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.200704004
Effects 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 mullite–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. Composite 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 environmental factors [1]. Ceramic–matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high temperatures, continue to attract attention as candidate materials for such applications. Additionally, 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 intermediate temperatures (800 C). The degradation involves oxidation of fibers, fiber coatings, and matrices and is typically 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 development of CMCs based on environmentally stable oxide constituents [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. RugglesWrenn). www.elsevier.com/locate/ijfatigue Available online at www.sciencedirect.com International Journal of Fatigue 30 (2008) 502–516 International Journalof Fatigue
M.B. Ruggles-Wrenn et al. International Journal of fatigue 30(2008)502-516 503 It is widely accepted that in order to avoid brittle frac- more dramatic deterioration of fatigue performance, reduc- ture behavior in CMCs and improve the damage tolerance, ing the time to failure in fatigue to the much shorter failure a weak fiber/matrix interface is needed, which serves to times observed in creep These results suggest that time- deflect matrix cracks and to allow subsequent fiber pull- dependent damage processes associated with creep may t[19-22]. It has been demonstrated that similar crack- influence cyclic lifetime. Therefore loading frequency is deflecting behavior can be achieved by using a matrix with likely to have an effect on fatigue durability of the finely distributed porosity instead of a separate interface N720/A composite between matrix and fibers [23]. This concept has been suc- The objective of this study is to investigate effects of cessfully demonstrated for oxide-oxide composites loading frequency and steam environment on fatigue [10, 14, 18, 24-28]. Resulting oxide/oxide CMCs exhibit behavior of N720/A, an oxide-oxide CMC, at 1200C in damage tolerance combined with inherent oxidation resis- air and in steam environments at the loading frequencies tance. An extensive review of the mechanisms and mechan- of 0.1-10 Hz. Results show that the loading frequenc ous matrix CMCs is given in [29] has a marked effect on fatigue life, especially in steam envi- ical properties o ial applications, CFCCs will be subject ronment. The composite microstructure, as well as damage In many potential ap to fatigue loading under a wide range of frequencies. Sev- and failure mechanisms are discussed. eral studies examined high-temperature fatigue perfor mance of CMCs at loading frequencies <10 Hz [30-36]. 2. Experimental procedure At higher frequencies(ranging from 10 to 375 Hz), a strong fect of loading frequency on fatigue life has been demon- 2. 1. Material strated for CMCs with weak fiber-matrix interfaces tested at room temperature [37-39). It was reported that fatigue The material studied was NextelTM720/Alumina life decreased sharply as the loading frequency increased. (N720/A), a commercially available oxide-oxide ceramic This decrease in fatigue life was attributed to frictional composite(COI Ceramics, San Diego, CA), consisting of heating and interface and fiber damage. More recently, it a porous alumina(Al2O3) matrix reinforced with Nex has been shown that the room-temperature fatigue life of telTM720 mullite-alumina fibers composed of 85% Al2O3 certain ceramic-matrix composites with a strong fiber- and 15% Sio2 by weight. The composite was supplied in matrix interface shows little dependence on the loading fre- a form of 2.8 mm thick plates, comprised of twelve 0/900 quency [40]. Vanswijgenhoven et al. [41] found that at woven layers, with a density of 2.77 g/cm and a fiber vol 1200C the fatigue limit of a Nicalon-fabric-reinforced ume of approximately 45%. Matrix porosity was N24% CMC was unaffected by the loading frequency, while the The fiber fabric was infiltrated with the matrix in a sol number of cycles to failure increased and the time to failure gel process. The laminate was dried with a"vacuum bag decreased with increase in frequency technique under low pressure and low temperature, ther Porous matrix oxide/oxide CMCs exhibit several behav- pressureless sintered [46]. No coating was applied to the ior trends that are distinctly different from those exhibited fibers. The damage tolerance of the N720/A composite is by traditional CMCs with a fiber-matrix