Fracture studies of selected dental restorative composites

Objectives. The purpose of this study was to evaluate the flexure strength, elastic modulus, and fracture toughness (mode I, mode II, and mixed mode) of resin and four specially made dental restorative composite materials. Methods. Testing was done on prismatic bars in flexure and disk specimens in diametral compression. Fracture strengths were analyzed using Weibull statistics. Statistical analysis consisted of a one-way analysis of variance followed by a Tukey multiple means analysis for each of the materials. In addition, the fracture strengths were analyzed using Weibull statistics due to the brittle behavior exhibited by these materials. Results. The experimental results showed that the addition of fillers resulted in a significant three-fold increase in flexure modulus and a significant 30–50% increase in fracture toughness from the resin. As was indicated by the different Weibull modulus values, strength data obtained from four-point bending were not related with strength data from three-point bending. A straight notch vs. a relatively sharp V-notch gave higher fracture toughness values. Fracture toughness was dependent on the depth of a straight notch and was practically independent of the V -notch depth. Mode I and II fracture toughness in two composites (75Sr and 75Sr10) were carried out on precracked disk specimens in diametral compression. The results of mode I toughness were close to those obtained from the flexure testing. The mode II toughness values were greater than the mode I values by more than 30%. The data fit an equation of the form KI/KIC + (KII/KIIC)2 =1 (where KI, KII, are the mode I and II stress intensity factors and KIC, KIIC are the respective critical values). Significance. Notching technique, testing configuration (three-point vs. four-point loading), and method of testing (bar vs. disk) have significant effect on the fracture properties.