NASA/TM—2006-214088 Life Limiting Behavior in Interlaminar Shear of Continuous Fiber-Reinforced Ceramic Matrix Composites at Elevated Temperatures Sung R. Choi University of Toledo, Toledo, Ohio Anthony M. Calomino, Narottam P. Bansal, and Michael J. Verrilli Glenn Research Center, Cleveland, Ohio January 2006 The NASA STI Program Office . . . in Profile Since its founding, NASA has been dedicated to • CONFERENCE PUBLICATION. Collected the advancement of aeronautics and space papers from scientific and technical science. The NASA Scientific and Technical conferences, symposia, seminars, or other Information (STI) Program Office plays a key part meetings sponsored or cosponsored by in helping NASA maintain this important role. NASA. The NASA STI Program Office is operated by • SPECIAL PUBLICATION. Scientific, Langley Research Center, the Lead Center for technical, or historical information from NASA’s scientific and technical information. 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Verrilli Glenn Research Center, Cleveland, Ohio National Aeronautics and Space Administration Glenn Research Center January 2006 Acknowledgments This work was supported by the Ultra-Efficient Engine Technology (UEET) Project, NASA Glenn Research Center, Cleveland, Ohio. The authors are grateful to R. Pawlik for experimental work during the course of this work. This report is a formal draft or working paper, intended to solicit comments and ideas from a technical peer group. Trade names or manufacturers’ names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Available from NASA Center for Aerospace Information National Technical Information Service 7121 Standard Drive 5285 Port Royal Road Hanover, MD 21076 Springfield, VA 22100 Available electronically at http://gltrs.grc.nasa.gov Life Limiting Behavior in Interlaminar Shear of Continuous Fiber-Reinforced Ceramic Matrix Composites at Elevated Temperatures Sung R. Choi University of Toledo Toledo, Ohio 43606 Anthony M. Calomino, Narottam P. Bansal, and Michael J. Verrilli National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135 Abstract Interlaminar shear strength of four different fiber-reinforced ceramic matrix composites was determined with double-notch shear test specimens as a function of test rate at elevated temperatures ranging from 1100 to 1316 °C in air. Life limiting behavior, represented as interlaminar shear strength degradation with decreasing test rate, was significant for 2-D crossplied SiC/MAS-5 and 2-D plain-woven C/SiC composites, but insignificant for 2-D plain-woven SiC/SiC and 2-D woven Sylramic SiC/SiC composites. A phenomenological, power-law delayed failure model was proposed to account for and to quantify the rate dependency of interlaminar shear strength of the composites. Additional stress rupture testing in interlaminar shear was conducted at elevated temperatures to validate the proposed model. The model was in good agreement with SiC/MAS-5 and C/SiC composites, but in poor to reasonable agreement with Sylramic SiC/SiC. Constant shear stress-rate testing was proposed as a possible means of life prediction testing methodology for ceramic matrix composites subjected to interlaminar shear at elevated temperatures when short lifetimes are expected. 1. Introduction Successful development and design of continuous fiber-reinforced ceramic matrix composites (CMCs) are dependent on understanding their properties such as deformation, fracture, delayed failure (fatigue, slow crack growth, or damage accumulation), and environmental durability. Particularly, accurate evaluation of delayed failure behavior under specified loading/environment conditions is prerequisite to ensure accurate life prediction of structural CMC components at elevated temperatures. Although fiber-reinforced CMCs have shown improved resistance to fracture and increased damage tolerance compared with monolithic ceramics, inherent material/processing defects or cracks in the matrix-rich interlaminar regions can still cause delamination under interlaminar normal or shear stress, resulting in loss of stiffness or in some cases structural failure. Strength behavior of CMCs in shear has been characterized in view of their unique interfacial architectures and its importance in structural applications (refs. 1 to 4). Because of the inherent nature of CMCs, it would be highly feasible that interlaminar defects or cracks are susceptible to delayed failure particularly at elevated temperatures, resulting in strength degradation or shortened time to failure. Although delayed failure is one of the important life-limiting phenomena, few studies have been done on this subject for CMCs under shear at elevated temperatures. In a previous study (ref. 5), both interlaminar and in-plane shear strengths of a unidirectional Hi-Nicalon™ SiC fiber-reinforced barium strontium aluminosilicate (SiC/BSAS) composite were determined at 1100 °C in air as a function of test rate using double-notch shear test specimens. The composite exhibited a significant effect of test rate on shear strength, regardless of orientation. The shear strength degraded by about 50 percent as test rate decreased from the highest (102 MPa/s) to the lowest NASA/TM—2006-214088 1 (10–2 MPa/s). A phenomenological, life-prediction model has been proposed and formulated to account for the shear strength degradation of the composite. This paper, as an extension of the previous study, describes life limiting behavior in interlaminar shear of four different fiber-reinforced CMCs at elevated temperatures, including three SiC fiber- reinforced CMCs and one carbon-fiber reinforced CMC. Interlaminar shear strength of each composite was determined in double notch shear as a function of test rate using constant stress-rate testing. The interlaminar shear strength was analyzed using the power-law type of crack growth model proposed previously in order to quantify the rate dependency/delayed failure of the composites under interlaminar shear. Stress rupture testing in interlaminar shear was also conducted at elevated temperatures with three selected CMCs in order to validate the proposed model. Some of the data in this paper have been reported previously (ref. 6). 2. Experimental Procedures Four different CMCs—three SiC fiber-reinforced and one carbon fiber-reinforced—were used in this study, including Nicalon™ SiC 2-D crossplied fiber-reinforced magnesium aluminosilicate (designated SiC/MAS-5), Nicalon™ SiC 2-D plain-woven silicon carbide (designated SiC/SiC), Sylramic SiC 2-D woven silicon carbide (designated Sylramic SiC/SiC), and T300™ carbon-fiber 2-D plain-woven silicon carbide (designated C/SiC). The SiC/MAS-5 composites were fabricated through hot pressing followed by ceraming of the composites by a thermal process. The matrix was doped with 5 vol% of borosilicate glass to increase oxidation resistance and interfacial shear strength at the fiber/matrix interfaces. The silicon carbide matrix in the SiC/SiC composite was processed through chemical vapor infiltration (CVI) into the fiber preforms. Silicon carbide was also chemically vapor deposited onto the composite panels to cover the residual porosity. More detailed information regarding the processing of these SiC/MAS-5 and SiC/SiC composites can be found elsewhere (ref. 7). The Sylramic cloth preforms in the Sylramic SiC/SiC composite were stacked and chemically vapor infiltrated with a thin BN-based interface coating followed by SiC matrix over-coating. Remaining matrix porosity was filled with SiC particulates and then with molten silicon at 1400 °C, a process termed slurry casting and melt infiltration (ref. 8). Mechanical properties and fabrication and testing of vane subelements using the Sylramic SiC/SiC composites have been evaluated and addressed extensively in recent studies (refs. 9 to 11). The carbon fiber performs in the C/SiC composite were coated with pyrolytic carbon as an interface prior to CVI SiC infiltration (ref. 12). Stress/life behavior of the C/SiC composites has been characterized in a low partial pressure of oxygen at elevated temperatures (refs. 12 and 13). Fiber volume fraction, laminate architecture, nominal dimensions of shear test specimens, and other information of the CMCs used in this work are summarized in table 1. Weaving patterns of the CMCs are also shown in figure 1. The double-notch-shear (DNS) test specimens were machined from panels of each CMC. Typically, test specimens were 13 to 15 mm wide (W) and 30 mm long (L). The thickness of test specimens was the same as a nominal thickness of