An approach to the design of composites for service at elevated and nonsteady temperatures
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INTRODUCTION
IT is well documented that temperature cycling can increase the creep rate of a stressed composite by orders of magnitude compared to the creep rates observed at the same stress and the highest temperature of the thermal cycle, t~-6] Furthermore, when subjected to large temperature cycles, many composites show strain per temperature cycle being proportional to the applied stress. [1'5-7l Generally, this has been attributed to the internal stresses that develop upon a temperature change due to the difference between the coefficients of thermal expansion of the matrix and the reinforcement, t~,7-gj Several relatively simple analyses 17,8,~~ have been carried out to show how the thermal cycling creep behavior is expected to vary with such variables as thermal cycle amplitude, matrix strength, reinforcement volume fraction, t~~ and reinforcement shape. [~4] But due to the nature of the problem, many effects or variables must be quantified or estimated in order to make accurate predictions. These should at least include matrix strength as a function of temperature, strain and history, reinforcement morphology, and volume fraction. Other effects may also be important as well, such as the rate dependency of matrix strength, interfacial sliding, and the detailed constitutive behavior of the matrix, such as kinematic hardening and anisotropy. As one last caveat, for sufficiently small particles, continuum approaches may begin to break down. t~5,~6] Despite all of the complications mentioned previously, there is a need to make at least order-of-magnitude predictions of composite creep rates under thermal cycling conditions because there is a great desire to use these materials in elevated temperature applications. KARIM F. ELFISHAWY, Graduate Student, and GLENN S. DAEHN, Associate Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, 43210. This article is based on a presentation made in the symposium entitled "Creep and Fatigue in Metal Matrix Composites" at the 1994 TMS/ASM Spring meeting, held February 28-March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASMMSD Composite Materials Committee. METALLURGICAL AND MATERIALSTRANSACTIONS A
Furthermore, in designing a composite for a given application, it is of great utility to be able to predict how a given change in structure may affect thermal cycling creep resistance. Because of all the complications mentioned previously it is also desirable, wherever possible, to have relatively simple experimental checks on the composite behavior instead of fully analytical approaches. In this article, a somewhat simplified approach to the problem of how stress and thermal expansion mismatch affect the "creep" behavior of reinforced deformablematrix composites is taken. It is assumed throughout that the matrix exhibits time-independent, elastic, perfectly plastic behavior.* Given these assumptions, it will be *The analysis presented could easily be extended to include strain har
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