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REVIEW |
337A College of Dentistry, Department of Restorative Dentistry m/c 555, 801 South Paulina Street, University of Illinois at Chicago, Chicago, IL 60612-7212, USA; drummond{at}uic.edu
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ABSTRACT |
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 TOP ABSTRACT BACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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KEY WORDS: dental composites • cyclic loading • aging • multi-axial compression • 3D tomography
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BACKGROUND |
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TOPABSTRACTBACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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). In addition to silica (SiO2), these composites also incorporate barium (Ba) and strontium (Sr) glasses, which add x-ray opacity to facilitate the radiological monitoring of the composite in vivo.
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Fracture mechanics is an important tool in understanding and predicting the life of materials. In its most basic form, fracture mechanics can be applied to relate the maximum permissible applied loads acting upon a structural component to the size and location of a crack (either real or hypothetical) in the component (Kanninen and Popelar, 1985). Fracture mechanics can also be used to predict the rate at which a crack can approach a critical size in fatigue or by environmental influences, and can be used to determine the conditions under which a rapidly propagating crack can be arrested. Fracture occurs when the stress concentration inside the material reaches the critical level known as the "plane strain fracture toughness". The "plane strain fracture toughness", KIC, is a measure for the crack resistance of a material. Characterization of this material property can thus help prevent catastrophic failures. Procedures for plane strain fracture toughness testing are standardized by the American Society for Testing and Materials (ASTM, 1990, 1997). A test method that has been used extensively in the study of fracture properties of brittle materials is the diametral compression test, also referred to as the ‘Brazilian disk test’ or the ‘indirect tension test’ (Awaji and Sato, 1978; Dimatteo, 1996; Huang et al., 1996). The Brazilian disk test involves loading a disk specimen in compression (edgewise) along a diameter. The loading generates a bi-axial stress state in the specimen, with a compressive principal stress in the direction of loading and a transverse tensile stress. For a valid plane strain fracture toughness measurement, in addition to the linearity of the load displacement curves and the plane strain conditions, the crack tip should be "atomically" sharp. The crack tip condition is difficult to satisfy in brittle materials, due to problems associated with the growing of a sharp crack normal to the applied load. Researchers have implemented various techniques to introduce sharp notches in brittle specimens (Sanchez, 1979; Shetty et al., 1986). One group analyzed the case of combined mode fracture via the Brazilian disk test and provided expressions for the stress intensity factor (Atkinson et al., 1982).
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AGING ENVIRONMENTS OF DENTAL COMPOSITES |
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TOPABSTRACTBACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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Aging in water may have a beneficial effect on dental composites, since the water is absorbed into the resin matrix, making the composite more flexible, resulting in an apparent increase in mechanical properties. However, over time, the leaching of the components, the swelling and degradation of the cross-linked matrix in the dental composite, and hydrolysis of the filler-matrix interfaces eventually lead to a decrease in mechanical properties (Ferracane et al., 1995, 1998; Takeshige et al., 2007). Other theories as to the cause of the degradation of the dental resin include the formation of microcracks through repeated sorption/desorption cycles, leading to hydrolytic degradation of the polymer (Musto et al., 2002; Yiu et al., 2004). With respect to fracture toughness, water seems to lower the yield stress, release internal stress accumulated during polymerization shrinkage, and increase the plastic zone ahead of the crack, which causes the increase in observed fracture toughness (Indrani et al., 1995; Takeshige et al., 2007). A five-week study on water absorption of dental composites suggested that the decrease in the mechanical properties was a result of residual stress between the wet and dry regions of the composite (Oshida et al., 1995). Whether this might be a carryover to possible breakdown of the resin matrix or separation of the filler particles from the resin matrix for complete sorption of water following a longer aging time was not discussed.
Aging in an ethanol and water mixture results in probable absorption of ethanol and water, resulting in penetration of the cross-linked matrix, weakening the resin, and decreasing the mechanical properties (Ferracane and Berge, 1995). Hydroxyapatite as micrometric or nanometric particles has been used as a filler in a Bis-GMA/TEGDMA resin. The flexure strength was increased over that of the unfilled resin, but was still lower than with traditional glass filler particle composites. The nanoparticles also tended to agglomerate, making uniform dispersion in the resin difficult, and the agglomeration led to reduced mechanical properties (Domingo et al., 2001).
