Fracture toughness and microhardness of a composite: Do they correlate?

Objectives. Chipping and bulk fracture are major contributors in clinical failures of composite restorations. Fracture toughness (K_Ic) quantifies susceptibility for fracture, but experimental determination is complicated. It would be beneficial for the dental community if a relatively simple experiment, such as microhardness (HK), could be used to screen composites for fracture resistance. This study explores a possible correlation between K_Ic and HK. Methods. Composite cylinders (4mm diameter and ∼7mm long) were cured for five combinations of light intensity (I, mW/cm²) and curing time (T, s) to achieve a range of different total light energy densities (I×T=100×10, 100×20, 300×20, 300×40, and 700×60mWs/cm²). A chevron-notch was cut in the median plane of the cylinders for the fracture toughness test, which was executed in a displacement control mode at 6μm/s cross-head speed (sample size 4). Knoop hardness was determined at the median plane of the cylinders (sample size 6). The tests were performed 15min and 24h after curing. Results. Both the K_Ic and HK increased with increased light energy density and storage time. Linear regression analysis indicated a strong correlation between HK and K_Ic tested at the same time period (R²=0.97 and 0.90 for 15min and 24h, respectively). The correlation became weaker between the different storage times (R²=0.71), indicating a change in fracture toughness and/or microhardness mechanisms. Conclusion. Fracture toughness of a composite cannot be simply extrapolated from microhardness.