SBIR-STTR Award

Hot structures fabricated from CMCs are an attractive design option for specialized components of future military aerospace vehicles and propulsion systems to reduce weight and increase survivability. However, thermal-structural components fabricated from CMCs suffer from inadequate interlaminar strength, thereby increasing their vulnerability to delamination when subjected to high through-thickness thermal gradients, acoustic/high cycle fatigue, impact damage, and/or applied normal loads. Although there are Industry standards for Interlaminar Shear strength testing, no such consensus exists for Interlaminar Tension [ILT] strength testing particularly at elevated temperatures. Better test methods are needed to characterize the interlaminar strength of CMCs to facilitate the design of efficient structures, reduce the risk of premature hardware failures, and support production Quality Assurance. Hyper-Therm HTC is proposing an innovative approach for measuring the elevated temperature ILT strength of CMCs. The test configuration directly applies an interlaminar tension force to the midplane of the laminate thereby creating a clean, ILT stress state in the specimen that does not require knowledge of the interlaminar properties in order to calculate the failure stress. Specimens can be fabricated inexpensively from flat plate stock, and because the test method does not require adhesive bonding, it can be used at temperatures up to 4000F.

Benefit:
The simple specimen configuration affords the economical testing of substantial quantities for the development of a statistically significant design database as well as its use for the tag end testing of components for production Quality Assurance. Because the test fixtures and procedure can be used at both RTA and elevated temperature test conditions, test results can be directly compared for a more meaningful assessment of environmental effects. Compatible with standard laboratory mechanical test apparatus, the fixtures and method can be adapted for elevated temperature lifing tests, such as fatigue and stress rupture, with relatively simple test setup modifications. By providing a basis for Industry consensus, the development of the proposed test method for the elevated temperature characterization of CMCs will inherently benefit both military and civilian applications for CMCs such as aero-engines.

Keywords:
ceramic matrix composites, ceramic matrix composites, Interlaminar Tensile Strength, Elevated Temperature, test methods

Hot structures fabricated from ceramic composite materials are an attractive design option for components of future high-speed aircraft and propulsion systems to reduce weight and increase performance. However, available ceramic composite components fabricated from laminated fiber preforms can suffer from inadequate interlaminar tensile strength, thereby increasing their vulnerability to delamination when subjected to high thru-thickness thermal gradients, acoustic/high cycle fatigue, impact damage, and/or normal loads. The ability to intelligently design components to avoid premature interlaminar failures requires a fundamental understanding of the interlaminar properties of the material; not only at ambient temperatures, but more importantly, at elevated temperatures. Although there are Industry standards for Interlaminar Shear strength (ILS) testing of ceramic composites, no such consensus exists for Interlaminar Tension (ILT) strength testing - particularly at elevated temperatures. Under Phase I, the feasibility of an innovative test method for characterizing the elevated temperature ILT strength properties of ceramic composites was successfully demonstrated. The test method enables using small, flat material samples of representative thickness (~0.125-in) and was shown to correlate well with that obtained by flatwise tension testing of epoxy-bonded circular samples consistent with the ASTM standard (C-1468) at ambient temperature. The objective of this Phase II program is to optimize the test sample geometry and test method to suit a variety of different available ceramic composite systems (e.g., CVI, MI, PIP, etc.). This effort will culminate into the development and verification of an industry acceptable ASTM Test Standard for elevated ILT testing of ceramic composites.

Benefit:
Fiber-reinforced ceramic composites are recognized as an enabling class of materials for a variety of high-temperature applications in rocket engine throat inserts, combustion chambers and nozzles; turbine combustors, blades, vanes, and exhaust nozzles; hypersonic airframe hot structure and thermal protection systems; spacecraft re-entry heatshields; and a variety of industrial power generation radiant burner and heat exchanger tubes. The simple specimen configuration affords the economical testing of substantial quantities for the development of a statistically significant design database as well as its use for the tag end testing of components for production Quality Assurance. Because the test fixtures and procedure can be used at both RTA and elevated temperature test conditions, test results can be directly compared for a more meaningful assessment of environmental effects. Compatible with standard laboratory mechanical test apparatus, the fixtures and method can be adapted for elevated temperature lifing tests, such as fatigue and interlaminar stress rupture, with relatively simple test setup modifications. By providing a basis for Industry consensus, the development of the proposed test method for the elevated temperature characterization of ceramic composites will inherently benefit both military and civilian applications for CMCs such as aero- and land-based turbine engines.

Keywords:
SiC/SiC Composites, Elevated Temperature, Fiber-Reinforced Ceramics, Interlaminar Tensile Strength, Mechanical Test Methods, High Temperature Mechanical Testing, ceramic matrix composites, Oxide/Oxide Composites