interface. Most enabled by a porous matrix. Representative micrographs Sic-fiber-containing CMCs exhibit longer life under static of the untested material are presented in Fig. I. Fig. la loading and shorter life under cyclic loading [42]. For these shows 00 and 90 fiber tows as well as numerous matrix materials, fatigue is significantly more damaging than cracks. In the case of the as-processed material, most are creep. Zawada et al. [43] examined the high-temperature shrinkage cracks formed during processing rather than mechanical behavior of a porous matrix Nextel 610/Alumi- matrix cracks generated during loading. Porous nature of nosilicate composite Results revealed excellent fatigue per- the matrix is seen in Fig. Ib formance at 1000C. Conversely, creep lives were short indicating low creep resistance and limiting the use of that 2. 2. Mechanical testing CMC to temperatures below 1000C. Ruggles-Wrenn et al [44] demonstrated that NextelTM720/Alumina(N720/A) A servocontrolled MTS mechanical testing machine composite exhibits excellent fatigue resistance in laboratory equipped with hydraulic water-cooled collet grips, a com air at 1200C. The fatigue limit(based on a run-out condi- pact two-zone resistance-heated furnace, and two tempera tion of 105 cycles)was 170 MPa(88% UTS at 1200C). ture controllers was used in all tests. An MTS TestStar Furthermore, the composite retained 100% of its tensile digital controller was employed for input signal generation strength. However, creep loading was found to be consid- and data acquisition. Strain measurement was accom- erably more damaging. Creep run-out (defined as 100 h plished with an MTS high-temperature air-cooled uniaxial at creep stress) was achieved only at stress levels belov extensometer. For elevated temperature testing, thermo- 50% UTS Mehrman et al. [45] reported that introduction couples were bonded to the specimens using alumina of a short hold period at the maximum stress into the fati- cement(Zircar)to calibrate the furnace on a periodic basis gue cycle significantly degraded the fatigue performance of The furnace controller(using a non-contacting thermocou- N720/A composite at 1200C in air. In steam, superpose- ple exposed to the ambient environment near the test spec tion of a hold time onto a fatigue cycle resulted in an even imen) was adjusted to determine the power setting needed
It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pullout [19–22]. It has been demonstrated that similar crackdeflecting behavior can be achieved by using a matrix with finely distributed porosity instead of a separate interface between matrix and fibers [23]. This concept has been successfully demonstrated for oxide–oxide composites [10,14,18,24–28]. Resulting oxide/oxide CMCs exhibit damage tolerance combined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical properties of porous matrix CMCs is given in [29]. In many potential applications, CFCCs will be subject to fatigue loading under a wide range of frequencies. Several studies examined high-temperature fatigue performance of CMCs at loading frequencies 610 Hz [30–36]. At higher frequencies (ranging from 10 to 375 Hz), a strong effect of loading frequency on fatigue life has been demonstrated for CMCs with weak fiber–matrix interfaces tested at room temperature [37–39]. It was reported that fatigue life decreased sharply as the loading frequency increased. This decrease in fatigue life was attributed to frictional heating and interface and fiber damage. More recently, it has been shown that the room-temperature fatigue life of certain ceramic–matrix composites with a strong fiber– matrix interface shows little dependence on the loading frequency [40]. Vanswijgenhoven et al. [41] found that at 1200 C the fatigue limit of a Nicalon-fabric-reinforced CMC was unaffected by the loading frequency, while the number of cycles to failure increased and the time to failure decreased with increase in frequency. Porous matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by traditional CMCs with a fiber–matrix interface. Most SiC-fiber-containing CMCs exhibit longer life under static loading and shorter life under cyclic loading [42]. For these materials, fatigue is significantly more damaging than creep. Zawada et al. [43] examined the high-temperature mechanical behavior of a porous matrix Nextel 610/Aluminosilicate composite. Results revealed excellent fatigue performance at 1000 C. Conversely, creep lives were short, indicating low creep resistance and limiting the use of that CMC to temperatures below 1000 C. Ruggles-Wrenn et al. [44] demonstrated