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

The interphase, or transition region between fibre and matrix in compo site material systems, is known to play an essential role in the perfo rmance of composite components. In fact, it has recently been discover ed that the interaction and cooperative action of the constituents in the system control the durability of such systems, in stark contrast t o 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, t his 'systems effect' has become the focus of much attention, and the o bject of modelling efforts. However, modelling the interphase with pro perly set boundary value problems is a considerable challenge, especia lly because the properties in these transition regions are often a fun ction 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 genera l problem of the 'systems effect', discuss the practical issues of how the interface enters micromechanical models of strength and durabilit y, and present models for the interphase region that include the varia bility of material properties in those transition regions. Special att ention will be given to the contrasts between these considerations and the rule-of-mixtures concepts that currently pervade the field, and t o the importance of these differences to the practical opportunities o ffered by the ability to properly model and 'design' interphase region s in polymer-matrix composites.