Methods: 60 short-rod specimens for fracture toughness test and 30 composite blocks for microtensile bond strength test was fabricated. Specimens were allocated into 6 groups according to the combination of surface treatment(none, air abrasion, diamond bur) and repair time(fresh cut, 2wks aging). 24 hours after repair, composite blocks were sectioned into three slabs and trimmed to an hourglass shape (1mm2 area). Slabs and short-rod specimens were tested under tension. Mean values analyzed with one-way analysis of variance/Duncan tests (a = 0.05). Pearson's coefficient was used to verify the correlation between the mean of fracture toughness and microtensile bond strength. Fe-SEM was employed on fractured surface to examine the crack propagation.
Results: 1. There was statistically significant difference between fresh resin groups and aged resin groups in µTBS test (p < 0.05). 2. In µTBS test, result of NF group was similar to those of SF and BF (p > 0.05). Result of NA group was significantly different from those of SA and BA (p < 0.05). 3. In fracture toughness test, mechanical treatment groups showed higher values than those of none treatment groups (p < 0.05). 4. There was correlation between fracture toughness and µTBS for composite repair within significance level (p < 0.001), but it was considered meaningless (r=0.535, r2 = 0.286). 5. Results of FE-SEM observation showed that the crack can propagate through composite as well as adhesive layer and demonstrated toughening mechanism of material. Conclusion: Surface treatment by mechanical interlocking is more important for effective composite repair. Fracture toughness test is more appropriate to study fractural behavior.