Fibre fracture during equal-channel angular extrusion of short fibre-reinforced thermoplastics

Customised short fibre composite materials with optimal strength, stiffness and fracture energy are only possible when the fibre length is controlled. Current industrial processes produce a broad distribution in fibre length because the fibre flaw distribution controls the fracturing process. Glass fibre/polyacetal rods compounded by conventional melt processing/extrusion were processed by equal-channel angular extrusion (ECAE) to determine its capability for fibre length control. ECAE is a novel process for affecting the microstructure of a material by extreme shear deformation. Material in the solid state passes through a die with a sharp change in flow direction. Billets exit with the same shape they initially had. The process performed at 73 °C did not significantly reduce initial mean fibre length of 105.6 μm. Processing at room temperature reduced the mean fibre length to 81.0 μm showing the potential for length control by thermal manipulation of the matrix shear strength or the fibre/matrix interface strength. Repeating the process at room temperature had no further effect on the fibre length indicating that the process is limited by the critical length of fibres for fracture at a given temperature and that a single pass may be sufficient to control the fibre length.