On the influence of ply-angle on damping and modulus of elasticity of a metal-matrix composite
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INTRODUCTION
THE development of large space structures requires the use of materials having a high strength-to-weight ratio, dimensional stability over a broad range of thermal environments, and resistance to deterioration by thermal or mechanical damage, for example, outgassing and micrometeorite impact. Due to the inability of amorphous composites to adequately satisfy these requirements over a reasonable service life, metal-matrix composites have become a favorable candidate for this application. The size of proposed space structures makes them prone to vibration. Therefore, the damping properties of metal-matrix composites are of interest. This subject is addressed herein. Experimental data are presented, showing the variation of flexural damping and flexural modulus with ply-angle for a specific metal-matrix composite, namely, Pitch 55 graphite fibers in a 6061 aluminum matrix (P55Gr/6061AI). The beam specimens, having ply-angles ranging from zero to 90 deg in increments of 15 deg, were cut from balanced, symmetric, four-ply laminate plates. Experimental results are compared with the behavior predicted by the Ni and Adams model, [lj which is rooted in classical laminate theory. [2] All experiments were carried out in vacuum using a cantilevered flexural apparatus. Damping, in general, is a function of temperature, frequency (i.e., strain rate), and strain amplitude. Since in
VIKRAM K. KINRA, Professor, is with the Department of Aerospace Engineering, Texas A & M University, College Station, TX 77843. GRAEME G. WREN, formerly Graduate Student, Department of Aerospace Engineering, Texas A & M University, is Captain, Aeronautical Research Laboratories, Melbourne, Victoria 3001, Australia. SURAJ P. RAWAL, Senior Research Engineer, and MOHAN S. MISRA, Manager, Structures and Materials, are with Martin Marietta Aerospace Corporation, Denver, CO 80201. This paper is based on a presentation made in the symposium "Acoustic/Vibration Damping Materials" presented during the TMS Fall Meeting, Indianapolis, IN, October 1-5, 1989, under the auspices of the TMS Physical Metallurgy Committee.
METALLURGICAL TRANSACTIONS A
the present work we are only interested in studying the influence of ply-angle on damping and modulus, all tests were conducted at room temperature, 35 Hz and 55/xe. Whenever it was desired to reduce the resonant frequency of the specimen, a mass was attached to the tip of the beam. A carefully conducted calibration study showed that the addition of an end-mass to a cantilevered beam does not affect the measurement of damping or modulus. [3] Experimental data were obtained using a digital acquisition system interfaced with a computer, and damping was determined by the least-squares error method, t4] The results are presented in terms of the specific damping capacity, qJ = AW/W, where AW is the energy dissipated during each loading cycle and W is the maximum stored energy. Our estimate of error in the measured values of specific damping is ---1 • 10 -3, and that in the measured values of flexural modul
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