Fiber Push-Out Testing of Intermetallic Matrix Composites at Elevated Temperatures
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FIBER PUSH-OUT TESTING OF INTERMETALLIC MATRIX COMPOSITES AT ELEVATED TEMPERATURES JEFFREY I. ELDRIDGE NASA Lewis Research Center, Cleveland, OH 44135 ABSTRACT A newly developed apparatus for performing fiber push-out testing at elevated temperatures has been applied towards testing fiber-reinforced intermetallic and metal matrix composites. This new capability shows the effects of the relief of residual stresses and increased matrix ductility with increasing temperature on fiber debonding and sliding behavior. INTRODUCTION Fiber push-out testing has been successfully applied towards evaluating fiber debonding and sliding behavior in fiber-reinforced composite materials at room temperature. The benefits of extending these measurements to elevated temperatures include (1)generating data at composite service temperatures which could be used to optimize interfacial mechanical behavior at those temperatures and (2) evaluating the effects of residual stresses and matrix ductility on fiber debonding and sliding. This paper describes a newly developed apparatus for performing elevated temperature fiber push-out tests and reports results for two intermetallic composites and one metal matrix composite. The results are discussed in terms of residual stresses, interfacial wear, matrix ductility, and changing modes of interfacial failure. EXPERIMENT Fiber Push-Out Apparatus Extending fiber push-out testing to elevated temperatures added two basic requirements to the previously developed apparatus [1]: (1) heating the sample and indenter and (2) providing a nonoxidizing environment. Fig. I shows a schematic of the new elevated temperature fiber push-out apparatus. Controlled indenter displacement is performed using an Instron frame with a crosshead speed of 0.98 1tm/s. The sample and indenter are located inside a cubical stainless steel chamber with conflat-flanged ports on each face. This test chamber is evacuated to a base pressure of x10"6 torr, which prevents significant sample oxidation. The indenter is a flat-bottomed cylindrical tungsten carbide punch. The indenter/load cell assembly is coupled to the Instron crosshead, moving inside a collapsible bellows. The specimen is mounted with a spring-loaded clamping device. The sample support block has a set of three 300 Jm wide grooves underneath the sample which allow fibers to be pushed out without resistance from the support block. Sample heating is achieved using a quartz halogen lamp inside an ellipsoidal reflector with the lamp at one focal point and the sample at the other where the radiation is focused. The reflector is bisected by the chamber's quartz window. Sample heating up to 1000°C takes less than 10 min. Sample temperature is monitored by a thermocouple attached to the Ta sample support block. A water-cooled plate below the sample support keeps the sample translation stage below 30°C. Fiber/indenter alignment is performed remotely, using motorized translation stages to bring an individual fiber beneath the indenter. Alignment is attained by monitoring the video
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