Miniature specimens for fracture toughness evaluation of dental resin composites

Objective

Millimeter-scale (“miniature”) specimens enable in-situ evaluation of mechanical properties of engineering materials at reduced cost. Here three such specimens for measuring fracture toughness (K_C) are developed and implemented to new dental materials. The latter include concurrent methacrylate-based and new ether-based resin composites designed to reduce polymerization stress and enhance service life in restored teeth.

Fracture toughness and cyclic fatigue resistance of resin composites with different filler size distributions

Objectives

To verify the influence of filler size distributions on fracture toughness (K_Ic), initial fracture strength (IFS) and cyclic fatigue resistance (CFR) of experimental resin composites.

Methods

Four composites were prepared with same inorganic content (78 wt%), in which 67 wt% was constituted by glass particles with d₅₀ of 0.5; 0.9; 1.2; 1.9 μm K_Ic of the composites was determined by the single-edge notched beam (SENB) method. To evaluate the IFS and the CFR a biaxial bending test configuration was used. The CFR was determined under cyclic loading for 10⁵ cycles using the ‘staircase’ approach. The fracture surfaces of IFS and CFR specimens were analyzed under scanning electron microscope (SEM).

Results

There was a positive linear correlation between d₅₀ vs. K_Ic and statistical difference was found only between C0.5 (1.24 ± 0.10 MPa m^0.5) and C1.9 (1.41 ± 0.17 MPa m^0.5). There were no statistical differences among IFS means, which ranged from 155.4 ± 18.8 MPa (C0.9) to 170.7 ± 23.1 MPa (C1.2). C0.5 (93.0 ± 18.6^a MPa) showed the highest and C0.9 the lowest CFR (82.5 ± 8.0^c MPa). There was no correlation between CFR with d₅₀ values or with K_Ic means. SEM images showed the morphology with brittle fracture patterns for the surfaces of IFS specimens and a more smooth fracture surface for CFR specimens.

Significance

Resin composites showed different failure mechanisms for quasi-static and fatigue loading. For K_Ic and IFS, composites with larger filler size distributions showed better results due to crack deflection; while under cyclic loading, viscous behavior was predominant and composites with smaller particles showed higher fatigue resistance.     

Ion releasing direct restorative materials: Key mechanical properties and wear

ObjectivesTo investigate the depth of cure (DoC), fracture toughness (K_IC) and wear of ion releasing resin-based composite (RBC) restorative materials.MethodsTwo ion releasing RBCs, Activa (ACT) and Cention-N (CN) were compared to a conventional RBC (Z350) and a resin-modified glass ionomer cement (Fuji-II-LC). The DoC was measured in a 10-mm deep semi-circular metal mold with a 2-mm internal radius (n = 8). The molds were irradiated from one end for 20-s. The Knoop hardness (KH) was measured at 0.5-mm intervals from the surface after the specimens had been stored at 37 °C for 24-h. To measure the K_IC, single-edge-notched specimens (n = 15/group) were prepared (25×5x2.5-mm) for a 3-point bending test and then stored for either 1 or 30-days in water at 37 °C. Disk-shaped specimens (n = 10) were subjected to 250,000-load cycles of 49-N using a chewing simulator against spherical steatite antagonists. DoC and wear data were analyzed by one-way ANOVA and Tukey post hoc tests (p ≤ 0.05). K_IC data were analyzed by two-way ANOVA and one-way ANOVA, and the Tukey post hoc test (p ≤ 0.05). In addition, an independent t-test was used to determine if storage time had any effect (α = 0.05) on the K_IC of each material.ResultsMaximum hardness value was the highest for Z350 and the lowest for ACT. The depth at which 80% of the maximum KH, was the highest for CN (9.2 mm) and the lowest for Z350 (2 mm). All tested materials met the manufacturers’ claims for DoC. After 1-day, the highest K_IC values were recorded for ACT and the lowest for Fuji-II-LC. Water storage (30-days) significantly reduced the K_IC value for all materials except Fuji-II-LC. The highest wear rate values were recorded for CN followed by ACT.SignificanceAll tested materials met their manufacturers’ claims for DoC. Water storage for 30-days significantly reduced the fracture toughness for ACT and CN. Wear was significantly higher for ACT and CN.     

