material properties, strength, and pressures are considered random variables. The variations in properties and strength
are based on the actual experimental data generated in house. The mean values for the pressures on the upper surface and the
lower surface are known but their distributions are unknown. In the present analysis the pressures are considered normally
distributed with a nominal variation. Temperature profile on the vane is obtained by performing a CFD analysis and is assumed
to be deterministic.
Author (revised)
Probability Theory; Structural Failure; Failure Analysis; Ceramic Matrix Composites; Turbine Blades; Vanes
20040112049 NASA Glenn Research Center, Cleveland, OH, USA
The Effect of Stress Rate on Slow Crack Growth Parameter Estimates
Salem, Jonathan A.; Jenkins, Michael G.; [2004]; 21 pp.; In English; ASME Turbo Expo, 14-17 Jun. 2004, Vienna, Austria;
No Copyright; Avail: CASI; A03, Hardcopy
The use of dynamic fatigue testing to generate data for life prediction assumes that test specimen lifetime is dominated
by a single region of the slow crack growth (SCG) curve. This assumption breaks down at excessive stressing rates when a
significant portion of the crack growth occurs in all three regions of the SCG curve. The resultant averaging of data
representing all three regions of the SCG curve results in an error, and such data may not be accurate for lifetime predictions.
Dynamic fatigue tests covering five orders of magnitude were performed on an alumina. Statistical analysis indicates a
significant effect of stress rate on the SCG parameter n for stress rates above several MPa/s, in agreement with analytical
results of Sudreau et al. Recommendations for dynamic fatigue testing are given with regard to the current procedures in
ASTM C 1368.
Author
Stress Analysis; Crack Propagation; Fatigue Tests
20040112053 NASA Glenn Research Center, Cleveland, OH, USA
Estimation and Simulation of Slow Crack Growth Parameters from Constant Stress Rate Data
Salem, Jonathan A.; Weaver, Aaron S.; Mechanics of Ceramics; [2003]; Volume 12; 21 pp.; In English; 8th International
Symposium on Fracture Mechanics of Ceramics, 26-28 Feb. 2003, Houston, TX, USA
Contract(s)/Grant(s): WBS 400-35-40-07; No Copyright; Avail: CASI; A03, Hardcopy
Closed form, approximate functions for estimating the variances and degrees-of-freedom associated with the slow crack
growth parameters n, D, B, and A(sup *) as measured using constant stress rate (’dynamic fatigue’) testing were derived by
using propagation of errors. Estimates made with the resulting functions and slow crack growth data for a sapphire window
were compared to the results of Monte Carlo simulations. The functions for estimation of the variances of the parameters were
derived both with and without logarithmic transformation of the initial slow crack growth equations. The transformation was
performed to make the functions both more linear and more normal. Comparison of the Monte Carlo results and the closed
form expressions derived with propagation of errors indicated that linearization is not required for good estimates of the
variances of parameters n and D by the propagation of errors method. However, good estimates variances of the parameters
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