Reinforcement of thermoplastics using carbon fibres

Abstract

The tensile mechanical properties of carbon fibre reinforced nylon 6.6 and polypropylene have been studied. Two distinct ranges of fibre length and a range of fibre volume fraction from 0 to 0.2 have been examined in each matrix. Longer fibres are more effective in improving strength and stiffness than are equivalent fractions of short fibres. The fibre matrix bond strengths are higher than the yield strength of unfilled matrix for both systems and it is suggested that this is due to a modification in matrix structure occurring at the fibre/matrix interface. A critical fibre length exists such that fibres of critical length or longer strain by the same amount as the matrix at their centres. The value of critical length is related to the strain in the matrix and the fibre matrix bond strength. Fibres which are subcritical in length carry a constant stress, and fibres which become subcritical by fracture continue to carry stress at the level appropriate to their new length as strain increases. Some specimens of each material failed at strains which exceed the mean fibre failure strain. In these cases considerable multiple fracture of the fibres occurred. In specimens failing below the mean fibre fracture strain less fibre breakdown occurred. It is suggested that failure in these composites may be initiated in the matrix and not by fibre fracture.