Practical and theoretical considerations on the fracture toughness testing of dental restorative materials

Background

An important tool in materials research, development and characterization regarding mechanical performance is the testing of fracture toughness. A high level of accuracy in executing this sort of test is necessary, with strict requirements given in extensive testing standard documents. Proficiency in quality specimen fabrication and test requires practice and a solid theoretical background, oftentimes overlooked in the dental community. Aims: In this review we go through some fundamentals of the fracture mechanics concepts that are relevant to the understanding of fracture toughness testing, and draw attention to critical aspects of practical nature that must be fulfilled for validity and accuracy in results. We describe our experience with some testing methodologies for CAD/CAM materials and discuss advantages and shortcomings of different tests in terms of errors in testing the applicability of the concept of fracture toughness as a single-value material-specific property.     

Fracture toughness and brittleness of novel CAD/CAM resin composite block

ObjectivesTo evaluate the fracture toughness and brittleness of a newly developed CAD/CAM resin composite block and compare it with five other resin composite blocks and one polymer-infiltrated ceramic block.MethodsFracture toughness was determined through single-edge notched beam (SENB) method according to ASTM D5045-14. Parallelepiped specimens (thickness × width × length = 1.8 × 3.6 × ～18 mm) with a V notch (length = ～1.8 mm) (n = 10) were made from block materials using a low-speed water-cooled diamond precision saw and razor blade. Brittleness index (BI) was calculated from Vickers hardness and fracture toughness. Data were analyzed using one-way ANOVA and post-hoc Tukey tests (p ≤ 0.05).ResultsThe experimental block showed significantly higher fracture toughness than all commercial block materials (p < 0.001) and has a brittleness index similar to most commercial blocks (p > 0.05) which have significantly lower brittleness index than Vita Enamic (p < 0.001). A moderate or strong correlation was observed between fracture toughness and flexural strength (Pearson’s correlation coefficient R = 0.66) or diametral tensile strength (R = 0.86) or filler loading (R = 0.66), and between brittleness and Vickers hardness (R = 0.87).SignificanceThe new composite block exhibited significantly higher fracture toughness and lower brittleness among the commercial CAD/CAM composite block materials tested, indicating a lower tendency to fracture and marginal chipping, and better machinability. The new composite block with higher fracture toughness and lower brittleness is suitable to use in the fabrication of CAD/CAM indirect restorations with potential long-term clinical success.     

Effect of storage media and time on the fracture toughness of resin-based luting cements

Background: Resin‐based cements are brittle materials and the major shortcomings of these materials are manifested in their sensitivity to flaws and defects. Although various mechanical properties of resin luting cements have been described, few fracture toughness test data for resin‐based luting cements using the short rod design have been published. Methods: Specimens were prepared from five resin luting cements. For each material a total of 36 disc‐shaped specimens were prepared using a custom‐made mould. Specimens were randomly divided into six groups of six, immersed in two solutions: distilled water and 0.01 mol/L lactic acid at 37 °C for 24 hours, 1 month or 3 months. The specimens were loaded using a universal testing machine. The maximum load at specimen failure was recorded and the K_Ic (MPa. M 0.5) was calculated. Results: There was a relationship between material, storage solution and time (p < 0.05). Nexus 3 showed the highest K_Ic followed by Panavia F, Calibra, Smart Cem2 and seT. Conclusions: The fracture toughness of the resin luting cements was affected by both time and storage solution. Comparable fracture toughness of conventional resin cement and self‐adhesive resin cements was observed.     

