Interphase properties and their effects on the compression mechanics of polymeric composites

Experimental and analytical investigations have shown that the interphase, with properties different from either the fiber or matrix, has a considerable influence composite material strength. The greatest obstacle, however, to this work lies in identifying the specific properties present in the interphase region of a composite and knowing how specific properties affect the performance of the material given particular service conditions. In this work. the author attempts to identify specific properties of two distinct interphases developed from fiber sizings and study their effect on the static and fatigue compression performance.

Two different interphases, as formed by the introduction of contrasting fiber sizings, were studied in this dissertation. The interphase produced from the amorphous thermoplastic polyvinylpyrrolidone (PVP) size showed a graded morphology very different from that formed in the presence of the unreacted epoxy size in a thermoplastic toughened epoxy matrix. However, these morphological differences were not introduced in an untoughened epoxy matrix, although many of the differences in composite performance between the two interphase materials were similar. Model studies of PVP modified epoxy showed that at low weight percent loadings of PVP failure strain and mode I toughness were increased but significantly decreased for loadings approaching 20 w%.

The effect of this altered interphase zone on elastic and inelastic properties of uniaxial tests was significant. The PVP interphase increased failure strain and strength for both tension and compression. Only the inelastic properties of the off-axis tests were affected by the contrasting interphases, and were similar for both toughened and untoughened epoxy matrix systems.

The PVP interphase material showed significant improvements in low cycle notched cross-ply compressive fatigue (one order of magnitude) and fatigue limit (10% of UCS) as compared to the conventional epoxy interphase. These enhancements were the result of the inherent toughness and damage tolerance of the material as influenced by the improved inelastic properties of the PVP interphase. Thus, it is the inelastic characteristics of the interphase (and not strength alone) which are dominant in the understanding of compression strength, as is made further evident by the initial postbuckling description of the fiber-binder system. Completion of the nonlinear (kinematic and constitutive) buckling problem by asymptotic approximation will lead to a fundamental description of the failure mode and strength as related to local inelastic properties and initial imperfections.