yielding stress of the matrix and interface is chosen to represent the influence of the chemical, mechanical, and thermodynamical nature of the bounding process between matrix and fiber materials. the shear stress-strain behavior of interface and matrix material is modeled as elastic-perfectly plastic. a local stress analysis model is used to represent the influence of interface and matrix yielding on stress distribution resulting from fiber fractures. the model is combined with the concept of multiple fiber fractures in the development of a mechanistic representation of tensile strength. in this strength formulation, fibers are assumed to be a statistical quantity, and as the material is loaded, fibers fracture randomly throughout the body causing localized stress concentration. the accumulation of these breaks results in overall failure. the resulting analysis is applied to numerical studies of the influence of interface/matrix yielding on stress and damage characteristics of unidirectional composites under quasistatic uniaxial loading. a discussion of optimal design parameters for composites is also presented