The variation in fracture of dental composites with different fillers is illustrated in Figs. 2–4. These Figs. represent a microhybrid (Fig. 2), Renew (Bisco Inc., Schaumburg, IL, USA); a nanofiller (Fig. 3), Filtek Supreme (3M ESPE, St. Paul, MN, USA); and a fiber filler (Fig. 4), Restolux (Lee Pharmaceutical, South El Monte, CA, USA). Renew is by weight 28% resin and 72% glass filler particles, with an average particle size distribution of 5% 0.004 µm, 62% 0.7 µm, and 5% 3–7 µm particles. Filtek is 78.5% by weight, with 25- to 75-nm filler particles and 21.5% resin. Restolux is 85% filler and 15% resin by weight, with the filler composed of 3- to 4-µm particles (~ 27%) and 80- to 120-µm fibers (~ 52%). The Figs. represent aging for 6 mos in 3 media: air, distilled water, and a 50/50 (by volume) mixture of ethanol and distilled water. The trend appeared to be the same for all materials, in that the fracture tended to occur in the resin matrix between the silanated filler for the specimens aged in air and distilled water. For those specimens aged in the 50/50 mixture of distilled water and ethanol, the resin was severely weakened, such that the failure was in the resin for Renew and Filtek, and for Restolux, the fiber filler was completely separated from the resin matrix (Figs. 4c, 4f). This separation of the fiber from the resin matrix may be unique for the Restolux composite, in that the fiber filler is much larger than any current fiber filler being used, and may be an issue of polymerization shrinkage stress separating the fiber from the resin matrix, in addition to the aging in the 50/50 mixture. Another feature of the nanofiller composite is that, due to the small size of the nanoparticles, the actual filler is in the form of clusters, around 5 µm in size (Fig. 3), and serves as the limiting factor with respect to the mechanical properties.
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, 6). The best results obtained were 74% by weight of the fused-silica whiskers, which resulted in a doubling of the fracture toughness to 3 MPa-m1/2 and better wear resistance compared with that of the controls (Xu, 1999, 2000; Xu et al., 2000, 2002a, 2003, 2004). The silica-fused whiskers demonstrated toughening and crack deflection. Aging of these whisker composites for 730 days in water showed an increased resistance to flexure strength loss compared with that of a control: silica-fused SiN4 whiskers 185 (33) MPa, SiC whiskers 146 (44) MPa, and the control 67 (23) MPa (ArtglassTM, Heraeus Kulzer GmbH, Wehrheim, Germany). The decrease in strength of the non-silica-fused SiN4 whiskers was attributed to the breakdown of the SiN4 whiskers, allowing for their fracture and no crack deflection reinforcement. When SiC whiskers thermally fused with silica particles were used to reinforce dental resin composites, no decrease in flexure strength was observed after 105 thermal cycles between 5°C and 60°C (Xu et al., 2002b). An experimental and theoretical study of a fiber-matrix interface using polypropylene with 40% in mass of short glass fibers for tensile and creep tests indicated zones of behavior that were linear, non-linear without damage, and nonlinear with damage for either treated or untreated fibers (Ségard et al., 2003). This would seem to indicate that the fracture is more complicated than just crack deflection (Manhart et al., 2000).
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Silanes such as methacryloxypropyltrimethoxysilane (MPS) and other silanes are used to strengthen dental composites by forming a covalent bond between the glass filler particles and the various methylmethacrylate-based compounds comprising the resin (Liu et al., 2001). Control of silanation will affect the wear properties of the dental composite (Condon and Ferracane, 1997), presumably by modulating the silane bonding at the organic-inorganic interface of the filler-resin composite. For example, control of moisture in the silanation of silica is also known to control the extent of silane polymerization, which competes with the initial surface bonding (Ulman, 1996). One end of MPS will form Si-O-Si bonds with hydroxy groups on silica and other oxides of the filler particles. Hydrolysis of the Si-O-Si bond by water is a well-known phenomenon, which is expected to weaken the polymer-filler interface during aging (Xiao et al., 1998; Lateef et al., 2002). The other end of the silane also strengthens the composite by forming covalent bonds with the resin matrix (Liu et al., 2001). Hydrolysis of the ester linkage that serves as the silane-resin bond is also feasible, and therefore represents another potential, but largely unexplored, degradation mechanism of dental composites.
The effect of silanization is to move the fracture of the dental composite from between the filler particles to the resin composite adjacent to the filler particles (Jandt, 1999; Lin et al., 2000; Debnath et al., 2004). The silanization also results in an increase in the mechanical properties of the composite. The bonding of glass to resin through silane agents (formation of oxane bonds), other than simple chemical reactivity, is best explained by interdiffusion and interpolymer network formation in the interphase region (Plueddemann, 1988). In dental composites, studies on the effects of interphase on the overall properties have been limited.