Fiber reinforcement of a resin modified glass ionomer cement

ObjectivesUnderstand how discontinuous short glass fibers and braided long fibers can be effectively used to reinforce a resin modified glass ionomer cement (RMGIC) for carious lesion restorations.MethodsTwo control groups (powder/liquid kit and capsule) were prepared from a light cured RMGIC. Either discontinuous short glass fibers or braided polyethylene fiber ribbons were used as a reinforcement both with and without pre-impregnation with resin. For the former case, the matrix was the powder/liquid kit RMGIC, and for the latter case the matrix was the capsule form. Flexural strength was evaluated by three-point beam bending and fracture toughness was evaluated by the single-edge V-notch beam method. Compressive strength tests were performed on cylindrical samples. Results were compared by analysis of variances and Tukey’s post-hoc test. Flexural strength data were analyzed using Weibull statistical analysis.ResultsThe short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase of ∼50% in the mean flexural strength and 160–220% higher fracture toughness compared with the powder/liquid RMGIC control. Reinforcement with continuous braided fibers gave more than a 150% increase in flexural strength, and pre-impregnation of the braided fibers with resin resulted in a significant flexural strength increase of more than 300% relative to the capsule control. However, for the short fiber reinforced RMGIC there was no significant benefit of resin pre-impregnation of the fibers. The Weibull modulus for the flexural strength approximately doubled for the fiber reinforced groups compared to the control groups. Finally, compressive strength was similar for all the groups tested.SignificanceBy using a RMGIC as a matrix, higher flexural strength was achieved compared to reported values for short fiber reinforced GICs. Additionally, the short fibers provided effective toughening of the RMGIC matrix by a fiber bridging mechanism. Finally, continuous braided polyethylene fibers gave much higher flexural strength than discontinuous glass fibers, and their effectiveness was enhanced by pre-impregnation of the fibers with resin.     

Fracture toughness of resin-modified glass ionomer restorative materials: effect of powder/liquid ratio and powder particle size reduction on fracture toughness

OBJECTIVES: The objectives of the current investigation were two-fold: (1) to examine the effect of different powder/liquid (P/L) ratios on the fracture toughness of commercial resin-modified glass ionomer cement and conventional glass ionomer cement, and (2) to evaluate the effect of powder size reduction on the fracture toughness of experimental resin-modified glass ionomers in order to improve their physical properties. METHODS: The P/L ratios of the glass ionomer and resin-modified glass ionomers were varied from the manufacturer's recommended ratio to 2.0 and 1.0 by weight. The powder particle sizes for the experimental resin-modified glass ionomers tested were 2, 5, 10 and 25 micro m in diameter. Fracture toughness was determined on ring-shaped specimens with a fatigued pre-crack. RESULTS: The fracture toughness of the resin-modified glass ionomers was significantly higher (p<0.005) than that of the glass ionomer and was not greatly influenced by the P/L ratio. For the experimental resin-modified glass ionomers, it was observed that fracture toughness gradually decreased as the powder particle size became finer. SIGNIFICANCE: The resin components in the liquid play an important role in the improvement of the physical properties of the resin-modified glass ionomer. A reduction in the powder particle size of up to 10 micro m, which resulted in a smoother surface, can maintain high fracture toughness. The high fracture toughness values of the resin-modified glass ionomer may be one of the factors contributing to a favorable clinical outcome in high stress-bearing areas.     

Evaluation of physical properties of fiber-reinforced composite resin

ObjectivesThis study aimed to investigate physical properties of a fiber-reinforced CAD/CAM resin disc, which included woven layers of multi-directional glass fibers.MethodsFiber orientations of CAD/CAM specimens (TRINIA, SHOFU) were specified as longitudinal (L), longitudinal-rotated (LR), and anti-longitudinal (AL). A fiber-reinforced composite (everX posterior, GC (E)) and a conventional composite (Beauti core flow paste, SHOFU (B)) were also tested.A three-point bending test and a tensile test with notchless prism-shaped specimens were conducted using a universal testing machine (AUTOGRAPH AG-IS, Shimadzu). A water absorption test was also carried out after the specimens were stored in water for 24 h or 1 week. Flexural strength and fracture toughness were obtained by conducting a three-point bending test.ResultsTRINIA L and LR groups showed significantly high flexural strength (254.2 ± 22.3 and 248.8 ± 16.7 MPa, respectively). Those were approximately 2.5 times higher than those in AL, E, and B groups (96.8–98.0 MPa) (p < 0.05, ANOVA and Tukey HSD test). No significant difference was shown in flexural modulus among the experimental groups. The fracture toughness in L group (9.1 ± 0.4 MPa/m^1/2) was found to be significantly higher than those in other groups (1.9–3.0 MPa/m^1/2; p < 0.05). TRINIA group demonstrated significantly lower water absorption (4.7 ± 1.9 μg/mm³) than did E (16.1 ± 3.1 μg/mm³) and B (17.3 ± 3.7 μg/mm³) groups (p < 0.05).SignificanceTRINIA demonstrated distinct anisotropy. TRINIA can be used as a superior restorative material when specifying directions of its fiber mesh layers.     

