applied fracture mechanics

134Applied Fracture MechanicsAdhesion at the titanium–porcelain interface using a fracture mechanics approach has alsobeen used to investigate the bonding mechanism of such systems [70]. In that study theyused specimens of five different titanium–porcelain and one base metal–porcelain bondingsystems on which they performed a four-point bending interfacial delaminating test. Thepre-cracked specimen was subjected to load and the strain energy release rate (G) was calculatedfrom the critical load to induce stable crack extension in each system. The strain energyrelease rate of titanium–porcelain with a Gold Bonder interface layer was highest among thefive different systems. No attempt was made to explain the experimental findings.In two studies by Ichim et al. [71, 72] they looked at a typical non-carious cervical lesion, aso called abfraction, treated with a glass ionome or a combination of glassionomer andcomposite. The approach they used was that they used a nonlinear fracture mechanicalapproach simulated by use of FEA. They used a novel Rankine and rotating crack model totrace the fracture failure process of the cervical restorations. The approach involves anautomatic insertion of an initial crack, mesh updating for crack propagation and self contactat the cracked interface. The results were in good agreement with published clinical data, interms of the location of the fracture failure of the simulated restoration and the inadequacyof the dental restoratives for abfraction lesions.In their second study [72] they investigated the influence of the elastic modulus (E) on thefailure of cervical restorative materials and tried to identify an E value that would minimizemechanical failure under clinically realistic loading conditions. What they found was that therestorative materials currently used in non-carious cervical lesions are largely unsuitable interms of resistance to fracture of the restoration. They suggested that the elastic modulus ofsuch a material should be in the range of 1 GPa rather than several GPa that is usually the case.Despite an obvious advantage to approach adhesives and their performance from a fracturemechanics point of view, traditional bond studies usually focus on bond strength values. Bycomparing such strength values, it is noticed that large variations exist among differentreports. These variations are due to differences among operators, but also on the day acertain tester performed a test. The standard deviation is 25-50 % of the mean value, whichsuggests that defects present in the adhesive region may be of a bigger concern than the trueadhesive strength.In an attempt to resolve the questions related to the large variability in strength values andtheir clinical meaning, The Academy of Dental Materials at their annual meeting in 2010,focused that meeting on the value of bond strength measurements. In one presentation,Scherrer et al. [73] presented a literature search based on all dentin bond strength data obtainedfor six adhesives evaluated with four tests (shear, microshear, tensile and microtensile)and critically analyzed the results with respect to average bond strength, coefficient ofvariation, mode of failure and product ranking. The PubMed search was carried out for theyears between 1998 and 2009. The six adhesive resins that were selected included three stepsystems (OptiBond FL, Scotch Bond Multi-Purpose Plus), two-step (Prime & Bond NT, SingleBond, Clearfil SE Bond) and one step (Adper Prompt L Pop). By pooling the results from