that Nextel720/Alumina (N720/A) composite exhibits excellent fatigue resistance in laboratory air at 1200 C. The fatigue limit (based on a run-out condition of 105 cycles) was 170 MPa (88% UTS at 1200 C). Furthermore, the composite retained 100% of its tensile strength. However, creep loading was found to be considerably more damaging. Creep run-out (defined as 100 h at creep stress) was achieved only at stress levels below 50% UTS. Mehrman et al. [45] reported that introduction of a short hold period at the maximum stress into the fatigue cycle significantly degraded the fatigue performance of N720/A composite at 1200 C in air. In steam, superposition of a hold time onto a fatigue cycle resulted in an even more dramatic deterioration of fatigue performance, reducing the time to failure in fatigue to the much shorter failure times observed in creep. These results suggest that timedependent damage processes associated with creep may influence cyclic lifetime. Therefore loading frequency is likely to have an effect on fatigue durability of the N720/A composite. The objective of this study is to investigate effects of loading frequency and steam environment on fatigue behavior of N720/A, an oxide–oxide CMC, at 1200 C in air and in steam environments at the loading frequencies of 0.1–10 Hz. Results show that the loading frequency has a marked effect on fatigue life, especially in steam environment. The composite microstructure, as well as damage and failure mechanisms are discussed. 2. Experimental procedure 2.1. Material The material studied was Nextel720/Alumina (N720/A), a commercially available oxide–oxide ceramic composite (COI Ceramics, San Diego, CA), consisting of a porous alumina (Al2O3) matrix reinforced with Nextel720 mullite–alumina fibers composed of 85% Al2O3 and 15% SiO2 by weight. The composite was supplied in a form of 2.8 mm thick plates, comprised of twelve 0/90 woven layers, with a density of 2.77 g/cm3 and a fiber volume of approximately 45%. Matrix porosity was 24%. The fiber fabric was infiltrated with the matrix in a sol– gel process. The laminate was dried with a ‘‘vacuum bag’’ technique under low pressure and low temperature, then pressureless sintered [46]. No coating was applied to the fibers. The damage tolerance of the N720/A composite is enabled by a porous matrix. Representative micrographs of the untested material are presented in Fig. 1. Fig. 1a shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of the as-processed material, most are shrinkage cracks formed during processing rather than matrix cracks generated during loading. Porous nature of the matrix is seen in Fig. 1b. 2.2. Mechanical testing A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet grips, a compact two-zone resistance-heated furnace, and two temperature controllers was used in all tests. An MTS TestStar digital controller was employed for input signal generation and data acquisition. Strain measurement was accomplished with an MTS high-temperature air-cooled uniaxial extensometer. For elevated temperature testing, thermocouples were bonded to the specimens using alumina cement (Zircar) to calibrate the furnace on a periodic basis. The furnace controller (using a non-contacting thermocouple exposed to the ambient environment near the test specimen) was adjusted to determine the power setting needed M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516 503
M. B. Ruggles-Wrenn et al. International Journal of Fatigue 30(2008)502-516 Fig. 1. As-received material: (a)overview, optical microscope and (b) porous nature of the matrix is evident (SEM to achieve the desired temperature of the test specimen. Fatigue run-out was defined as 10 cycles at 0.1 and The determined power setting was then used in actual tests. 1.0 Hz, and as 10 cycles at 10 Hz. The 10 cycle count The power setting for testing in steam was determined by value represents the number of loading cycles expected in placing the specimen instrumented with thermocouples in aerospace applications at that temperature. Fatigue run- steam environment and repeating the furnace calibration out limits were defined as the highest stress level, for which procedure. Thermocouples were not bonded to the test run-out was achieved. Note that in the case of all run-out specimens after the furnace was calibrated. Tests in steam tests, the failure of specimen did not occur when the test environment employed an alumina susceptor (tube with was terminated. Cyclic stress-strain data were recorded end caps), which fits inside the furnace. The specimen gage throughout each test. Thus stiffness degradation as well section is located inside the susceptor, with the ends of the as strain accumulation with fatigue cycles and/or time specimen passing through slots in the susceptor. Steam is could be examined. All specimens that achieved run-out introduced into the susceptor(through a feeding tube) in were subjected to tensile test to failure at 1200C in labo- a continuous stream with a slightly positive pressure, expel- ratory air to determine the retained strength and stifness ling the dry air and creating 100% steam environment inside the susceptor(see Fig. 2). 