| Recently, thermoplastic polymers have begun to replace thermosetting polymers as matrices in polymer-matrix composite materials. One significant difference between thermoplastic polymers and thermosetting polymers is that the mechanical properties (especially the failure properties) of thermoplstic polymers are very sensitive to changes in the processing conditions because these changes alter the microstructures of the polymer. It is not known, however, to what extent changes in the polymer microstructure affect the properties of the composite materials.;The purpose of this investigation was to determine how changes in processing parameters affect polymer microstructures in composite materials, what effect these changes have any effect on the failure properties of the composite materials, and, in general, what role the matrix plays in the failure of composite materials. This was accomplished by studying the processing-microstructure-failure property relationships in three composite systems, a compression-molded PEEK/continuous carbon fiber system a compression-molded polypropylene/glass fiber system and an injection-molded polypropylene/glass fiber system.;It was found that the toughness of the interfaces between groups of polymer crystal colonies and the toughness of interfaces between crystal colonies and fibers were the most important factors in determining the toughness of the composites. Decreasing the cooling rate of the consolidation greatly reduced these strengths. Other factors, such as melt temperature, had only a small effect on the failure properties of the materials. It was further found that the fibers in the short-fiber materials toughened the matrix by significantly increasing its stiffness without severely reducing its ductility. These two results imply that, considering only the effects of the matrix on the properties of the composite, rapid cooling of consolidated thermoplastic matrix composites should lead to maximum toughness. |
