Dental adhesives are usually tested in shear or tension even though neither of these approaches measures the local stress triggering failure. Because the stress level varies extensively over the bonded surface, it seems as a fracture mechanics approach would be more appropriate.
In this review different general aspects of fracture mechanics and adhesive joints were reviewed first. That review served as a foundation for a review of fracture toughness studies performed on dental adhesives. The dental adhesive studies were identified through a MEDLINE search using “dental adhesion testing AND enamel OR dentin AND fracture toughness” as search strategy.
The outcome of the review revealed that fracture toughness studies performed on dental adhesives are complex, both regarding technical performance as well as achieving good discriminating ability between different adhesives. The review also suggested that most fracture toughness tests of adhesives performed in dentistry are not totally reliable because they usually did not consider the complex stress pattern at the adhesive interface. However, despite these limitations, the review strongly supports the notion that the proper way of studying dental adhesion is by use a fracture mechanics. At the present time, it seems as the fracture energy of adhesives might be more appropriate to determine than their fracture toughness values.
Introduction to the review
The objective with this article was to critically review some of the literature regarding fracture mechanics and how fracture mechanics relates to the mechanics, geometry, load application and other testing parameters (i.e. speed) of fracture toughness adhesion tests. To do so, the topic was divided into two main sections. In Section 2.1 , fracture mechanics in general and how it can be applied to adhesives in general is presented. In Section 2.2 , a review of different fracture toughness studies performed on dental adhesives, focusing on specimen geometry, load application (i.e. speed) and other testing parameters is presented. In Section 3 , a summary of advantages and limitations of fracture toughness tests conducted on dental adhesives is presented.