Factors influencing the mode I interlaminar fracture resistance of toughened matrix composites.

Abstract

Fiber reinforced polymeric composites which provide high stiffness and strength to weight ratios have become important for the production of weight sensitive structures. A major problem with polymer composites based on brittle matrices is that they have a low interlaminar fracture toughness and are sensitive to out of plane impact. Simply introducing a toughened resin as a matrix material does not necessarily bring about a substantial improvement in the fracture toughness of a composite. In this study, composites were produced utilizing epoxy and thermoplastic matrix materials both in the neat and rubber modified conditions. These composites allow evaluation over the extremes of matrix fracture toughness and the identification of factors which influence the mode I interlaminar fracture toughness of toughened matrix composites. Bulk epoxy and polycarbonate resins were toughened using core shell impact modifiers. The bulk resins were evaluated in terms of mechanical properties, fracture toughness, and deformation mechanisms by using uniaxial tensile dilatometry tests, three point bend tests, surface (Optical Microscopy (OM) and Scanning Electron Microscopy (SEM)) and subsurface (OM) analysis. These resins were then introduced as matrices for unidirectional graphite fiber (58 vol.%) composites. The mode I interlaminar fracture toughness of the composites was evaluated using hinged double cantilever beam specimens. The deformation mechanisms which occur within the composites during fracture were evaluated using surface (SEM) and subsurface (OM) analysis. The results of this work have identified several factors which limit the interlaminar fracture toughness of a composite. These include: rubber particle agglomeration, solvent and molding induced crystallization of the matrix, and poor fiber matrix adhesion. It is observed that the plastic zone which develops in the composite is smaller than that of the bulk resin and extends beyond a single interfibrillar spacing. Improvements made in the fracture toughness of the matrix resins do not lead to proportional increases in the interlaminar fracture toughness of a composite due to poor fiber matrix adhesion. In order to produce tough composites utilizing toughened matrices, any improvements in the matrix toughness must be coupled with improvements in the fiber/matrix adhesion.