23. Microstructural characterization All tests were performed at 1200C. In all tests, a spec imen was heated to test temperature in 25 min, and held at Fracture surfaces of failed specimens were examined temperature for additional 15 min prior to testing. Dog using SEM(FEI Quanta 200 HV) as well as an optical bone shaped specimens of 152 mm total length with a 10- microscope(Zeiss Discovery V12). The SEM specimen mm-wide gage section shown in Fig. 2 were used in all tests. were carbon coated. In addition, energy-dispersive X-ray Tensile tests were performed in stroke control with a con- spectroscopy (EDS) analysis was performe using an stant displacement rate of 0.05 mm/s at 1200C in labora- EDAX Genesis 4000 EDS system tory air. The effects of frequency on the fatigue behavior were evaluated in tension-tension fatigue tests conducted 3 Results and discussion at the frequencies of 0. 1 and 10 Hz at 1200oC, in labora tory air and in steam environments. Fatigue data at 3.1. Monotonic tension 1.0 Hz from prior work [44] is included for comparison All fatigue experiments were carried out in load control Tensile results obtained at 1200C were consistent with with the ratio R(minimum to maximum stress) of 0.05. those reported earlier [44, 47]. The ultimate tensile strength (UTS)was 190 MPa, elastic modulus, 76 GPa, and failure strain, 0.38%. It is worthy of note that in all tests reported herein, the failure occurred within the gage section of the extensometer Creep was shown to be considerably more damaging than cyclic loading to oxide-oxide CMCs with porous Fig. 2. Test specimen, dimensions matrix [43, 44]. Recently Ehrman et al. [45] demonstrated
to achieve the desired temperature of the test specimen. The determined power setting was then used in actual tests. The power setting for testing in steam was determined by placing the specimen instrumented with thermocouples in steam environment and repeating the furnace calibration procedure. Thermocouples were not bonded to the test specimens after the furnace was calibrated. Tests in steam environment employed an alumina susceptor (tube with end caps), which fits inside the furnace. The specimen gage section is located inside the susceptor, with the ends of the specimen passing through slots in the susceptor. Steam is introduced into the susceptor (through a feeding tube) in a continuous stream with a slightly positive pressure, expelling the dry air and creating 100% steam environment inside the susceptor (see Fig. 2). All tests were performed at 1200 C. In all tests, a specimen was heated to test temperature in 25 min, and held at temperature for additional 15 min prior to testing. Dog bone shaped specimens of 152 mm total length with a 10- mm-wide gage section shown in Fig. 2 were used in all tests. Tensile tests were performed in stroke control with a constant displacement rate of 0.05 mm/s at 1200 C in laboratory air. The effects of frequency on the fatigue behavior were evaluated in tension–tension fatigue tests conducted at the frequencies of 0.1 and 10 Hz at 1200 C, in laboratory air and in steam environments. Fatigue data at 1.0 Hz from prior work [44] is included for comparison. All fatigue experiments were carried out in load control with the ratio R (minimum to maximum stress) of 0.05. Fatigue run-out was defined as 105 cycles at 0.1 and 1.0 Hz, and as 106 cycles at 10 Hz. The 105 cycle count value represents the number of loading cycles expected in aerospace applications at that temperature. Fatigue runout limits were defined as the highest stress level, for which run-out was achieved. Note that in the case of all run-out tests, the failure of specimen did not occur when the test was terminated. Cyclic stress–strain data were recorded throughout each test. Thus stiffness degradation as well as strain accumulation with fatigue cycles and/or time could be examined. All specimens that achieved run-out were subjected to tensile test to failure at 1200 C in laboratory air to determine the retained strength and stiffness. 