Thermal and mechanical anisotropy in compression molded carbon fiber/resin composites

A 53% (vol.) chopped carbon fiber (2.5 cm long) in resin composite was molded under uniaxial extensional deformation conditions (for a charge draw ratio (DR) of 1, 2, 4, and 8) to produce flat panels 4 mm thick. Axial direction tensile strength increased by 42% from DR 1 to 2 and rose slightly at higher DR, while a slight increase occurred for the transverse direction values over the whole range. Axial direction thermal conductivity (t.c.) increased 32% from DR 1 to 2 then, decreased at DR 4 and DR 8, while normal t.c. showed a moderate decrease and transverse t.c. showed no definite trend. Measurements of the Hermanns fiber orientation function in the core of the samples showed a gradual increase from 0.3 to 0.45 (with respect to draw direction) while surface values ranged from about 0.08-0.15 over the range of DR. Results for a 30% (vol.) carbon fiber (6.5 mm long) composite showed similar trends. A flow analysis based on the power law model indicates that surface shear effects tend to rotate fibers in the surface region out of the plane of the panel. This limits the axial t.c. and tensile strength at high DR for the 30% composite, while the decrease in axial t.c. at high DR is caused by orientation of the 3rd phase (filler) for the 53% composite. Results show that the nondestructive nature of the t.c. measurements described allows an indirect measure of tensile strength for molded components in service.