Abstract
The behavior of the crack healing has been studied in past 30 years in polymer and ceramics. Thermal healing is a kind of crack healing. Heat can heal cracks (defects) in polymer and ceramics. The polymer chains can move and rotate above Tg (glass transition temperature), so polymer defects can be healed above Tg. In this paper, we study the difference of internal and surface crack healing, and the mechanical strength recovery during surface crack healing in poly (methyl metharcylate) (PMMA). The ability of crack healing is dependent on the chance of the atom jumps from one side of the crack surface to the other. The heat increases the chance of the atom to move across the surface barrier. The crack healing shrinkage rate is dependent on the environmental temperature. The higher temperature is, the faster crack shrinkage rate in PMMA is. The internal crack can be healed between Tg and 170℃, but the surface crack can be healed above Tg. The activation energy of the internal crack healing is larger then the surface crack healing. When the temperature is above 170℃, the internal crack will change to void, grow and then form bubbles accompanying with the weight loss. The weight of PMMA is fast dropping at the small times and slow at long times. The mechanical strength of PMMA will be recovered to the original strength by the regeneration of polymer chain among crack surfaces. It is proportional to one-fourth power of the crack healing time.