2.3. Microstructural characterization Fracture surfaces of failed specimens were examined using SEM (FEI Quanta 200 HV) as well as an optical microscope (Zeiss Discovery V12). The SEM specimens were carbon coated. In addition, energy-dispersive X-ray spectroscopy (EDS) analysis was performed using an EDAX Genesis 4000 EDS system. 3. Results and discussion 3.1. Monotonic tension Tensile results obtained at 1200 C were consistent with those reported earlier [44,47]. The ultimate tensile strength (UTS) was 190 MPa, elastic modulus, 76 GPa, and failure strain, 0.38%. It is worthy of note that in all tests reported herein, the failure occurred within the gage section of the extensometer. 3.2. Tension–tension fatigue Creep was shown to be considerably more damaging than cyclic loading to oxide–oxide CMCs with porous matrix [43,44]. Recently Mehrman et al. [45] demonstrated Fig. 1. As-received material: (a) overview, optical microscope and (b) porous nature of the matrix is evident (SEM). R=50 50.0 76.0 8.0 9.0 5.0 Fig. 2. Test specimen, dimensions in mm. 504 M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516
M.B. Ruggles-Wrenn et al. International Journal of fatigue 30(2008)502-516 Summary of fatigue results for the N720/A composite at 1200C, in laboratory air and steam environments Test environment Max stress(MPa) Cycles to failure Time to failure(h) Fatigue at 0.1 HE Laboratory air 193 Laboratory air 136,1212 2.132 Steam 56,093 35 Steam 48.6 115 150 75 21 12 0.03 Fatigue at 1.0 Hz 33. Laboratory air 40.7 1.144 Laboratory air ,47 52 Laboratory 09.436 25 Steam 100,780 0 0.714 Steam 08 Steam 150 l1,782 Steam 02 0.06 0.81 fatigue at 10 Hz 1,0000102 0.77 Run-out, failure of specimen did not occur when the test was terminated Data from Ruggles-Wrenn et al. [44] that introduction of a short hold period(at the maximum Furthermore, in steam the influence of the loading fre- stress)into the fatigue cycle significantly degraded the fati- quency on fatigue life becomes dramatic. In laboratory gue performance of N720/A composite at 1200C in air air, the high 170 MPa fatigue limit was obtained at both and in steam. These results suggest that the loading rate 0.1 and 1.0 Hz In steam, the fatigue performance was best plays a significant role in damage development. Therefore at the frequency of 10 Hz. Yet, even at 10 Hz the in-steam investigation of the effects of frequency on fatigue behav- fatigue limit is only 150 MPa(78% UTS at 1200 C), ior, especially when conducted in steam environment is noticeably lower than what could be expected in air. At critical to assessing the durability of a given porous matrix the loading frequency of 1.0 Hz, the fatigue limit drops oxide-oxide CMC to 125 MPa(69% UTS at 1200C). As the frequency Tension-tension fatigue tests were conducted at the fre- decreases by another order of magnitude, the fatigue per quencies of 0. I and 10 Hz at 1200C in air and in steam. formance deteriorates drastically. At 0.1 Hz, run-out was Results are summarized in Table l. Results are also pre- not achieved even at the low stress level of 75 MPa(39% sented in Fig. 3 as the stress vs. time to failure curves. UTS at 1200C) Results of fatigue tests at 1.0 Hz from the prior study [44]are included in Table I and in Fig 3 for comparison Data in Table I show that the loading frequency has little T=1200'C, Steam ffect on fatigue performance in air. For the frequencies of 0 I and 1.0 Hz the fatigue limit in air was 170 MPa(88% UTS at 1200 C UTS at 1200C). This fatigue limit is based on the run- out condition of 10 cycles, approximate number of load ing cycles expected in aerospace applications at 1200C. could have resulted in a lower fatigue limit. Because the o 0 It is recognized that a more rigorous run-out condition fatigue performance was expected to improve with increas- ing loading frequency, no tests were conducted in air at the 01.0 Hz, Ruggles-Wrenn et al, 2006 口10Hz frequency of 10 Hz. In view of the excellent fatigue resis- tance and high fatigue limit obtained in air at the frequen cies 0. 1 and 1.0 Hz, an equally high in-air fatigue limit ould be anticipated at 10 Hz. Presence of steam causes noticeable degradation in fatigue performance. At all load- Fig 3. Fatigue S-N curves for NextelT720/alumina ceramic composite at ing frequencies investigated, the fatigue limits obtained 1200C in steam environment. Fatigue data at 1.0 Hz from ruggles Wrenn et al. [44]. Arrow indicates that failure of specimen did not occur steam are significantly lower than those obtained in air. when the test was terminated
