applied fracture mechanics

Fracture of Dental Materials 137at 2 and 10 min after irradiation. Radial tensile stresses due to polymerization contraction atthe location of the cracks (σ-crack) were calculated from the incremental crack lengths andthe fracture toughness KC of the glass. Contraction stresses at the composite–glass bondedinterface (σ-interface) were calculated from σ-crack on the basis of the simple mechanics ofan internally pressurized thick-walled cylinder. The greater the distance or the shorter thetime following polymerization, the smaller was σ-crack. Distance, material, irradiationprotocol and time significantly affected σ-crack. Two-step irradiation resulted in asignificant reduction in the magnitude of σ-interface for all resin composites. Thecontraction stress in soda-lime glass propagated indentation cracks at various distancesfrom the cavity, enabling calculation of the contraction stresses.4. ConclusionBy reviewing enamel, dentin and their interfacial bond, it is obvious that the tooth evolvedin such a way it would be able to function in an optimal way without fracturing. With thesophisticated structure of both enamel and dentin, it becomes quite clear that existing manmade restorative materials are far from optimal in comparison to the biological hard tissues.The crack growth risk in ceramics needs to be reduced, something that can be achieved byuse of fracture tough ceramics such as alumina and zirconia. Unfortunately, as shown inLawn et al.’s[45] paper, rather extensive removal of existing tooth structure needs to beperformed in order to minimize future failures. Such an approach, though, does not makesense if one considers that a more sophisticated material is removed in order to replace itwith an inferior material.When it comes to dental composites, we have now reached a point when fractures of compositesare being judged as the most common reason for composite failures [48, 56]. To cometo grip with that problem, our understanding of the fracture mechanics of dental compositesneeds to be improved. The same is true regarding cements/adhesives. The particulate filledresins we are now using are rather primitive when compared to both enamel and dentin.However, it seems as this group of materials has the highest chance to evolve and approachthe properties of enamel and dentin.By looking at dentistry from a fracture mechanics point of view, it becomes quite clear thattraditional dentistry suffer from some major processing problems. The first problem is thatrestorations are individual units that differ in shape and size. These different sizes andshapes result in different levels and locations of localized stresses. The second problem isthat restorations are placed by individual dentists working under different conditions andintroducing different amounts and types of flaws during the different dental procedures.Considering that the theoretical strength is several magnitudes stronger than the realstrength values due to the presence of defects in materials suggest that processing defects,located in a material or at an interface is a significant dental problem. The third problem ispartly self-inflicted. During dental education, students learn to copy the anatomy of naturalteeth. The pits and fissures present in natural teeth act naturally as stress concentrators, butbecause of the sophisticated structure of a substrate such as enamel, such pits and fissures