FRACTURE CHARACTERISTICS OF CF AND GF REINFORCED PA6 SANDWICH MOLDINGS

摘要

The effects of interfacial shear stress between fiber and matrix, and fiber surface morphology on the fracture toughness of carbon fiber (CF) and glass fiber (GF) reinforced PA 6 sandwich composites are investigated in this study. Both fiber length distribution measurement and single edge notch bending fracture tests were carried out. 6 configurations were produced for this study, i.e. PA 6, composites reinforced with CF (CF/PA 6), composites reinforced with GF (GF/PA 6), GF and CF blend (GF/CF/PA 6) produced via conventional injection molding techniques, whereas the sandwich moldings CF-PA 6 skin/GF-PA 6 core (CF_s/GF_c/PA6) and GF-PA 6 skin/CF-PA 6 core (GF_s/CF_c/PA6) which were produced using co-injection molding. As a result of rough CF surface, coupled with its capabilities to induce transcrystallinity in the matrix, CF/PA6 have higher interfacial shear stress, fracture toughness and fracture energy values compared to GF/PA 6 composites.. CF also tends to have a very low critical fiber length compared to GF. This is very obvious as the fracture toughness values of CF/PA6 were the highest among all composites. The GF/PA6 configuration was the second lowest, while PA 6 itself recorded the lowest fracture toughness values. Substantial retention of toughness is observed by blending both CF and GF together into GF/CF/PA6. Both CF_s/GF_c/PA 6 and GF_s/CF_c/PA 6 sandwich structures show nearly similar toughness values. This is due to the fact that both sandwich composites behave like laminates, i.e. GF/PA 6 and CF/PA 6 laminates combined into a single composite. As such, the superior properties of CF were not efficiently utilized. Fracture energy was calculated on the basis of K_c and G_c relation. CF/GF/PA 6 and GF_s/CF_c/PA 6 dominate the highest values of fracture energy. This is again attributed to the addition of CF into the sandwich composites.