Off-axis fatigue strengths for high-temperature composites and effects of resin ductility

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The influence of matrix resin systems with different strength and ductility-high-temperature thermoplastic polyimide and PEEK and a conventional thermosetting epoxy-on the off-axis static and fatigue strengths of unidirectional carbon fiber composites has been studied. The results obtained may be summarized as follows :1.Off-Axis Static Deformation and Fracture :The stress-strain curves at 100ﾟC for the T800H/Epoxy and AS4/PEEKcomposites were accompanied by a significant non-linearity, especially for the off-axis angles rheta=10ﾟ-60ﾟ.Forrheta=0ﾟ and 90ﾟ,they were almost linear up to final fracture. In spite of the great ductility of the polyimide resin, the manufactured T800H/Polyimide composite exhibited almost linear stress-strain curves for most off-axis angles tested except for rheta=10ﾟ and 30ﾟ,and it fractured at relatively low stress and strain. Among these composites, AS4/PEEK showd the highest toughness and strength at 100ﾟC for every off-axis angle. A significant non-uniform … More fiber distribution in the as-manufactured T800H/Polyimide composite canceled the high performance of the polyimide resin. The elastic properties and offaxis static fracture strengths for these carbon fiber composites were appropriately described using the orthotropic linear elastic theory and Tsai-Hill criterion, respectively.2.Off-Axis Fatigue Strength :For three kinds of UD composites, the off-axis fatigue strengths at 100ﾟC decreased as the off-axial angles increased. The S-N relations at 100ﾟC for the T800H/Epoxy and AS4/PEEK composites had linear relationships, irrespective of the off-axis angles. For T800H/Polyimide, however, the S-N relation could not be described using a single linear function for all fatigue strength ratios due to a significant bent appeared in the high cycle fatigue range for rheta=10ﾟ.Among all these composites, AS4/PEEK showed the highest off-axis fatigue strength for the whole range including high cycle fatigue. By taking the relationships between the fatigue strain range and the number of cycle to fatigue failure, it was found that the S-N data were separated into two groups associated with the fractures of fibers and fiber-matrix interfaces, respectively. The off-axis fatigue strengths for the T800H/Epoxy, AS4/PEEK,and T800H/polyimide composites were described using the Hashin's fracture model. Less