Comparison of four modes of fracture toughness testing for dental composites

Investigators have reported on the measurement of fracture toughness (K(lc)) of dental composites using a variety of testing methods. However, fracture toughness has been shown to be dependent upon several variables, including crack-tip sharpness and specimen geometry. This study was designed to compare the fracture toughness values obtained for two experimental and three commercial posterior composites, using four popular testing methods.

The four methods for testing fracture toughness included single-edge notched, compact tension, short rod with chevron notch and double torsion. The results were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05).

The values obtained from the short rod test were significantly higher than those from the three other tests. Evaluation of the load-deflection curves and fracture surfaces suggested that the data for this test may not have been valid because a stable crack growth region could not be identified. In general, the fracture toughness results obtained from the double torsion test were lower than values obtained from the single-edge notched and compact tension methods. The double torsion test was the most difficult to conduct, resulting in only a 50% success rate.

The double torsion test, though possibly the most technique-sensitive of the four fracture toughness methods evaluated provides the most information about crack initiation and propagation and may be most indicative of the true fracture toughness of dental composites.