Fiber push-out testing apparatus for elevated temperatures
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A newly developed apparatus has been designed for performing fiber push-out testing on continuous fiber-reinforced composites at elevated temperatures. This test measures the force at which a fiber resists being pushed by a flat-bottomed indenter moving at a constant speed. The applied load versus time curve characterizes the fiber debonding and sliding behavior. Extending measurements to elevated temperatures required incorporating sample/indenter heating in a nonoxidizing environment. With this new apparatus, fiber push-out tests have been performed up to 1100 °C in a vacuum of IQr6 Torr. A line-of-sight to the sample is maintained during the test which allows video monitoring of the push-out process. Results are shown for SCS-6 SiC fiber-reinforced T i - 2 4 A l - l l N b (at. %) and T i - 1 5 V - 3 C r - 3 S n - 3 A l (at. %) matrix composites. The results are discussed in terms of residual stresses, interfacial wear, matrix ductility, and changing modes of interfacial failure. The effect of temperature-dependent interfacial wear on the interfacial roughness contribution to frictional shear stresses during fiber sliding is examined.
I. INTRODUCTION Fiber push-out testing has become an important tool for characterizing fiber debonding and sliding behavior in fiber-reinforced composite materials.1"7 This interfacial behavior is important because it has a significant impact on the overall strength and toughness of the composite. Various models have been proposed which relate the mechanical properties of the interface to those of the composite.8'9 In addition to fiber push-out testing, several other methods have been used to evaluate mechanical properties of the fiber/matrix interface. These include fiber pull-out,10'11 fiber fragmentation,12 matrix cracking,13 and fiber/matrix displacements due to thermal cycling.14 While each of these methods has its advantages, none of them directly measures the dynamic response to mechanical loading except for fiber push-out and pull-out. There are counterbalancing benefits and drawbacks to consider in deciding between fiber push-out and pullout testing. Fiber pull-out testing has the advantage of more closely duplicating the stress states at the interface present in a composite under tension; however, it requires sample geometries which can be extremely difficult to prepare and which produce only one measurement per specimen.11 This difficult sample preparation makes routine measurements and accumulation of statistical distributions virtually impossible for many composite systems. In contrast, fiber push-out testing is conducive to more routine testing and generates many measurements per specimen, and thus, was chosen as the testing procedure for this paper. Because the targeted use of many of the composite materials is at elevated temperatures, the benefits of
extending these measurements to elevated temperatures are clear: (i) to generate data at composite service temperatures that could be used to optimize interfacial mechanical behavior at those temperatures and (ii) to
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