MODELING OF THE INTERPHASE IN POLYMER-MATRIX COMPOSITE-MATERIAL SYSTEMS

The interphase, or transition region between fibre and matrix in composite material systems, is known to play an essential role in the performance of composite components. In fact, it has recently been discovered that the interaction and cooperative action of the constituents in the system control the durability of such systems, in stark contrast to other behaviour characteristics such as stiffness and quasi-static (or 'instantaneous') properties wherein the constituents contribute to the properties, essentially, in proportion to their presence. Hence, this 'systems effect' has become the focus of much attention, and the object of modelling efforts. However, modelling the interphase with properly set boundary value problems is a considerable challenge, especially because the properties in these transition regions are often a function of spatial position, i.e., they are non-uniform as a function of position. The author has constructed models of the interphase region which admit this non-uniformity, and has achieved solutions to several classes of these problems. The present paper will describe the general problem of the 'systems effect', discuss the practical issues of how the interface enters micromechanical models of strength and durability, and present models for the interphase region that include the variability of material properties in those transition regions. Special attention will be given to the contrasts between these considerations and the rule-of-mixtures concepts that currently pervade the field, and to the importance of these differences to the practical opportunities offered by the ability to properly model and 'design' interphase regions in polymer-matrix composites.