Fracture toughness of a short‐fiber reinforced thermoplastic

Abstract

AbstractThin plates of carbon short‐fiber reinforcement polycarbonate were injection molded. The mold was designed to produce a uniform melt flow across the cavity and an extended knock‐out pin was incorporated to form a circular hole at the center of the molded plate. The elastic constants of the plaques were determined using sections cut from the plate at different angles to the direction of flow. Analysis of the data showed that the plates could be treated macroscopically as being orthotropic. Microscopic observations revealed that the fiber orientation was primarily in the flow direction and was tangential in the vicinity surrounding the hole. The fracture toughness, as measured by the stress intensity (K), was determined using the compliance method. Experimental calibration curves were constructed at 0° and 90° to the axis of flow by loading specimens containing saw cuts of varying length. The resultant curves were non‐dimensionalized by incorporation of the elastic moduli, thickness, and width. The fracture toughness values were determined using a razor notch as a starter crack. The crack growth during testing was found to be stable, which could allow several determinations to be made on each plate. The effects of crack length, flow in the cavity, and fiber orientation around the hole were investigated. The fracture toughness was found to decrease with increasing crack length, but was not found to reach a limiting value within the practical range of testing. The effect of flow was also found to be significant. Specimens oriented 90° to the axis of flow showed higher toughness values. This was attributed to the fibers being oriented perpendicular to the axis of the crack. The samples tested with razor notches cut at the edge of the molded holes had still higher apparent toughness values. Similarly, this effect was explained by the higher fiber orientation shown with photomicrographs of specimens cut near the edge of the hole.