Effective elastic moduli of fiber-matrix interphases in high-temperature composites
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INTRODUCTION
I T is well established that the fiber-matrix interphase plays an important role in determining composite performanceY ,2J The interphase not only allows load transfer between fibers and matrix but also provides matching of chemical and thermal compatibility between the constituents. The latter is particularly important for high-temperature composites in order to increase the composite fracture resistance and to accommodate residual stresses. The interphase microstructure and its reactions with other composite constituents have received increasing attentionJ 3,4] Despite great effort in the development of special fiber coatings to tailor the interphase,tSl interphase mechanical properties remain difficult to measure and interpret. One typical method for interphase characterization is to measure the interracial shear strength and debond energy by the fiber pullout and pushout tests. E6]The last test, based on an indentation technique,t7,81 is very popular for characterization of interphases in high-temperature composites. The sample used in these tests is a slice so cut from a composite that the faces are perpendicular to the fiber axis. The nature of these measurements prohibits their application as a nondestructive tool for quality control in composite processing. To characterize the interphase nondestmctively, several techniques have been suggested for deducing interphase
properties from measurements of ultrasonic velocities and attenuation in the composite, t9-13] the composite thermal conductivity,t14] or the ultrasonic reflectivity from the interphase. I~51Matikas and Karpur~lsl studied the local reflectivity of a single fiber to estimate the interface shear stiffness. In contrast, our approach, using ultrasonic velocity measurements, focuses on average interfacial stiffnesses in the volume insonified by the ultrasonic beam. In addition to shear stiffness, radial and axial stiffnesses are determined. The procedure for finding interphasial moduli from ultrasonic phase velocity measurementstmLl is illustrated schematically in Figure 1. In this approach, the interphase is considered as a single or multilayer with elastic properties distinct from those of the fiber and matrix (such as the fiber coating). The ultrasonic phase velocities in the comDETERMINATION
OF INTERPHASIAL PROPERTIES
ULTRASONIC VELOCITY DATA
INVERSION OF MULTI-PHASE -> MICROMECHAN CAL MODELS
-2; COMPOSITE ELASTIC MODULI
ELASTIC MODULI OF INTERPHASIAL LAYERS
-2; Y.C. CHU, Graduate Research Associate, and S.I. ROKHLIN, Professor, are with the Nondestructive Evaluation Program, Department of Industrial, Welding, and Systems Engineering, The Ohio State University, Columbus, OH 43210 Manuscript submitted July 22, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
AVERAGE TRANSVERSE MODULI Fig. l--Flow chart for ultrasonic interphase characterization. VOLUME 27A, JANUARY 1996--165
posites are first measured and used to find the composite elastic moduli. Next, we calculate the average composite transverse moduli from the measu
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