Availableonlineatwww.sciencedirect.com Internationa ° Science Direct Journalof Fatique ELSEVIER International Journal of Fatigue 30(2008)502-516 www.elsevier.com/locate/ijfatigue Effects of frequency and environment on fatigue behavior of an oxide-oxide ceramic composite at1200°C查 M. B. Ruggles-Wrenn a, *. G. Hetrick a. SS.Baek Department of Aeronautics and Astronautics, Air Force Institute of Technology Patterson Air Force Base. OH 45433-7765. USA Agency for Defense Decelopment, Daejeo Received 7 August 2006: received in revised form 3 April accepted 8 April 2007 Available online 27 April 2007 Abstract The effect of frequency on fatigue behavior of an oxide-oxide continuous fiber ceramic composite( CFCC) was investigated at 1200C In aboratory air and in steam environment. The composite consists of a porous alumina matrix reinforced with laminated, woven mull- ite-alumina(NextelM720)fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance Tension-tension fatigue tests were performed at frequencies of 0. I and 10 Hz for fatigue stresses ranging from 75 to 170 MPa. Fatigue run-out was defined as 10 cycles at the frequency of 0. 1 Hz and as 10 cycles at the frequency of 10 Hz. The CFCC exhibited excellent fatigue resistance 170 MPa(88% UTS at 1200C). The material retained 100% of its tensile strength. Presence of steam significantly degraded the fatigue performance, with the degradation being most pronounced at 0. 1 Hz. Com posite microstructure, as well as damage and failure mechanisms were investigated. Examination of fracture surfaces revealed higher degrees of fiber pull-out in specimens tested at 10 Hz, indicating weakening of the fiber/matrix interface. a qualitative spectral anal showed evidence of silicon species migration from the fiber to the matrix. Published by elsevier Ltd Keywords: Ceramic-matrix composites(CMCs): Oxides; Fatigue: High-temperature properties; Mechanical testing: Fractography 1. Introduction temperatures, together with a reduced need for cooling air, allow for improved high-temperature performance Advances in aerospace technologies have raised the when compared to conventional nickel-based superalloy mand for structural materials that exhibit superior [2]. Advanced reusable space launch vehicles will likely long-term mechanical properties and retained properties incorporate CMCs in critical propulsion components [3]. under high temperature, high pressure, and varying envi- However, these applications require exposure to oxidizing ronmental factors [1]. Ceramic-matrix composites(CMCs), environments. Therefore the thermodynamic stability and capable of maintaining excellent strength and fracture oxidation resistance of CMCs are vital issues. toughness at high temperatures, continue to attract atten Non-oxide fiber/ non-oxide matrix composites generally tion as candidate materials for such applications. Addition- exhibit poor oxidation resistance [4, 5], particularly at inter ally, the lower densities of CMCs and their higher use mediate temperatures(800C). The degradation involves oxidation of fibers, fiber coatings, and matrices and is typ- ically accelerated by the presence of moisture [6-8]. Using a w The views expressed are those of the authors and do not reflect the non-c oxide fiber/oxide matrix or oxide fiber/non-oxide official policy or position of the United States Air Force, Department of matrix composites generally does not substantially improve Defense or the US government the high temperature oxidation resistance [9]. The need for Corresponding author. Tel: +l 937 255 3636x4641: fax: +1 937 656 environmentally stable composites motivated the develop- E-mail address: marina. ruggles-wrenn(@afit. edu(M.B. Ruggles. ment of CMCs based on environmentally stable oxide con- stituents [10-18] 0142-1123S-see front matter Published by Elsevier Ltd. doi:10.1016/ .fatigue.200704004Effects of frequency and environment on fatigue behavior of an oxide–oxide ceramic composite at 1200 C q M.B. Ruggles-Wrenn a,*, G. Hetrick a , S.S. Baek b a Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, USA b Agency for Defense Development, Daejeon, South Korea Received 7 August 2006; received in revised form 3 April 2007; accepted 8 April 2007 Available online 27 April 2007 Abstract The effect of frequency on fatigue behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) was investigated at 1200 C in laboratory air and in steam environment. The composite consists of a porous alumina matrix reinforced with laminated, woven mull￾ite–alumina (Nextel720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue tests were performed at frequencies of 0.1 and 10 Hz for fatigue stresses ranging from 75 to 170 MPa. Fatigue run-out was defined as 105 cycles at the frequency of 0.1 Hz and as 106 cycles at the frequency of 10 Hz. The CFCC exhibited excellent fatigue resistance in laboratory air. The fatigue limit was 170 MPa (88% UTS at 1200 C). The material retained 100% of its tensile strength. Presence of steam significantly degraded the fatigue performance, with the degradation being most pronounced at 0.1 Hz. Com￾posite microstructure, as well as damage and failure mechanisms were investigated. Examination of fracture surfaces revealed higher degrees of fiber pull-out in specimens tested at 10 Hz, indicating weakening of the fiber/matrix interface. A qualitative spectral analysis showed evidence of silicon species migration from the fiber to the matrix. Published by Elsevier Ltd. Keywords: Ceramic–matrix composites (CMCs); Oxides; Fatigue; High-temperature properties; Mechanical testing; Fractography 1. Introduction Advances in aerospace technologies have raised the demand for structural materials that exhibit superior long-term mechanical properties and retained properties under high temperature, high pressure, and varying envi￾ronmental factors [1]. Ceramic–matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high temperatures, continue to attract atten￾tion as candidate materials for such applications. Addition￾ally, the lower densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-temperature performance when compared to conventional nickel-based superalloys [2]. Advanced reusable space launch vehicles will likely incorporate CMCs in critical propulsion components [3]. However, these applications require exposure to oxidizing environments. Therefore the thermodynamic stability and oxidation resistance of CMCs are vital issues. Non-oxide fiber/non-oxide matrix composites generally exhibit poor oxidation resistance [4,5], particularly at inter￾mediate temperatures (800 C). The degradation involves oxidation of fibers, fiber coatings, and matrices and is typ￾ically accelerated by the presence of moisture [6–8]. Using a non-oxide fiber/oxide matrix or oxide fiber/non-oxide matrix composites generally does not substantially improve the high temperature oxidation resistance [9]. The need for environmentally stable composites motivated the develop￾ment of CMCs based on environmentally stable oxide con￾stituents [10–18]. 0142-1123/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.ijfatigue.2007.04.004 q The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the US Government. * Corresponding author. Tel.: +1 937 255 3636x4641; fax: +1 937 656 4032. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles￾Wrenn). www.elsevier.com/locate/ijfatigue Available online at www.sciencedirect.com International Journal of Fatigue 30 (2008) 502–516 International Journalof Fatigue