Fracture toughness versus micro-tensile bond strength testing of adhesive-dentin interfaces

The objective of the study was to assess interfacial fracture toughness of different adhesive approaches and compare to a standard micro-tensile bond-strength (µTBS) test. Chevron-notched beam fracture toughness (CNB) was measured following a modified ISO 24370 standard. Composite bars with dimensions of 3.0x4.0x25 mm were prepared, with the adhesive-dentin interface in the middle. At the adhesive-dentin interface, a chevron notch was prepared using a 0.15 mm thin diamond blade mounted in a water-cooled diamond saw. Each specimen was loaded until failure in a 4-point bend test setup and the fracture toughness was calculated according to the ISO specifications. Similarly, adhesive-dentin micro-specimens (1.0x1.0x8-10 mm) were stressed in tensile until failure to determine the µTBS. A positive correlation (r²=0.64) was observed between CNB and µTBS, which however was only nearly statistically significant, mainly due to the dissimilar outcome of Scotchbond Universal (3M ESPE). While few µTBS specimens failed at the adhesive-dentin interface, almost all CNB specimens failed interfacially at the notch tip. Weibull moduli for interfacial fracture toughness were much higher than for µTBS (3.8-11.5 versus 2.7-4.8, respectively), especially relevant with regard to early failures. Conclusions: Although the ranking of the adhesives on their bonding effectiveness tested using CNB and µTBS corresponded well, the outcome of CNB appeared more reliable and less variable. Fracture toughness measurement is however more laborious and requires specific equipment. The µTBS nevertheless appeared to remain a valid method to assess bonding effectiveness in a versatile way.status: publishe