that introduction of a short hold period (at the maximum stress) into the fatigue cycle significantly degraded the fatigue performance of N720/A composite at 1200 C in air and in steam. These results suggest that the loading rate plays a significant role in damage development. Therefore investigation of the effects of frequency on fatigue behavior, especially when conducted in steam environment is critical to assessing the durability of a given porous matrix oxide–oxide CMC. Tension–tension fatigue tests were conducted at the frequencies of 0.1 and 10 Hz at 1200 C in air and in steam. Results are summarized in Table 1. Results are also presented in Fig. 3 as the stress vs. time to failure curves. Results of fatigue tests at 1.0 Hz from the prior study [44] are included in Table 1 and in Fig. 3 for comparison. Data in Table 1 show that the loading frequency has little effect on fatigue performance in air. For the frequencies of 0.1 and 1.0 Hz the fatigue limit in air was 170 MPa (88% UTS at 1200 C). This fatigue limit is based on the runout condition of 105 cycles, approximate number of loading cycles expected in aerospace applications at 1200 C. It is recognized that a more rigorous run-out condition could have resulted in a lower fatigue limit. Because the fatigue performance was expected to improve with increasing loading frequency, no tests were conducted in air at the frequency of 10 Hz. In view of the excellent fatigue resistance and high fatigue limit obtained in air at the frequencies 0.1 and 1.0 Hz, an equally high in-air fatigue limit could be anticipated at 10 Hz. Presence of steam causes noticeable degradation in fatigue performance. At all loading frequencies investigated, the fatigue limits obtained in steam are significantly lower than those obtained in air. Furthermore, in steam the influence of the loading frequency on fatigue life becomes dramatic. In laboratory air, the high 170 MPa fatigue limit was obtained at both 0.1 and 1.0 Hz. In steam, the fatigue performance was best at the frequency of 10 Hz. Yet, even at 10 Hz the in-steam fatigue limit is only 150 MPa (78% UTS at 1200 C), noticeably lower than what could be expected in air. At the loading frequency of 1.0 Hz, the fatigue limit drops to 125 MPa (69% UTS at 1200 C). As the frequency decreases by another order of magnitude, the fatigue performance deteriorates drastically. At 0.1 Hz, run-out was not achieved even at the low stress level of 75 MPa (39% UTS at 1200 C). Table 1 Summary of fatigue results for the N720/A composite at 1200 C, in laboratory air and steam environments Test environment Max. stress (MPa) Cycles to failure Time to failure (h) Failure strain (%) Fatigue at 0.1 Hz Laboratory air 170 100,017a 278a 1.93a Laboratory air 170 136,121a 378a 2.13a Steam 75 56,093 156 3.35 Steam 100 17,498 48.6 1.80 Steam 125 1850 5.14 1.15 Steam 150 75 0.21 0.67 Steam 170 12 0.03 0.53 Fatigue at 1.0 Hzb Laboratory air 100 120,199a 33.4a 0.63a Laboratory air 125 146,392a 40.7a 1.14a Laboratory air 150 167,473a 46.5a 1.66a Laboratory air 170 109,436a 30.4a 2.25a Steam 100 100,780a 28.0a 0.71a Steam 125 166,326a 46.2a 1.08a Steam 150 11,782 3.27 1.12 Steam 170 202 0.06 0.81 Fatigue at 10 Hz Steam 150 1,000,010a 27.8a 0.77a Steam 170 11,387 0.32 1.03 a Run-out, failure of specimen did not occur when the test was terminated. b Data from Ruggles-Wrenn et al. [44]. 0 50 100 150 200 250 0.01 0.1 1 10 100 1000 Time to failure (h) ) aP M( ssert S 0.1 Hz 1.0 Hz, Ruggles-Wrenn et al, 2006 10 Hz UTS at 1200 ˚C T = 1200 ˚C, Steam Fig. 3. Fatigue S–N curves for Nextel720/alumina ceramic composite at 1200 C in steam environment. Fatigue data at 1.0 Hz from RugglesWrenn et al. [44]. Arrow indicates that failure of specimen did not occur when the test was terminated. M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516 505
506 M.B. Ruggles-Wrenn et al. International Journal of Fatigue 30 (2008)502-516 Evolution of the hysteresis response of N720/A with Fig 4 reveal that ratcheting, defined as progressive increase fatigue cycles is typified in Fig. 4, which shows hysteresis in accumulated strain with increasing number of cycles stress-strain loops for tests conducted in steam at various continues throughout the test. Effects of loading frequency loading frequencies In all tests, regardless of the frequency, and test environment on hysteresis response are illustrated the most extensive damage occurs on the first cycle, where in Figs. 5a and b, respectively. It is seen that the permanent considerable permanent strain is seen upon unloading. strain produced during the first cycle decreases with Afterwards hysteresis loops stabilize quickly. Results