Fracture toughness of Composites, Comparison of CNB and SEVNB Methodologies

Fracture toughness is a key parameter to study chipping fractures and wear. Because of their heterogenic nature, the fracture toughness of dental composites is often hard to assess. Objectives: To determine the fracture toughness of 4 composites using 2 different test methodologies. Methods: For the Chevron-Notched Beam (CNB) test, the ISO 24370:2005(E) for ceramics was slightly adapted to comply with resin-composite specimens. Composite blocks of 25x3x35-50 mm were prepared and cut into 25x3x4 mm specimens. After 1 week of water storage, a notch was prepared using a precision low-speed saw (Accutom 50, Struers) mounted with a 150-µm thick diamond blade. Next, the fracture toughness was determined by subjecting the specimens to a four-point bending setup in a universal testing machine (5848, Instron). For the single-edge V-notched beam (SEVNB) test, eight 16x3x2 mm specimens were prepared for each composite according to the ASTM Designation E 399-83. Notch tip radii of 5–10 µm were prepared and specimens with notch depths of 0.8–1.4 mm were used. Specimens were then loaded to fracture in a three-point bending test setup (4400, Instron), after which KIC was calculated. Results: Fracture toughness measured using the CNB method resulted in significantly higher values than that measured using the SEVNB method (2-way ANOVA, p<0.0001). Nevertheless, both methods correlated very well (Pearson's correlation coefficient = 0.9779; p=0.0221), so that ranking and the observed differences between the different composites were preserved.

Conclusion: Fracture toughness of dental composites can be determined by the CNB as well as the SEVNB methodology; despite the difference in specimen size, the outcome of both methods correlated very well. The easier specimen preparation of the CNB methodology renders the outcome less technique sensitive and enables to prepare more specimens with less effort.