Fracture strength and fracture patterns of root filled teeth restored with direct resin restorations

Objective

To compare fracture characteristics of root-filled teeth with variable cavity design and resin composite restoration.

Methods

80 extracted intact maxillary premolars were divided randomly into eight groups; (1) intact teeth; (2) unrestored MOD cavity; (3) unrestored MOD cavity plus endodontic access through the occlusal floor; (4) unrestored MOD plus endodontic access with axial walls removed; (5) MOD restored with resin composite; (6) MOD plus endodontic access, resin composite; (7) MOD plus extensive endodontic access, resin composite; (8) MOD plus extensive endodontic access, GIC core and resin composite. A ramped oblique load was applied to the buccal cusp in a servohydraulic testing machine. Fracture load and fracture patterns were recorded. Fracture loads were compared statistically using 1-way ANOVA, with Dunnett test for multiple comparisons.

Results

Unrestored teeth became progressively weaker with more extensive preparations. Endodontic access confined within the occlusal floor did not significantly affect strength compared to an MOD cavity. Loss of axial walls weakened teeth considerably [292 + 80 N vs 747 + 130 N for intact teeth]. Restoration increased the strength of prepared teeth particularly in teeth without axial walls. Teeth with a GIC core were not significantly weaker than intact teeth [560 + 167 N]. Failures were mostly adhesive at the buccal interface, with the fracture propagating from the buccal line angle of the occlusal floor (MOD and MOD plus access groups) or of the proximal box (axial wall removed).

Conclusions

Direct restorations increased fracture resistance of root filled teeth with extensive endodontic access. Both restored and unrestored teeth showed similar fracture patterns.     

Effect of silane coupling agent and concentration on fracture toughness and water sorption behaviour of fibre-reinforced dental composites

ObjectivesThis paper presents the effect of silane treatment of S-2 Glass fibres on the fracture toughness and water sorption/solubility behaviour of fibre-reinforced flowable dental composites. The effect of epoxy- and methacrylate-based silane coupling agents (SCAs) on the mechanical strength and hydrolytic properties were investigated. The concentration of the selected SCAs on the mechanical and physical properties were investigated. The influence of molecular structure and concentration in the interfacial adhesion at the fibre-matrix interfaces was also studied.MethodsShort S-2 Glass fibres of 250 µm in length and 5 µm in diameter were etched with acid to remove any impurities and roughen the surface. The acid-etched fibres were silane treated with 3MPS, 3GPS, and 8MOTS at different concentrations by weight (%). The silane-treated fibres were incorporated at 5 % into the dental resin mixture. Untreated fibres were added at 5 % to the dental resin mixture and served as the control group. The physical properties such as water sorption, solubility, and desorption along with mechanical properties such as fracture toughness and total fracture work of the fibre-reinforced dental composites grafted with the above-mentioned SCAs were evaluated. The surface morphology of the fractured surface was studied and analysed.ResultsThe fracture toughness tests showed that the dental composites grafted with optimum weight per cent (wt. %) concentration of the SCA had a better stress intensity factor (K_IC) when compared to the 2.0 wt. % and 3.0 wt. % concentration. The K_IC value of dental composites grafted with untreated surface etched glass fibres was less than the K_IC values of dental composites grafted with optimum concentrations of 3MPS, 3GPS, and 8MOTS by 81.6 %, 38.6 %, and 110.5 %, respectively. A similar trend was found while investigating the total work of fracture of the dental composites, between optimum concentration, 2.0 wt. % and 3.0 wt. % concentration of respective SCA. The increase in silane concentration also led to an increase in the water sorption/solubility characteristics. The absorption of water was most severe in the fibre-reinforced dental composites without silane treatment (32.9 µg/mm³). The ANOVA results showed that the fibre-reinforced dental composites grafted with 8MOTS at optimum concentration showed an increase in fracture toughness when compared to optimum concentrations of 3GPS and 3MPS by 51.9 % and 15.9 %, respectively. The enhanced mechanical and physical characteristics are due to the increased adhesion between the fibre and silane achieved from the optimum wt. % concentration of 8MOTS. Similarly, dental composites grafted with 8MOTS at optimum concentration showed a decrease in water sorption characteristics when compared to optimum concentrations of 3GPS and 3MPS by 18.2 % and 0.6 %, respectively. The decreased water sorption characteristics at the optimum concentration of 8MOTS could be due to the reduced availability of reactive hydroxyl groups and the hydrophobic characteristics of 8MOTS.SignificanceSilane coupling agents (SCAs) are important components of dental composites. The type and concentration of SCA have a significant effect on material properties. The current study focuses on understanding the effects of different SCAs and wt. % concentrations on the interfacial fracture behaviour and the influence of different SCAs on the water sorption and solubility behaviour of S-2 Glass fibre-reinforced flowable dental composites.     