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CRACKS AND FLAWS AND CYCLIC LOADING |
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TOPABSTRACTBACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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In many instances, fracture and failure of dental composites occur from a surface or subsurface crack or flaw oriented at an angle with respect to that applied load (mode I and mode II loading) (Ferracane et al., 1992). Fracture initiation for bar specimens results from cracks, voids, inclusions, or other defects, most likely resulting from the processing of the dental composite or the fabrication (polishing, grinding) of the specimens (Rodrigues et al., 2007). Thus, normal (mode I) and shear (mode II) loads drive the crack originating from such defects. Inclined flaws or cracks are also observed in wear loading in many composite materials. Baran et al.(1994, 1998), using an indenter to study wear, observed that although the majority of surface cracks ran originally orthogonal to the surface, they changed direction to run 20° to 30° to the horizon in the direction of the indenter movement. In fact, it is believed that fracture characteristics of any composite material could be realistically investigated under combined fracture modes, because the highly heterogeneous materials’ microstructures give rise to curved crack paths. In addition, it offers a more realistic approach to the study of the fatigue and fracture of dental restorations, since it is more likely that a flaw is at an angle with the force of mastication.
The traditional approach to the testing of brittle materials has been monotonic loading (Guiberteau et al., 1993). These materials are polycrystalline and have enhanced toughening from crack-bridging from various monophase and multi-phase components. Many materials, especially those in the oral cavity, are subject to concentrated contact stresses at the microstructural level, rather than macroscopically distributed stresses, as represented in conventional crack propagation and strength tests (Lawn and Wilshaw, 1975). Lawn and his group have proposed a new procedure for studying fatigue properties of brittle ceramics using a spherical indenter. The initial load of stress fields is purely elastic, and beyond a critical load, the material undergoes permanent deformation and/or fracture (Hertz, 1896; Lawn and Wilshaw, 1975). For a well-behaved, highly brittle material (glasses and ultra-fine polycrystalline ceramics), a well-defined, cone-shaped crack (Hertzian fracture) occurs around the contact circle and spreads downward and outward into the material. In less brittle material, the material deforms plastically beyond the elastic limit, and the cracks form radial or lateral geometries.
Cyclic loading of materials has gained increased importance, since it has been realized that a static evaluation of a material may not be as important as cyclic fatigue values for materials utilized in the oral cavity. Numerous dental restorative materials have shown susceptibility to cyclic loading: ceramics, glass ionomers, fiber-reinforced resins, and composites (Drummond, 1989; Braem et al., 1994, 1995; Drummond et al., 1995, 1998; Bapna et al., 2002). A recent study on flexural fatigue behavior concluded that static strengths do not correlate with fatigue values, and also that contact fatigue is different from flexural fatigue (McCabe et al., 2000).
Flexural cyclic loading results in lower observed flexure strengths (30–50%) than static testing and is considered more sensitive for evaluating the performance of clinical materials (Yoshida et al., 2003; Lohbauer et al., 2006). Papadogiannis et al.(2007), using a dynamic mechanical technique to relate to the viscoelastic properties of dental composites, concluded that fatigue strength is related not only to the type of filler, but also to the silanization of the fillers, and that the resin matrix plays a role.
Mixed-mode loading conditions have been used to investigate the effects of cyclic loading and environmental aging on 3 dental resin composites with different filler compositions: a fiber filler, a hybrid filler, and a microfill (Ravindranath et al., 2007). Diametral (Brazilian) disk specimens 25 mm in diameter and 2 mm in thickness were used in this study. The specimens were aged for 4 mos in air, water, artificial saliva, and a 50/50 (by volume) mixture of ethanol and water at room temperature in sealed polyethylene containers. Both unaged and aged specimens were subjected to cyclic loading at a frequency of 5 Hz with sinusoidal loads cycling for 1, 1000, and 100,000 cycles at a load level 60% of the fracture load for non-cycled specimens. Test results showed that aging in a 50/50 alcohol-water mixture lowered the fracture toughness of dental resin composite, which was further reduced by cyclic loading (Fig. 7). The loads at failure were used as input into a finite element model. After the stress field in the specimens was obtained by the finite element method, the mixed-mode stress intensity factors were calculated by an interaction energy integral method. Good agreement was obtained between the fracture envelope predicted by the maximum tensile stress criterion and the experimental fracture toughness data. Hence, it can be concluded that it is necessary to characterize only the mode I fracture toughness to characterize fully the mixed-mode behavior of dental resin composites. Scherrer et al.(2000) found no difference in the KIC of composites using diametral specimens after aging for 12 mos; however, the specimens were not subjected to cyclic loading. Tikare and Choi (1993) found a range of 0.7–2.0 for KIC, with different fracture behaviors for different fracture-initiating flaws. They stated that indentation methods of crack formation were unable to obtain pure mode II fracture data from flexure-loading conditions, and that fracture toughness was dependent on the microstructure.
