A thermoplastic matrix of polypropyleneas seen under the electron microscope
A new Institute study of how neighboring fibers in composite materials interact under stress may contribute to an improvement of everything from bowling lanes and bicycle frames to auto parts and spacecraft.
More and more modern conveniences make use of composite materials, in which some type of reinforcement -- usually in the form of thin fibers (typically 10 microns in diameter) -- is embedded into a matrix, most often polymeric. The composites are designed to exhibit properties that are superior to those of their individual components.
A critical factor in the performance of these materials is their response to stress. In order to assess the interaction of various fibers under conditions of stress, Prof. Daniel Wagner of the Department of Materials and Interfaces created a unique model microcomposite containing a small number of fibers that could be placed at varying distances from one another by means of precise micropositioning equipment. By applying stress to the system and monitoring the progress of the resultant breaks in the fibers, Wagner found that when the fibers were closer together, the breaks in one fiber were specifically correlated with those in its neighbors. This finding enabled him to formulate a "stress concentration factor" showing the amount of load that is transmitted from one fiber to its neighbors when a break occurs. Such information may lead to a more efficient positioning of fibers within a matrix and to a better understanding of fracture physics in such materials.
In a related study, Prof. Wagner is investigating how crystal formation affects the interface between a fiber and a thermoplastic melt. When the melt is cooled down quickly to a constant temperature, crystals are formed at the interface and grow perpendicularly to the fiber -- a phenomenon that, if better understood, may some day be exploited to improve the properties of thermoplastic composites.
Prof. Wagner -- a recent recipient of the prestigious Fiber Society Award for Distinguished Achievement -- has been invited to join an international effort geared to probing composite interfaces, sponsored by the French Office National d'Etudes et de Recherches Aerospatiales and the British Royal Aerospace Establishment. His research is funded in part by the United States-Israel Binational Science Foundation.
Prof. Daniel Wagner