Effect of microstructure and loading-rate on the mode I fracture behavior of poly (lactic acid)

Poly (lactic acid) was molded under four different kinds of condition to obtain samples with different microstructures. A single crystal was not observed in the specimen molded at 0°C. The size of spherulites became larger with increase of molding temperature. The density of spherulites increased with increase of molding time. Mode I fracture toughness, K_IC, was measured at a static (1 mm/min) and an impact loading rate (1 m/s). K_IC became lower with increase of degree of crystallization at the static loading rate. This is mainly due to suppression of craze formation in the vicinity of crack-tip. On the other hand, at the impact loading rate, K_IC increased with increase of degree of crystallization. Thus, the results showed that crystallization has significant influence on K_IC, and its effect on the static K_IC was different from that on the impact one. Relationship between fracture toughness and fracture micromechanism was discussed on the basis of the results of fracture tests and microscopies.