Advanced gas turbine engines will operate at higher temperatures than current engines to improve engine efficiency. Consequently, the designers of these engines need materials that can withstand elevated temperatures and provide sustained performance. Ceramic matrix composites (CMCs) are the preferred class of materials because of their high-temperature capability and high specific strength. In fact, a woven, melt-infiltrated SiC/SiC composite was identified as a candidate combustor liner material for advanced gas turbine engines (ref. 1). Since gas turbine engine components are subjected routinely to cyclic loads at elevated temperatures, the fatigue durability of the CMC needed to be characterized under different mechanical loading conditions and temperatures.
At the NASA Glenn Research Center, the cumulative fatigue behavior of the candidate material, a woven, melt-infiltrated SiC/SiC composite, was investigated. The influence of R-ratio (minimum load/maximum load in a cycle) on the fatigue life of the woven SiC/SiC composite (manufactured in September 1999 and designated as N22) was initially established by conducting fatigue tests at R-ratios of 0.05 and 0.50. Two test temperatures (1038 and 1204 °C) were used, and five tests were conducted for each temperature and R-ratio combination. Tensile properties of the CMC at these two temperatures are reported in reference 2. For all the fatigue tests, a maximum stress of 179 MPa (which is higher than the proportional limit strength of the CMC at both temperatures) and a frequency of 0.33 Hz were used. At both temperatures, higher R-ratio tests (with larger mean stresses) yielded longer fatigue lives than lower R-ratio tests (see the bar chart). The error bars in the bar chart represent observed scatter in the fatigue life data.
Mean logarithmic fatigue lives and extreme values for the woven SiC/SiC composite. Maximum stress, 179 MPa; frequency, 0.33 Hz.
Cumulative fatigue tests, at an R-ratio of 0.05 followed by an R-ratio of 0.50 on the same specimen, were subsequently conducted at the same two temperatures. In these tests, a life fraction n1/N1 (where n1 is the number of applied cycles and N1 is the previously established average cyclic life) of 0.1, 0.25, and so forth, was first applied at an R-ratio of 0.05. Each test was then switched to an R-ratio of 0.50 and continued until the specimen failed or accumulated 106 cycles (runout) at the second loading condition. The number of cycles sustained by the specimen at the second loading condition n2 and the corresponding average cyclic life N2 were used to calculate the second life fraction n2/N2 and to analyze the cumulative fatigue data (see the graph). In this graph, the initial life fraction n1/N1 is plotted against the sum of life fractions, (n1/N1) + (n2/N2), from both loading conditions, and the upward arrow indicates a runout. Miner’s linear damage rule, developed originally for metallic alloys, dictates that the sum of life fractions should be near unity for cumulative fatigue tests. For the loading conditions investigated, the majority of the cumulative fatigue data (except for two tests) were above unity, indicating that Miner’s rule constituted a lower bound for this CMC in most cases.
Evaluation of the applicability of Miner’s linear damage rule with cumulative fatigue results from the woven SiC/SiC composite.
Last updated: October 11, 2006
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