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MULTI-AXIAL COMPRESSION AND 3D TOMOGRAPHY |
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TOPABSTRACTBACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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To overcome these issues of uni-axial compression, investigators have used a method of multi-axial compression loading. Dental materials are commonly tested for strength by the use of a bar specimen in a three- or four-point bend configuration. However, dental composites, when placed in natural teeth, are subject to radial as well as axial stresses, thereby introducing a three-dimensional (3D) compressive stress state. Therefore, it is of interest for these materials to be experimentally examined when subjected to multi-axial compression loads, rather than to uni-axial compression. With a method described by Ma and Ravi Chandar (2000), to characterize materials constitutively under confined compression, it is possible to determine the principle components of stress and strain. The loading configuration that composite specimens experience in this method of multi-axial testing better replicates the loading that restorations experience in the oral cavity, and may yield greater insight into the failure mechanisms of dental composites.
This method also allows for better control of compressive load conditions, with the dental composite fabricated as cylindrical specimens 3.7 mm in height and 2.4 mm in diameter in aluminum ring molds, with a strain gauge placed on the outer wall of the aluminum (Al) confining ring to record hoop strain during compression. Assuming plane stress conditions for a thick-walled cylinder subject to internal pressure, the confining stress can be calculated at the interface between the inner wall of the ring and the sample during elastic deformation of the ring (Kotche et al., 2008).
Following mechanical loading, tomographic data were generated for each specimen by the microtomography system at beamline 2-BM of the Advanced Photon Source (APS) (Argonne National Laboratory). Preliminary results are extremely encouraging, yielding clean images with few artifacts (Figs. 8a–8d) (Kotche et al., 2008). Multi-axial compression also eliminates any concern for splintering of the specimen during mechanical loading, since the sample is confined. Furthermore, multi-axially loaded specimens are more consistent between specimens, permitting reasonable baseline thresholds to be established for an entire sample population, thus allowing for a more reliable, automated method of processing the tomographic images, and allowing for more accurate reconstructions of the 3D cracking pattern (Figs. 8e, 8f). Data from this multi-axial approach are given in the Table. The preliminary data are from the investigation of Renew, a hybrid microfilled composite. The objective of this study was to quantify the crack edge area/total volume of multi-axial confined compression specimens of dental composite. The specimens were controls and specimens subjected to 6 and 12% strain, two different loading ring configurations, and cyclic loading at 400 N for 100,000 cycles. The multi-axial compression specimens were contained within Al rings, similar to a Class I occlusal restoration, and loaded via stainless steel plungers on the composite only. The axial load was known, and the constrained load was measured via a strain gauge on the exterior of the Al ring. The Al rings varied with respect to the inner diameter vs. the outer diameter, a variable known as 
, 2.0, and 2.7. The higher the value, the higher the load on the confined composite. The cracks developed during loading were quantified by image analysis of the datasets obtained at the APS. This preliminary 3D image analysis indicated that, as the loading conditions were intensified, from controls to confined compression at 6 and 12%, and an increase in from 2.0 to 2.7, the amount of cracking observed in the composite also increased. No effect was observed for cyclic loading at 400 N under these conditions. Analysis of these data would indicate that Class I occlusal restorations are subject to increased cracking, depending on the thickness of the surrounding tooth structure. It is hoped that continuation of this technique will allow for comparison of the work of strain (the area under the stress-strain curve during loading of the multi-axial confined compression specimen) with the amount of cracking measured in the 3D tomography analysis.
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ACKNOWLEDGMENTS |
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Received January 24, 2008; Last revision April 14, 2008; Accepted April 23, 2008
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REFERENCES |
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TOPABSTRACTBACKGROUNDAGING ENVIRONMENTS OF DENTAL...CRACKS AND FLAWS AND...MULTI-AXIAL COMPRESSION AND 3D...REFERENCES |
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