Comparison of two fracture toughness testing methods using a glass-infiltrated and a zirconia dental ceramic

PURPOSE

The objective of this study was to compare the fracture toughness (K_Ic) obtained from the single edge V-notched beam (SEVNB) and the fractographic analysis (FTA) of a glass-infiltrated and a zirconia ceramic.

MATERIALS AND METHODS

For each material, ten bar-shaped specimens were prepared for the SEVNB method (3 mm × 4 mm × 25 mm) and the FTA method (2 mm × 4 mm × 25 mm). The starter V-notch was prepared as the fracture initiating flaw for the SEVNB method. A Vickers indentation load of 49 N was used to create a controlled surface flaw on each FTA specimen. All specimens were loaded to fracture using a universal testing machine at a crosshead speed of 0.5-1 mm/min. The independent-samples t-test was used for the statistical analysis of the K_Ic values at α=0.05.

RESULTS

The mean K_Ic of zirconia ceramic obtained from SEVNB method (5.4 ± 1.6 MPa·m^1/2) was comparable to that obtained from FTA method (6.3 ± 1.6 MPa·m^1/2). The mean K_Ic of glass-infiltrated ceramic obtained from SEVNB method (4.1 ± 0.6 MPa·m^1/2) was significantly lower than that obtained from FTA method (5.1 ± 0.7 MPa·m^1/2).

CONCLUSION

The mean K_Ic of the glass-infiltrated and zirconia ceramics obtained from the SEVNB method were lower than those obtained from FTA method even they were not significantly different for the zirconia material. The differences in the K_Ic values could be a result of the differences in the characteristics of fracture initiating flaws of these two methods.     

Mechanical degradation of contemporary CAD/CAM resin composite materials after water ageing

ObjectiveThe aim of the present study was to assess the effect of water storage on the quasi-static properties and cyclic fatigue behavior of four contemporary CAD/CAM resin composite materials.MethodsThe CAD/CAM resin composites Grandio Blocs, Lava^TM Ultimate, Cerasmart^TM and Brilliant Crios, as well as the direct resin composite Grandio SO, were evaluated. Rectangular plates were cut from the blocks or fabricated using a silicon mold to obtain specimens for fracture toughness (K_Ic, n = 10), biaxial strength (σ₀, n = 30) and cyclic fatigue testing (n = 30). Half of the specimens was stored for 24 h in dry conditions and the other half was aged for 60 days in distilled water at 37 °C. K_Ic was determined using the Compact-Tension (C(T)) method and σ₀ and cyclic fatigue were tested using the Ball-on-Three-Balls assembly. Additional disc-shaped specimens (n = 5) were produced to obtain water sorption curves of the materials. Weibull statistics and two-way ANOVA with Tukey’s post-hoc test were used for data assessment.ResultsThe highest water sorption was observed for Lava^TM Ultimate (42.6 μg/mm³), whereas Grandio SO displayed the lowest uptake (14 μg/mm³). A statistically significant drop in K_Ic and σ₀ was measured for all materials after water storage, except for the σ₀ of Cerasmart^TM. Water ageing had a dissimilar effect on the cyclic fatigue behavior, increasing the slow crack growth susceptibility of Lava^TM Ultimate, but decreasing it for Cerasmart^TM and Brilliant Crios.SignificanceContemporary CAD/CAM resin composites are susceptible to water driven degradative processes, although differences in filler content and resin matrix constitution play an important role in how it impacts their mechanical properties.     

