Relevance of mode mixity when contrasted with adhesion variability in dental restorations.

Dental repair treatments often involve a bonded ceramic prosthesis, where the bonded interface constitutes a weakness subjected to complex mechanical stresses, leading to mixed interface loading. In such cases, the mixed-mode interface properties are of interest in predicting the ultimate moment of failure, however, their characterization can be laborious. This paper shows that the influence of mode mixity is insignificant with respect to interface adhesion variability, even for interfaces created under laboratory conditions. This result is obtained from miniature mixed-mode bending tests on bonded dental assemblies with varying mode mixity. In situ microscopy images were used, in combination with digital image correlation, to measure the crack propagation, which plays a critical part in computing the interface toughness. Next, a full uncertainty analysis of all error sources provides an upper bound for the expected variability. In contrast, the influence of mode mixity was much smaller than the measured interface toughness variability leading to the conclusion that the actual interface has a spread in toughness which is likely due to surface roughness and chemical variations along the adhesion surface, instead of due to mode mixity. This interface variability is shown to be much larger than the influence of mode mixity, if present at all. Consequently, in the short term, mode mixity analyses have little impact in the understanding of these interfaces, allowing more attention to be given to the source of the interface variability.