INSTRUMENTED IMPACT FRACTURE AND RELATED FAILURE BEHAVIOR IN SHORT-GLASS-FIBER-REINFORCED AND LONG-GLASS-FIBER-REINFORCED POLYPROPYLENE

The fracture behavior of injection-moulded, polypropylene (PP) composites reinforced with chopped, short glass fibers (SGF) and long glass fibers (LGF) was studied in instrumented high-speed impact bending tests. Testing was carried out on Charpy specimens of different notching direction in order to elucidate microstructural effects. The microstructural parameters were included in a reinforcing effectiveness parameter, R, which considers the fiber volume fraction, the fiber layering and fiber orientation therein, and the fiber aspect ratio and its distribution. From tests performed at -40, 20 and 60-degrees-C, fracture mechanical parameters such as fracture toughness, K(d), initiation fracture energy, G(d,i), and Young's modulus, E(d), were derived. A distinction was made between crack initiation and propagation stages based on the fractograms by using the ductility index, DI. Addition of LGF considerably improved K(d), G(d,i) and DI, while E(d) was unaffected when compared to the related values of SGF-PP. In addition, K(d), G(d,i) and DI did not change significantly with temperature. A reasonable correlation was found between fracture toughness of the composite, K(d,c), and of the matrix, K(d,m), by considering the above R parameter according to the microstructural efficiency (M) concept. The failure modes of the materials were studied by fractography and are discussed. It was concluded that the matrix fails by crazing, whereas among the fiber-related failure events fracture and debonding occur in the LGF composites, while, debonding and pull-out dominate in the SGF-PP.