[Comparison of three testing methods of fracture toughness using indirect composite].

PURPOSE: The aim of this study was to determine the adaptability of fracture toughness evaluation and the relationship between three testing methods: single-edge notched beam fracture toughness test (SENB), indentation fracture toughness test (IF), and notchless triangular prism fracture toughness test (NTP). METHODS: Two types of indirect composite, Ceramage and Estenia C&B, were used. Four types of experimental specimens were prepared under two curing conditions for each composite resin. One curing condition was only light curing and the other was heat curing after light curing. The fracture toughness value (K(Ic)) of the experimental specimens was measured by each of the three fracture toughness tests. K(Ic) of each test was compared by ANOVA and Tukey's multiple comparison test for the adaptability of fracture toughness evaluation. Then the correlation of two tests was evaluated by a simple linear regression analysis. RESULTS: Evaluation of the load-strain curve (or load-deflection curve) and fracture surface observation of SEM suggested the plane strain fracture phenomenon. K(Ic) values obtained by the three testing methods were significantly different (p<0.01). SENB could separate significantly these experimental specimens into four groups. IF could separate them into two groups. NTP could separate them into three groups. The fracture toughness evaluation of NTP was almost the same as that of SENB. The relationship between SENB and NTP showed a significant correlation (R(2)=99.7%) and its equation was NTP (K(Ic))=0.82 SENB(K(Ic))+1.05. IF did not show a significant correlation. CONCLUSION: These indirect composites tested exhibited different fracture toughnesses. This difference occurred due to the contents and curing conditions. The adaptability of fracture toughness evaluation showed different values depending on the testing method. The relationship of SENB and NTP showed a significant correlation.