panels of each composite (see table 1). Two notches, 0.3 mm wide (h) and 6 mm (Ln) away from each other, were made into each test specimen such that the two notches were extended to the middle of each specimen within ±0.05 mm so that shear failure occurred on the plane between the notch tips. Schematics of DNS test specimen showing a notch configuration is shown in figure 2. Monotonic shear testing for DNS test specimens was conducted at elevated temperatures with different test rates in ambient air (relative humidity of about 45 percent), using an electromechanical test frame (Model 8562, Instron, Canton, Massachusetts) under load control. This type of testing, employing with different test rates, is called constant stress-rate or “dynamic fatigue” testing that is used for advanced monolithic ceramics and other brittle materials such as glasses and glass ceramics in order to evaluate their slow crack growth behavior in flexure or in tension (refs. 14 and 15). Test temperatures were 1100, 1200, 1316, and 1200 °C, respectively, for SiC/MAS-5, SiC/SiC, Sylramic SiC/SiC, and C/SiC composites. NASA/TM—2006-214088 2 TABLE 1.—FIBER-REINFORCED CERAMIC MATRIX COMPOSITES USED IN THIS WORK Composites Weaving Fiber Fiber Panels/notches Manufacturer volume fraction SiC/MAS-5 2-D cross- Nicalon# 0.39 16 plies; t = 3.2 mm; W = 12.7 Corning, Inc. (’93) plied (0/90°) SiC mm; L = 6 mm (Corning, NY) n SiC/SiC (’92) 2-D plain Nicalon 0.39 6.7 epc§; 12 plies; t = 3.5 mm; DuPont Lanxide Composites (now GE Power woven SiC W = 13.0; L = 6 mm Systems Composites, GE PSC) (Newark, DE) n Sylramic 2-D woven Sylramic* 0.36 7.9 epc; 5 HS; 8 plies; t = 2 mm; Honeywell Advanced Composites, Inc. SiC/SiC (’99) SiC W = 12.7/6mm; L = 6 mm (now GE PSC) (Newark, DE) n C/SiC (’97) 2-D plain T300$ 0.46 7.5 epc; 26 plies; t = 3.3 mm; Honeywell Advanced Composites, Inc. woven carbon W = 15.4 mm; L = 6 mm (now GE PSC) (Newark, DE) n # Nippon Carbon Co. (Japan) *Dow Corning (Midland, MI) $ Toray Industries, Inc. (Japan) § epc = end per centimeter. 4 mm (a) (b) (c) (d) Figure 1.—Weaving patterns of continuous fiber-reinforced ceramic matrix composites used in this work: (a) SiC/MAS-5 (2-D cross- plied); (b) SiC/SiC (2-D plain-woven); (c) Sylramic SiC/SiC (2-D woven); (4) C/SiC (2-D plain-woven). NASA/TM—2006-214088 3 R 0.15mm L L n h 0.30mm Notch detail W t (a) P Upper fixture Test specimen Guides (optional) Thermocouple Lower fixture (b) Figure 2.—(a) Configurations of double notch shear (DNS) test specimen; (b) A schematic showing test fixture and test specimen used in this work. A total of three to four applied interlaminar shear stress rates ranging from 10–4 to 101 MPa/s were used for a given composite, depending on the type of composite. Typically, three to ten test specimens were tested at each test rate, again depending on type and availability of materials. A simple test-fixture configuration consisting of SiC upper and lower fixtures, as shown in figure 2, was used for test specimens whose thickness was ≥3 mm. With this specimen thickness and the tight machining tolerances, the test specimens could stand alone and be subjected to negligible bending (≤4 percent) due to misalignment, geometrical inaccuracies, and/or buckling. By contrast, for thin test specimens whose thickness was about 2 mm (e.g., Sylramic SiC/SiC composite), anti-buckling guides were used with specially designed ring-shaped fixtures. Each test specimen was kept for about 20 min at test temperature for thermal equilibration prior to testing. Basically, test specimen configurations and testing procedures were followed in accordance with ASTM test method C 1425 (ref. 16). The interlaminar shear fracture stress—the average interlaminar shear stress at failure—was calculated using the following relation P f τ = (1) f WL n NASA/TM—2006-214088 4 where τ is the interlaminar shear strength, P is the fracture force, and W and L are the specimen width f f n and the distance between the two notches, respectively (see fig. 2). The applied interlaminar shear stress rateτ(cid:5) was given as follows: (cid:5) P τ(cid:5) = (2) WL n (cid:5) where Pis applied shear (or compressive) force rate employed via a test frame in load control. Additionally, stress rupture testing in interlaminar shear was conducted with DNS