This paper presents a theory of the effective response of composites contai
ning general, nonlinearly separating inclusion-matrix interfaces. The direc
t method of composite materials theory is utilized to pass from local nonli
near behavior of a solitary inclusion problem to nonlinear aggregate respon
se. Interaction effects at finite volume concentration are captured in the
representative solitary problem by employing the Mori-Tanaka mean field est
imate. The resulting model falls within the conceptual framework of continu
um damage mechanics in that nonlinear effective response depends on interna
l variables that are governed by local evolution equations. The damage vari
ables turn out to be the expansion coefficients arising in an eigenfunction
representation of the displacement jump at a representative inclusion-matr
ix interface. Interfaces are generally modeled according to a nonlinear for
ce-separation law that allows for both normal and shear decohesion (X-.P. X
u, A. Needleman, Modell. Simul. Mater. Sci. Eng. 1 (1993) ill). Detailed ca
lculations of effective stress-strain response are carried out for the case
of transverse shear and plane dilatation of unidirectional fiber composite
s at various values of concentration and interface constitutive constants.
The effects of the various parameters on bifurcation of equilibrium separat
ion in the solitary inclusion problem and on overall composite stability ar
e demonstrated as well. (C) 2000 Elsevier Science Ltd. All rights reserved.