Stress-strain behaviour of IM7/977-2 and IM7/APC2 carbon fibre composites at low and high strain rates

Abstract
The dynamic compressive behaviour of two IM7 carbon fibre composite systems has been studied at room temperature at strain rates from about 10 ^-3 s ^-1 to 10 ³ s ^-1. A Hopkinson bar method was used for high rates and a conventional screw machine for low rates. One composite system (IM7/977-2) has a thermoset epoxy resin matrix toughened with thermoplastic material; the other system (IM7/APC2) has a thermoplastic PEEK matrix. Three types of fibre configuration were used for each system: (a) longitudinal unidirectional, 0° (LU), (b) transverse unidirectional, 90° (TU), and (c) transverse isotropic, (0°/90°/ 45°) (TI). At low rates, both systems showed similar stress-strain behaviour. For the LU types, the fracture stresses were about 790 MPa, while for the TU types the values were about 235 MPa. For the Tl specimens, the maximum available stress of 950 MPa was insufficient for fracture. At high rates, the fracture stress was about 25% greater than the quasistatic values for both LU types, and about 30% higher for both TU types. The major difference between the two systems was with the TI types: the energy density to fracture was about 65 MJ m ^-3 for the epoxy composite, while it was about 40 MJ m ^-3 for the PEEK composite.