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

The aim of this study is to characterize the fracture behavior of biodegradable poly(lactic acid) (PLA). Especially, the effects of crystallinity and loading-rate on the fracture behavior are emphasized. Annealing was performed to control the crystallinity of the PLA samples prepared, and then their fracture toughness values were measured under quasi-static and impact loading conditions. The results showed that the quasi-static fracture toughness of PLA decreases with increase of crystallinity; on the other hand, the impact fracture toughness tends to increase with crystallinity. The crack growth behaviors of the PLA specimens having different crystallinity were also observed by polarizing and scanning electron microscopies. The microscopic results exhibited that under quasi-static loading, disappearance of multiple crazes in the crack-tip region results in the decrease of the fracture toughness with crystallinity. On the contrary, under impact loading, the increase of the fracture toughness with crystallinity is considered to be related to the increase of fibril formation.