This paper explores the nonlinear effective behavior of unidirectional fiber-reinforced composites due to yielding of interphases and/or of the continuous matrix phase. The dispersive fiber phase is assumed to be linear elastic. The yielded regions in the matrix surrounding the fiber are modeled as co-axial discrete interphasial subregions of small but finite thickness. A generalized self-consistent Mori-Tanaka scheme, developed by the authors (Dasgupta and Bahandarkar, 1992) for unidirectional composites with multi-phase cylindrical reinforcements, is utilized in an incremental manner in this study, to investigate the nonlinear effective response of the composite to axisymmetric loading such as longitudinal extension and transverse hydrostatic tension. Since the matrix The discrimination of the matrix into multiple interphases enables more accurate estimates of the nonlinear behavior, than was previously possible with Tandon and Weng's (1988) plasticity models for composite materials. Instead of using the gross average-matrix theory employed by Tandon and Weng, this study uses a piece-wise continuous version of the Mori-Tanaka theory.

Examples are provided to illustrate the nonlinear behavior of Aluminum matrix composites reinforced with Tungsten fibers. The aluminum matrix is approximated as a power-law hardening material whose stress-strain curve is approximated with a piece ¨Cwise linear curve. The tungsten fibers are assumed to be linear elastic. The instantaneous longitudinal modulus, plane-stress and plane-strain transverse bulk module. And the longitudinal as well as transverse coefficients of thermal expansion are computed for incremental loading.
 

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