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Attempts to toughen composites by using matrix materials of high toughness have not met expectations. The rationales given generally fall into two categories: size effect and constraints. This paper critically examines these issues and proposes approaches to test these rationales. The constraint effect is considered in light of recent advances in the understanding of the toughening mechanisms in rubber-modified plastics and epoxies. It is argued that cavitation processes are inherently capable of relieving the constraint created by the fibers, and that shear localization processes are effective in increasing the size of the plastic zone. The material properties giving rise to stable shear band propagation are discussed. The assumption of the necessary rubber particle size to effect toughening is examined by comparison with past experiences in toughening epoxies and certain thermoplastics. Several examples of successful toughening of resins by submicron rubber particles are cited.
composites, fracture, polymers, toughening, impact
professor, University of Michigan, Dow Bldg., Ann Arbor, MI