Fracture studies of adhesion

Effect of dentine depth on the fracture toughness of dentine-composite adhesive interfaces

OBJECTIVES

The fracture toughness test was recently introduced as a clinically relevant method for assessing the fracture resistance of the dentine-composite interface. The objective of this study was to evaluate the effect of dentine depth on the interfacial fracture toughness test of several dentine-composite interfaces using some new proprietary dentine bonding agents.

METHODS

Miniature short rod fracture toughness specimens containing a chevron-shaped dentine-composite-bonded interface were prepared for each group (n = 12). Six different dentine bonding agents and two dentine depths were the variables assessed at the dentine-composite interfaces. After 24 h at 37 degrees C in water, the specimens were tested by loading at 0.5 mm/min in the Instron Universal Testing Machine. The interfacial KIC results were analysed by ANOVA, unpaired Student's t-tests and Fisher's LSD test (P < 0.05).

RESULTS

The interfacial KIC results in MN.m-3/2 (S.D.) on superficial and deep dentine, respectively, were: All-Bond 2, 0.80 (0.21), 0.44 (0.13); Bond-lt, 0.75 (0.20), 0.38 (0.19); Prime and Bond, 0.56 (0.11), 0.28 (0.10); Scotchbond Multi-Purpose, 0.45 (0.23), 0.26 (0.15); One-Step and OptiBond, insufficient results due to premature specimen failures.

CONCLUSIONS

The results from this study should contribute to the development of the fracture toughness test as a method for assessing the integrity of the dentine-composite interface. The interfacial fracture toughness test determined significant differences among the different dentine bonding agents and between the superficial and deep dentine substrates. The dentine bonding agents showed significantly reduced interfacial fracture toughness results when bonding to deep versus superficial dentine.

Effects of dentin surface treatments on the fracture toughness and tensile bond strength of a dentin-composite adhesive interface

It has been proposed that the fracture toughness test provides an appropriate method for assessing the fracture resistance of the dentin-composite interface. The plane-strain fracture toughness test was therefore applied to a dentin-composite interface, with use of a specific dentinal adhesive, so that the effects of various dentin surface treatments on dentin-bond integrity could be studied. Interfacial fracture toughness (KIC) values were determined following 24h and 180 days of specimen aging in distilled water at 37 degrees C. Tensile bond strength (TBS) results following 24-hour aging were also obtained for comparison with the 24-hour KIC results. In general, the fracture resistance of the dentin-composite interface was highest when the dentin surface was conditioned with acid but not air-dried, intermediate when the dentin surface was conditioned with acid and subsequently air-dried, and lowest when the dentin was not conditioned with acid. The tensile bond strength results differed from the fracture toughness results in indicating differences in surface preparation effects and the type of interfacial failure observed.

Analysis of the "shear" bond strength of pretreated aged composites used in some indirect bonding techniques

Investigations of the repair of composites have shown that bond strengths can be significantly reduced at an interface involving an aged composite. Brackets placed by some indirect bonding techniques also have an interface involving an aged composite. This study investigated some of the properties of sealant-composite interfaces and parameters affecting these properties independent of other regions and interfaces found in the total enamel-sealant-composite-bracket system using a shear bond-strength test and fractographic analysis. The effects of various pretreatments of 7-day-old composite surfaces on the bond strength and contact angle of a mixed sealant on these pretreated surfaces were determined. Fractured specimens with adhesive failure closest to the point of force application had low bond strengths (17.62 MPa), which could be correlated with surface pretreatment. Preliminary tests showed that acetone pretreatment produced the most consistently strong interfaces with the fewest adhesive failures of this type. Specimens with adhesive failure only in other regions had higher bond strengths (23.41 MPa), which showed no dependence on pretreatment. There was no statistical relationship between contact angle and bond strength. The data coupled with fracture path analysis suggest that: first, the critical region for fracture analysis is that part of the adhesive interface which is closest to the point of force application; second, failure during some dental shear bond strength tests probably occurs as a consequence of tensile stress induced by a bending moment rather than by shear stress; and third, specifying bond strength as breaking load/area may be incorrect.

The effect of corrosive environment on the porcelain-to-metal bond--a fracture mechanics investigation

Microcracks, flaws, and voids inside a metal-porcelain restoration may cause the restoration to fracture in service. Such cracks result in the concentration of stresses. The dynamic nature of the stresses due to mastication promotes crack growth. In addition, corrosive components of the oral environment enhance the growth rate. In the present investigation, fracture mechanics has been used to analyze the in vitro resistance to fracture of the porcelain-fused-to-metal restoration. The risk of a clinical failure of porcelain-fused-to-metal decreases with enlarged crack resistance (increasing work of fracture). The work of fracture represents an average of the energy for initiation and propagation of a crack through the interface separating porcelain and metal. This work also indicates a material's ability to stop a crack once it is moving. This study utilized the three-point bending test for the crack resistance measurement, and investigated one palladium and five base metal alloys. Corrosive components of the oral environment and the details of firing were of crucial importance for long-term bond stability.

