Thermally-induced crack healing in poly(methyl methacrylate)
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Julie P. Harmon Chemistry Department, University of South Florida, Tampa, Florida 33620-5250
Sanboh Lee Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan (Received 3 January 2002; accepted 3 March 2002)
A microscopic theory of thermally induced crack healing in poly(methyl methacrylate) is presented. Both laser-induced cylindrical cracks and knife-induced surface cracks were analyzed. For a given temperature, the crack closure rate was constant for both types of cracks. However, the crack closure rate was lower for samples with cylindrical cracks than for those with surface cracks. The former exhibited higher activation energy for crack closure than the latter, because the knife-induced cracks had sharper crack tips. Fracture stress was proportional to surface crack healing time to the one-fourth power for thermal healing at a given temperature. Based on the reptation model of polymer chains, the activation energy of chain diffusion was calculated. The healing process was monitored via fractography and crack closure was confirmed. The results were compared with solvent healing and thermal healing in the literature.
I. INTRODUCTION
Crack healing or rejoining two fracture surfaces or micro-voids is well documented in ceramic materials1–8 and polymeric materials.9–17 Crack healing behavior arises from atomic diffusion in ceramic materials and chain entanglement in polymeric materials. There are four methods of initiating crack healing: thermal healing, mechanical healing (or cold healing), solvent healing, and adhesive healing. Thermal healing and adhesive healing are observed both in ceramic materials1–7 and in polymeric materials,8 whereas solvent healing is observed only in polymeric materials.13–16 Mechanical healing is only documented in ionic crystals.8 Most recent research focuses on ceramic materials; there is a great need to increase crack healing studies on polymeric materials. Gupta18 proposed four stages during thermal healing in ceramic materials: (i) continuous recession of the coarse crack from the crack tip or discontinuous pinching off of the crack, (ii) cylindrization of the crack, (iii) break up of cylindrical voids into rows of isolated pores; and (iv) shrinkage of isolated pores. The first stage involves a wetting process. Wool and co-workers9–11 proposed five stages accompanying thermal annealing in polymeric materials: (i) surface rearrangement, (ii) surface approach, (iii) wetting, (iv) diffusion, and (v) randomization. They used a wetting function to describe healing modes. One mode is a line mode in which the crack heals J. Mater. Res., Vol. 17, No. 6, Jun 2002
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as the crack width decreases, and the other mode is a point mode in which the crack heals as the crack tip retreats. Crack tip recession is observed during the first stage of healing in ceramic materials and during wetting described by the point mode mechanism in polymers. Yu et al.19 proposed a model for constant crack closure ra
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