High strain rate behaviour of carbon fibre composites

This thesis describes a study of the dynamic mechanical properties of two types of carbon fibre composite: IM7/977-2 with 977-2 toughened thermoset resin, and IM7/APC2 with thermoplastic PEEK resin, both using Hercules IM7 carbon fibres with a volume fraction of 62%. The Split Hopkinson Pressure Bar (SHPB) is used to produce dynamic strain rates ranging from 103 to 5xl03 s-', while Instron and Hounsfield screw driven · machines are used to give a constant strain rate of about 2 x 10-3s-1 for comparative quasi-static loading. The Split Hopkinson Pressure Bar (SHPB) technique uses specimens in the form of a cylinder, sandwiched between two maraging steel bars. One of the maraging steel bars is subjected to an impact produced by a projectile. The Instron and Hounsfield machine used the same form of specimen. A strain gauge method has also been used to record the strain directly by attaching a strain gauge on to the specimen under test. Three different fibre orientations have been investigated: unidirectional fibre with loading parallel (0°) to the axis of the specimen; transverse loading (90°) where the fibres are perpendicular to the axis of specimen; and quasi-isotropic with transverse loading . The dynamic stress-strain properties of the carbon fib~e composites have been investigated experimentally at room temperature under quasistatic condition and at the high strain rates produced in the SHPB method using loading pulses of about 100 μS duration and rise time of 12 μS. The effects of strain rate on stiffness, yield stress and strength for the composites have been determined and compared with those obtained from a mathematical model (Rule of Mixtures). The longitudinal carbon fibre composite had the highest stiffness, followed by the quasi-isotropic composite, and then the transverse composite. For compressive strength, the quasi-isotropic had the highest strength followed by the longitudinal and then the transverse. The fracture energy has also been obtained indicating that for the quasi-isotropic composites, IM7/977-2 has a considerably high fracture energy than IM7/APC2. The bulk temperature rise has also been predicted up to the fracture stress.