in increasing frequency. It is also seen that larger permanent strain is produced in steam than in air Of importance in cyclic fatigue is the reduction in stifl- ness(hysteresis modulus determined from the maximum T=1200C, Steam and minimum stress-strain data points during a loa Omax= 125 MPa f=0.1 Hz cycle), reflecting the damage development during fatigue cycling. Change in modulus is shown in Fig. 6, where nor malized modulus (i.e. modulus normalized by the modulus obtained in the second cycle) is plotted vs. fatigue cycles. The first cycle was not used for normalization because of Cycle 1 the large permanent strain offset upon unloading. It is note- 00 ycle 50 worthy that although all in-air tests achieved run-out, a decrease in normalized modulus with cycling was still observed(Fig 6a) Modulus loss increased with increasing fatigue stress level. In air at the frequency of 1.0 Hz, the 1251.50 Strain(%) a200 1.0 Hz. Ruggles-Wrenn, 2006 T=1200C, Steam Cycle Omax= 125 MPa Cycle 2 Cycle 1025 T=1200 C, Steam 0250.500.751.00125 0.50 00 Strain (%) Strain(%) 1200℃c, Steam Cycle 1 T=1200"c Cycle 1000 gma=170 MPa 1.001.25150 0.25 0.75 1.00 Strain (%) Strain(%) Fig 4. Typical evolution of stress-strain hysteresis response of N720/A Fig. 5. The stress-strain response of N720/A ceramic composite at with fatigue cycles at 1200C in steam:(a)at 0. I Hz and 125 MPa, (b)at 1200C: (a)in steam environment at three different loading frequencies 1.0 Hz and 125 MPa, data from Ruggles-Wrenn et al. [44](c) at 10 Hz and (b) at 0. I Hz in air and in steam environments. Curves shifted by O1% and 170 MPa for clarity. Data at 1.0 Hz from Ruggles- Wrenn et al. [44]
Evolution of the hysteresis response of N720/A with fatigue cycles is typified in Fig. 4, which shows hysteresis stress–strain loops for tests conducted in steam at various loading frequencies. In all tests, regardless of the frequency, the most extensive damage occurs on the first cycle, where considerable permanent strain is seen upon unloading. Afterwards hysteresis loops stabilize quickly. Results in Fig. 4 reveal that ratcheting, defined as progressive increase in accumulated strain with increasing number of cycles, continues throughout the test. Effects of loading frequency and test environment on hysteresis response are illustrated in Figs. 5a and b, respectively. It is seen that the permanent strain produced during the first cycle decreases with increasing frequency. It is also seen that larger permanent strain is produced in steam than in air. Of importance in cyclic fatigue is the reduction in stiff- ness (hysteresis modulus determined from the maximum and minimum stress–strain data points during a load cycle), reflecting the damage development during fatigue cycling. Change in modulus is shown in Fig. 6, where normalized modulus (i.e. modulus normalized by the modulus obtained in the second cycle) is plotted vs. fatigue cycles. The first cycle was not used for normalization because of the large permanent strain offset upon unloading. It is noteworthy that although all in-air tests achieved run-out, a decrease in normalized modulus with cycling was still observed (Fig. 6a). Modulus loss increased with increasing fatigue stress level. In air at the frequency of 1.0 Hz, the 0.00 0.25 0.50 0.75 1.00 1.25 1.50 Strain (%) T = 1200 ˚C, Steam max = 125 MPa f = 0.1 Hz Cycle 1000 Cycle 1 Cycle 50 Cycle 1820 0 50 100 150 0.00 0.25 0.50 0.75 1.00 1.25 1.50 ) aP M( ssert S ) aP M( ssert S 0 50 100 150 ) aP M( ssert S T = 1200 ˚C, Steam max = 125 MPa f = 1.0 Hz Cycle 100000 Cycle 10000 Cycle 1025 Cycle 25 Cycle 2 Cycle 1 a b c 0 50 100 150 200 0.00 0.25 0.50 0.75 1.00 1.25 1.50 Strain (%) Strain (%) T = 1200 ˚C, Steam σmax = 170 MPa f = 10 Hz Cycle 1 Cycle 10000 Cycle 1000 Cycle 50 Cycle 10 Fig. 4. Typical evolution of stress–strain hysteresis response of N720/A with fatigue cycles at 1200 C in steam: (a) at 0.1 Hz and 125 MPa, (b) at 1.0 Hz and 125 MPa, data from Ruggles-Wrenn et al. [44], (c) at 10 Hz and 170 MPa. 0 50 100 150 200 0.00 0.25 0.50 0.75 1.00 Strain (%) ) aP M( ssert S T = 1200 ˚C, Steam σmax = 170 MPa 0.1 Hz 10 Hz 1.0 Hz, Ruggles-Wrenn, 2006 0.00 0.25 0.50 0.75 1.00 Strain (%) T = 1200 ˚C σmax = 170 MPa f = 0.1 Hz Air Steam a 0 50 100 150 200 ) aP M( ssert S b Fig. 5. The stress–strain response of N720/A ceramic composite at 1200 C: (a) in steam environment at three different loading frequencies and (b) at 0.1 Hz in air and in steam environments. Curves shifted by 0.1% for clarity. Data at 1.0 Hz from Ruggles-Wrenn et al. [44]. 506 M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516