Fracture-mechanics parameters of the composite-enamel bond

In a previous study, the critical values of the opening mode stress intensity factor (K1), its equivalent, the strain energy-release rate (G1), and the J integral (J1) (in the elastic case being equal to that of G1) were determined for resin composite. In this study, the strength of the composite-tooth interface was investigated. The critical values of K1 and J1 were measured with single-edge notched-bend (SENB) specimens of resin composite bonded to enamel, with the notch at midspan at the bonded interface. Due to enamel's anisotropy, the values of Klc and Jlc to be used in a fracture-mechanics application for failure prediction of a structure depend on the enamel prism orientation relative to the adhesive interface. Where interfacial failure is to be expected, the following values for Jlc and Klc can be used: Silux, Jlc = 145 +/- 35 Jm-2 and Klc = 0.84 +/- 0.16 MNm-3/2; P-30, Jlc = 163 +/- 13 Jm-2 and Klc = 1.02 +/- 0.07 MNm-3/2. Where enamel failure is expected or where the failure mode cannot be predicted, the following values can be applied: Silux, Jlc = 89 +/- 15 Jm-2 and Klc = 0.84 +/- 0.16 MNm-3/2; P-30, Jlc = 89 +/- 15 Jm-2 and Klc = 0.75 +/- 0.10 MNm-3/2.

Increase in compressive strength of glass ionomer restorative materials with respect to time: a guide to their suitability for use in posterior primary dentition

Glass ionomer restorative cements have been used in anterior restorations for a number of years but have not been considered strong enough for use in posterior restorations. The compressive strengths of a number of materials currently available were measured at 30 min, 1 hour and 24 hours. It was concluded that some of the materials might be considered for posterior restorations in deciduous teeth where no other considerations, such as radiopacity, apply.

Failure stress criteria for composite resin

In previous work (Peters and Poort, 1983), the stress distribution in axisymmetric models of restored teeth was analyzed by finite element analysis (FEA). To compare the tri-axial stress state at different sites, they calculated the Von Mises equivalent stress and used it as an indication for weak sites. However, the use of Von Mises' theory for material failure requires that the compressive and tensile strengths be equal, whereas for composite resin the compressive strength values are, on the average, eight times larger than the tensile strength values. The objective of this study was to investigate the applicability of a modified Von Mises and the Drücker-Prager criterion to describe mechanical failure of composite resin. In these criteria, the difference between compressive and tensile strength is accounted for. The stress criteria applied to an uni-axial tensile stress state are compared with those applied to a tri-axial tensile stress state. The uni-axial state is obtained in a Rectangular Bar (RB) specimen and the tri-axial state in a Single-edge Notched Bend (SENB) specimen with a chevron notch at midspan. Both types of specimens, made of light-cured composite, were fractured in a three-point bend test. The size of the specimens was limited to 16 mm x 2 mm x 2 mm (span, 12 mm). Load-deflection curves were recorded and used for linear elastic FEA. The results showed that the Drücker-Prager criterion is a more suitable criterion for describing failure of composite resins due to multi-axial stress states than are the Von Mises criterion and the modified Von Mises criterion.

Shear strength of lingual rest seats prepared in bonded composite

A study based on the single criterion of resistance to loading forces was conducted to investigate the suitability of lingual composite rest seats for anterior teeth. The minimum acceptable resistance level was chosen from a review of the literature; other basic criteria were described; and the desirability of this technique was discussed.

Fracture properties of human enamel and dentin in an aqueous environment

SEM fractography and work-of-fracture techniques were used to investigate the fracture properties of human enamel and dentin as a function of the temperature of an aqueous environment. Both enamel and dentin were found to be anisotropic with respect to mechanical failure but were found to have little variation in fracture properties with respect to temperatures from 0 degree to 70 degrees C. Fractographs of a naturally fractured tooth were consistent with experimental findings. The results of this research indicate that natural fracturing of teeth probably cannot be explained on the basis of a reduced work of fracture resulting from chewing hot or cold foods. It was also shown that if dentin surfaces can be kept below 100 degrees C during cavity preparation, the restored tooth will not be weakened by an irreversible reduction of its work of fracture.

Fracture properties of human teeth in proximity to the dentinoenamel junction

Under conditions of controlled failure, the fracture properties of dentin at and in proximity to the dentinoenamel junction (DEJ) were studied with a work-of-fracture (Wf) technique and by scanning electron microscopy (SEM). In 37 degrees C distilled water, fracture of normal human teeth at the DEJ was a difficult failure mode to investigate. Even with high stress concentration at the DEJ, only a few specimens showed apparent DEJ failure at localized regions. SEM fractography of these localized regions revealed surfaces similar in some respects to surfaces found in other studies of the DEJ of diseased teeth and decalcified or desiccated teeth. SEM fractographs varied depending upon the distance of the fractured surface from the DEJ. The results of this research suggest that the work of fracture of dentin increases for failure in the near vicinity of the DEJ, probably reaching a maximum at the DEJ.

Conventional and microfilled composite resins. Part II. Chip fractures

Dentists are accustomed to advantages and disadvantages in the materials at their disposal. This article was concerned with one disadvantage of microfilled composite resins, namely, chip fractures. Probably due to their higher coefficient of thermal expansion, higher water sorption, higher polymerization shrinkage, and lower tensile strengths, cohesive as well as adhesive chip fractures occur three to four times more often with microfilled composite resins than with conventional composite resins. Microfilled composite resins are indicated for esthetic purposes. They are contraindicated for Class IV and stress-bearing restorations. They are indicated for limited use in Class I restorations where esthetics is of primary importance. The technique of use must include acid-etching and intermediate bonding. The microfilled composite resins enjoy a smooth finish and high luster. This offers advantages in areas where smoothness is paramount. They may replace conventional composite resins for resurfacing existing restorations and veneering stained or mottled anterior teeth. They are indicated for treatment of cervical erosion.