Evaluation of fracture toughness of a fiber containing dental composite after flexural fatigue

Objective. The purpose of this study was to examine the effect of cyclic loading in four different environments on the fracture toughness of a fiber reinforced resin composite. Methods. The specimens were 3×3×25 mm3 bars polished with 320 grit SiC paper. A 60° v-notch was machined 1 mm deep at the midspan of each bar. The specimens were loaded in air, artificial saliva, water, and a 50/50 by volume mixture of ethanol and water at a frequency of 5 Hz with sinusoidal loads cycling between 10 and 20 N for 1, 1000, 10,000 and 100,000 cycles. Specimens were also aged for 4 months in each respective media. Following cyclic loading, the specimens were tested in three-point loading. Statistical analysis consisted of four 2-way ANOVAs followed by Tukeyʹs HSD inference, and two 3-way ANOVAs followed by two-sample t-tests. Results. The ANOVAs indicated no difference in the means of fracture toughness for the set numbers of cycles, but a significant difference for the aging solutions and the aging times. Significance. Statistical analysis showed a significant decrease of fracture toughness from unaged to aged specimens when aged in air, saliva, and 50/50 ethanol/water. The latter showed the most pronounced decrease. The effect of the 50/50 by volume mixture of ethanol and water would appear to attack the resin matrix and/or the bond between the resin matrix and the filler, which resulted in the observed decrease in fracture toughness. In addition, the significant decrease in fracture toughness following aging is attributed to residual matrix stress around the large fiber filler particles resulting in separation of the fiber filler from the resin matrix.