Improved Miller Fasteners Based on 3-D Braided Preforms

Miller Fasteners made from 2-D laminates represent the state of the art in joining CMC panels to metal frames. Despite its success, the extant Miller fastener design is limited by its construction from a laminate of 2-D fabric. The resultant fastener part has poor interlaminar shear strength, and its manufacture and application entails high machining costs and high material scrap rates. 3TEX, in collaboration with COI Ceramics, will design, model, and demonstrate CMC Miller Fasteners based on 3-D braided fiber architectures. This improved fastener addresses each of the concerns with the existing design. The intertwined 3-D braided fiber architecture has no layers, thus no fracture-prone interlaminar regions. This will lead directly to higher out-of-plane stiffness and strength, and increased fracture toughness. The braided preforms are near net shape and would reduc costs through less machining and material waste. In this effort carbon fiber 3-D braided preforms with integral head shaping and through hole attachments will be infiltrated with SiC matrix and tested. Design tools will be developed and validated to enable rapid adoption of the new construction of fasteners and speed the transition from carbon fiber based fasteners to ceramic fiber preforms in subsequent researchBENEFITS: CMC components allow gas turbine engines to operate at higher temperatures with attendant increases in output and efficiency. They also reduce the engine weight unless the added weight of the required attachments to join the CMC to the airframe structure overwhelms to savings of individual components. Simple, robust attachment schemes like the Miller Fastener enable the increased use of CMC’s within gas turbine engines. The proposed improvements to the Miller Fasteners will make them less costly and more robust, further increasing CMC usage with the attendant engine performance and component life increases. Additionally, the CMC technology based on 3-D braiding proposed here will extend to other gas turbine engine parts. 3BRAIDTM preforms offer a wide variety of integral cross-sectional shapes, each with high fracture toughness and interlaminar shear strength due to the intertwined nature of the fiber architecture. The design tools to be demonstrated in the Phase I option will enable quick, reliable design of CMC parts, with minimal expensive fabrication and test cycles.