Statistical Analysis of the Properties of Advanced Fibers Designed for Titanium Alloy and Intermetallic Reinforcement
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STATISTICAL ANALYSIS OF THE PROPERTIES OF ADVANCED FIBERS DESIGNED FOR TITANIUM ALLOY AND INTERMETALLIC REINFORCEMENT JOHN R. PORTER 1049 Camino Dos Rios, Rockwell International Science Center, Thousand Oaks, CA 91360 ABSTRACT The mechanical properties of new ceramic reinforcing fibers need to be well characterized before their incorporation into composite materials. Critical fiber properties include strength and Weibull modulus, both off the spool and after matrix extraction, bundle strength, modulus and creep resistance. Important composite properties include thermochemical stability, interface debond energy and interfacial sliding resistance. Tailoring these interfacial properties invariably involves the use of a fiber coating that can, in turn, influence fiber properties. Methods of measuring strength related properties are addressed and the results of a computer simulation to assess the quality of measured data using statistical methods are presented. The simulation was developed to determine the errors associated with a strength/Weibull modulus determination based on a limited number of samples. Finally, an assessment of the effect of mixing of high and low quality fiber on bundle strength and composite properties is made. INTRODUCTION The available monofilament fibers for reinforcing titanium alloys and intermetallics are mostly chemically vapor deposited (CVD) silicon carbide. Current aerospace programs have identified the need for new ceramic fibers as enabling materials for their success. Consequently, a number of programs have recently addressed the development of new fibers.t The new fibers are needed for several reasons including: having fibers with a CTE greater than that of SiC, to match more closely that of titanium alloys; having fibers that have higher strength and creep resistance; and having fibers that are chemically stable in new candidate matrices. These new programs necessitate the need for characterizing fibers and, in particular, assessing the quality of the property measurements that are made both during fiber development and for a property data base of a developed fiber. The strength of a ceramic fiber is statistical and is determined by the largest flaw (weakest link) in the flaw population of the tested length of fiber. This can have important implications for toughening a brittle matrix material such as an intermetallic. For toughening by continuous fiber reinforcement, fibers need to exhibit pull-out during matrix crack propagation, as shown schematically in Fig. 1. However, the stress distribution along a bridging fiber, as shown in the figure, has a maximum in the crack plane and for fiber failure to occur away from the crack plane, a prerequisite for pull-out, then a weak link must exist some distance away from the crack plane. 1 t
National Aerospace Plane (NASP), High Speed Civil Transport Enabling Materials Program (HSCT/EPM), Integrated High Performance Turbine Engine Technology (IHPTET) program. Mat. Res. Soc. Symp. Proc. Vol. 273. @1992 Materials Research Society
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