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Interlaminar Mode I and Mode II Fracture Toughnesses in Ceramic Matrix Composites (CMCs)

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OBJECTIVE: Develop and demonstrate innovative interlaminar Mode I and Mode II fracture toughness test methods for CMCs. DESCRIPTION: Military aircraft platforms are targeting CMCs for aeroengine applications with a goal of increase in specific power and performance. Concerns still exist regarding CMCs in terms of their transition, maturation, reliability, and environmental durability. In particular, due to their laminated architectures, CMCs are significantly lower in mechanical properties in interlaminar direction than in in-plane counterpart. The CMCs are also highly susceptible to delayed failure (stress rupture or fatigue) even in interlaminar shear at elevated temperatures. Consequently, these inferior interlaminar properties of CMCs have shown to have a significant effect to reduce components"lives and therefore are a design criterion in many cases (e.g., airfoils) rather than their"superior"in-plane properties. Therefore, interlaminar properties of CMC should be assessed accurately with appropriate test methods and used in component design and life prediction to ensure overall structural reliability and integrity. Currently, there are a number of test standards in Military Standards (MIL STD) and American Society for Testing and Materials (ASTM) for CMCs at both ambient and elevated temperatures, including interlaminar tension and shear strength test methods. However, there exist no standardized test methods for determination of interlaminar fracture toughness in CMCs. Although some previous work exists on interlaminar Mode I and Mode II fracture toughnesses of various types of CMCs, the test methods applied particularly in Mode II fracture toughness testing showed definite drawbacks and limitations. This is primarily due to non-existence of appropriate test methods in conjunction with complexities associated with an anisotropic nature of CMCs and thin configurations (typically<3 millimeter) of test coupons. Hence, an immediate, urgent need exists to develop innovative test methods to determine interlaminar Mode I and Mode II fracture toughnesses (KI and KII) or crack growth resistances (GI and GII) unique to CMC material systems. The pertinent test methods would provide ways to fabricate or tailor CMCs with a desired level of damage tolerances. Ultimately, the methods would allow one to establish reliable databases and to utilize them for design and reliability/life-prediction analyses of CMC areoengine structural components. Also consider potential applicability of the test methods at elevated temperatures up to 2400F (1316C). PHASE I: Design and develop initial concept models on interlamnar KI and KII test methods. Demonstrate the feasibility by analytical method (e.g., Finite Element Analysis) at ambient temperature. PHASE II: Develop and optimize the test methodologies and evaluate them by conducting fracture toughness testing using test coupons with different architectures of CMCs at ambient temperature. Demonstrate the feasibility of test methods applicable to elevated temperature testing. PHASE III: Perform validation and certification testing. Transition the approaches to Joint Strike Fighter (JSF), ASTM, and CMC propulsion applications. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: CMCs propulsion components have a great potential to transition to the civilian aeroengine applications. The resulting test methods development can allow the complete evaluation of interlaminar properties with which reliable component design and life prediction are feasible. The development also could provide national consensus test methods for MIL, ASTM, and related communities. REFERENCES: 1. (2012). ASTM C1292-10 Standard test method for shear strength of continuous fiber-reinforced advanced ceramics at ambient temperatures. Annual Book of ASTM Standards 15.01, doi:10.1520/C1292-10 2. (2012). ASTM C1468 06 Standard test method for transthickness tensile strength of continuous fiber-reinforced advanced ceramics at ambient temperature. Annual Book of ASTM Standards 15.01, doi:10.1520/C1468-06 3. Choi, S.R., & Bansal, N.P. (2006). Interlaminar tension/shear properties and stress rupture in shear of various continuous fiber-reinforced ceramic matrix composites. In N.P. Bansal, J.P. Singh & W.M. Kriven (Eds.), Advances in Ceramic Matrix Composites XI, Volume 175 (pp. 119-134). Hoboken: John Wiley & Sons Inc. doi:10.1002/9781118407844.ch11 4. Choi, S.R. & Kowalik, R.W. (2008). Interlaminar crack growth resistances of various ceramic matrix composites in mode I and mode II loading. J. Eng. Gas Turbines Power, 130(3), 031301-031308. doi:10.1115/1.2800349 5. Choi, S.R., Kowalik, R.W., Alexander, D.J., & Bansal, N.P. (2009). Elevated-temperature stress rupture in interlaminar shear of a Hi-Nic SiC/SiC ceramic matrix composite. Composites Science and Technology, 69(7-8), 890-897. doi:10.1016/j.compscitech.2008.12.006 6. Kumar, R.S., & Welsh, G.S. (2012). Delamination failure in ceramic matrix composites: numerical predictions and experiments. Acta Materialia, 60(6-7), 2886-2900. doi:10.1016/j.actamat.2012.01.053 7. Ojard, G., Barnett, T., Dahlen, M., Santhosh, U., Ahmad, J., & Miller, R. (2010). Mode I interlaminar fracture toughness testing of a ceramic matrix composite. In D. Singh, J. Salem, S. Mathur & T. Ohji (Eds.), Mechanical Properties and Performance of Engineering Ceramics and Composites V: Ceramic Engineering and Science Proceedings, Volume 3 (pp. 195-206). Hoboken: John & Wiley Sons, Inc. doi:10.1002/9780470944127.ch20

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