applied fracture mechanics

118Applied Fracture Mechanicsduring cavity preparation with conventional dental burs may serve as a principal source forpremature restoration failure.As the hardest and one of the most durable load bearing tissues of the body, enamel hasattracted considerable interest from both material scientists and clinical practitioners due toits excellent mechanical properties. In a recent article [8] possible mechanisms responsiblefor the excellent mechanical properties of enamel were explored and summarized. Whatthese authors emphasized was the hierarchical structure and the nanomechanical propertiesof the minor protein macromolecular components. The experimental and numerical resultssupported the made assumptions. For example, enamel showed to have lower elasticmodulus, higher energy absorption ability and greater indentation creep behavior thansintered hydroxyapatite material. These findings suggest that the structural andcompositional characteristics of the minor protein component significantly regulate themechanical properties of enamel in order to better match its functional needs.The fascinating aspect of enamel is that its structure seems to have evolved and adapted tothe need of the user of the teeth. For example, in some recent publications [9, 10], theseissues have been discussed. Lucas et al. [9] proposed a model based on how fracture anddeformation concepts of teeth may be adapted to the mechanical demands of diet, whileConstantino et al [10] used that model by examining existing data on the food mechanicalproperties and enamel morphology of great apes (Pan, Pongo, and Gorilla). They paidparticular attention to whether the consumption of fallback foods plays a key role ininfluencing great ape enamel morphology. Their results suggest that so is the case, and thattheir findings may explain the evolution of the dentition of extinct hominins.Along these lines, Lee et al.[11] did a comparative study of human and great ape molartooth enamel. They used nano-indentation techniques to map profiles of elastic modulusand hardness across sections from the enamel–dentin junction to the outer tooth surface.The measured data profiles overlapped between species, suggesting a degree of commonalityin material properties. Using established deformation and fracture relations, critical loadsto produce function-threatening damage in the enamel of each species were calculated forcharacteristic tooth sizes and enamel thicknesses. The results suggest that differences inload-bearing capacity of molar teeth in primates are less a function of underlying materialproperties than of morphology.From the above studies, it is quite clear that Nature has adapted the structure of enamel toresist fractures. In a study by Bajaj [4] the crack growth resistance behavior and fracturetoughness of human tooth enamel was determined. The results were quantified usingincremental crack growth measures and conventional fracture mechanics. The resultsrevealed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve)with crack extension from the outer to the inner enamel, and that the rise in toughness is afunction of distance from the dentin enamel junction (DEJ). The outer enamel exhibited thelowest apparent toughness (0.67± 0.12 MPa m 0.5 ), and the inner enamel exhibited a rise in thegrowth toughness from 1.13 MPa m 0.5 /mm to 3.93 MPa m 0.5 /mm. The maximum crackgrowth resistance at fracture (i.e. fracture toughness (KC)) ranged from 1.79 to 2.37 MPa m 0.5 .