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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.