test specimens for SiC/MAS-5 at 1100 °C, Sylramic SiC/SiC at 1316 °C, and C/SiC at 1200 °C in air. The test frame, test specimen configuration, and test fixture used in stress rupture were the same as those used in constant stress-rate testing. For each composite, the number of applied shear stresses was typically three, and the minimum number of test specimens used was six. This stress rupture testing was performed to determine life limiting behavior of the chosen composites under constant applied shear stress and to relate their stress rupture data to the respective constant stress-rate data in order to validate the phenomenological life prediction model proposed. 3. Experimental Results 3.1. Constant Stress-Rate Testing Without exception, all specimens tested failed in interlaminar shear mode along their prospective shear planes. A typical example of a tested specimen showing such interlaminar shear mode failure is presented in figure 3, together with a test specimen prior to testing for comparison. P τ τ Prospective shear plane (a) After test (b) Before test Figure 3.—(a) A typical example showing interlaminar shear failure of a Sylramic SiC/SiC composite double-notch-shear (DNS) specimen tested at 1316 °C in air at 5 MPa/s. The corresponding specimen configuration prior to test is shown in (b) for comparison. Note a slight change of specimen surface appearance due to high temperature exposure/oxidation. NASA/TM—2006-214088 5 3.1.1. SiC/MAS-5 Composite The results of monotonic interlaminar shear strength testing for the 2-D crossplied SiC/MAS-5 composite tested at 1100 °C are presented in figure 4, where interlaminar shear strength is plotted as a function of applied shear test rate. The solid line in the figure represents a best-fit regression based on the log (interlaminar shear strength) versus log (applied shear test rate) relation. The reason for using the log-log relation will be described in the “Discussion” section. The decrease in interlaminar shear strength with decreasing test rate, indicating a susceptibility to delayed failure or slow crack growth, was significant for this composite. The interlaminar shear strength degradation was about 50 percent when test rate decreased from the highest (5 MPa/s) to the lowest (0.005 MPa/s) value. A similar trend in shear strength degradation with decreasing test rate was also found from a previous study in an 1-D, Hi-Nicalon™ fiber-reinforced barium strontium aluminosilicate (SiC/BSAS) composite at 1100 °C in air (ref. 5). This trend in shear strength with respect to test rate was also analogous to that in ultimate tensile strength of various continuous fiber-reinforced CMCs including SiC/MAS-5, SiC/CAS (calcium aluminosilicate), SiC/BSAS, C/SiC, and SiC/SiC composites (refs. 17 and 18). These CMCs have exhibited significant degradation of ultimate tensile strength with decreasing test rates, with their degree of degradation being dependent on material and test temperature. Fracture surfaces of the SiC/MAS-5 composite showed that the mode of interlaminar shear failure was typified as delamination of fibers from matrix-rich regions, implying that the fiber-matrix interfacial architecture is the most influencing characteristic in controlling shear properties of the composite. More violent and rough fracture surfaces were seen from the specimens at higher test rate, sometimes exposing two more layers (0/90°) therein, while smoother surfaces were noted for the specimens at lower test rate with only one layer associated with delamination, as shown in figure 5. The presence of viscous flow/phases was obvious from fracture surfaces, particularly at low test rates in which more enhanced delayed failure occurred. The residual glassy phase might have been a major cause of delayed failure in the SiC/MAS-5 silicate composite, as observed previously in the SiC/BSAS silicate composite (ref. 5). ] a 80 P M 70 SiC/MAS-5 (DNS/1100 oC) [τf 60 h, 50 t g n 40 e r t s 30 r a e h s 20 r a n mi a rl e t 10 n I 10-4 10-3 10-2 10-1 100 101 102 Applied shear stress rate, τ (cid:5) [MPa/s] Figure 4.—Results of interlaminar shear strength as a function of applied shear stress rate for 2-D crossplied SiC/MAS-5 composite tested at 1100 °C in air. The solid line represents the best-fit. NASA/TM—2006-214088 6