Relationship Between Fracture Toughness and Fracture Mirror in Modern Polymer-Based Dental Composites

The mechanical behavior of dental composites depends on the sample size and stress configuration. This makes it difficult to extrapolate laboratory data to clinical restorations with significant variations in size and geometry. Intrinsic parameters, such as fracture toughness, are therefore of great importance, even if they are less common and more difficult to measure. The aim of this study was to apply principles of fractography and fracture mechanics to exploit the results obtained from a three-point bending test. The objectives include calculating a material-specific constant, validating the experimental findings, and establishing a correlation with fracture toughness. Forty representative composites with wide variation in filler quantity (65–83% by weight and 46.4–64% by volume), type (compact glasses and pre-polymerized), and composition were examined. Fracture toughness/KIc was evaluated in a notchless triangular prism test. Fracture type, origin, and mirror size were determined on 280 flexural fracture specimens (n = 20). The amount of filler strongly influences all measured parameters, with the effect strength varying in the sequence: mechanical work (ηP2 = 0.995), modulus of elasticity (ηP2 = 0.991), flexural strength (ηP2 = 0.988), fracture toughness (ηP2 = 0.979), and mirror constant (ηP2 = 0.965). Fracture surfaces allowed the delineation of the fracture mirror and the application of fracture mechanics approaches. The mirror constant was derived from the radius of the fracture mirror, measured in the direction of constant stress, using Orr’s equation, and correlates well with KIc (0.81). Larger confidence intervals were observed for the mirror constant data, while for 5 of 14 materials, the mirror constant was overestimated compared to KIc. The overestimation was attributed to the lower refractive index of the urethane methacrylate composition.