Mechanical properties and fracture behavior of flowable fiber reinforced composite restorations

Objective

The aim was to evaluate the effect of short glass-fiber/filler particles proportion on fracture toughness (FT) and flexural strength (FS) of an experimental flowable fiber-reinforced composite (Exp-SFRC) with two methacrylate resin formulations. In addition, we wanted to investigate how the fracture-behavior of composite restorations affected by FT values of SFRC-substructure.

Strength and fracture toughness of zirconia dental ceramics

Objective

The aim of the paper is to determine and discuss the correlation between the fracture toughness and the fracture stress in zirconia transforming ceramics with a small artificial crack. As an R-curve behaviour is usually present in transforming ceramics for both small and long cracks, predictions of the fracture stress can only be done with an accurate knowledge of the R-curve and crack dimensions.

Translucency,flexural strength,fracture toughness,fracture load of 3-unit FDPs,Martens hardness parameter and grain size of 3Y-TZP materials

ObjectiveThis investigation tested pre-shaded 3Y-TZP materials on optical, mechanical and structural properties and calculated correlations between these properties.MethodsSeven A2-shaded 3Y-TZP zirconia materials were investigated on translucency (T) via UV–vis-spectrophotometer, fracture load of 3-unit FDPs (FL), biaxial flexural strength (FS), Chevron-Notch Beam (CNB), fracture toughness (K_IC) and Martens parameter (hardness: HM and indentation modulus: E_IT). FL, FS and K_IC were measured in a universal testing machine. The grain size was evaluated by scanning electron microscopy (SEM). Data was analyzed using one-way ANOVA followed by post hoc Scheffé, Kruskal–Wallis-, Mann–Whitney-U- and Pearson-test (p < 0.05).ResultsFor translucency, negative correlations were found with results of facture load (R = −0.444, p < 0.001) and K_IC (R = −0.503, p < 0.001). While a positive correlation was found between translucency and flexural strength (R = 0.238, p = 0.019), between fracture load and E_IT (R = 0.227, p < 0.029), between fracture load and K_IC (R = 0.362, p < 0.001) as well as between fracture load and the grain size (R = 0.598, p = 0.007). While the grain size positively correlated with E_IT (R = 0.534, p = 0.017) as well as E_IT with HM (R = 0.720, p < 0.001).SignificanceDespite of being based on the same raw material, tested zirconia materials significantly differed regarding optical, mechanical (except biaxial flexural strength and Martens hardness) and structural properties. Materials with highest optical properties were those with lowest mechanical properties (CER, COP).     

In vitro evaluation of fracture strength of zirconia restoration veneered with various ceramic materials

PURPOSE

Fracture of the veneering material of zirconia restorations frequently occurs in clinical situations. The purpose of this in vitro study was to compare the fracture strengths of zirconia crowns veneered with various ceramic materials by various techniques.

MATERIALS AND METHODS

A 1.2 mm, 360° chamfer preparation and occlusal reduction of 2 mm were performed on a first mandibular molar, and 45 model dies were fabricated in a titanium alloy by CAD/CAM system. Forty-five zirconia copings were fabricated and divided into three groups. In the first group (LT) zirconia copings were veneered with feldspathic porcelain by the layering technique. In the second group (HT) the glass ceramic was heat-pressed on the zirconia coping, and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented onto their titanium dies with Rely X™ Unicem (3M ESPE) and loaded with a universal testing machine (Instron 5583) until failure. The mean fracture values were compared by an one-way ANOVA and a multiple comparison post-hoc test (α=0.05). Scanning electron microscope was used to investigate the fractured interface.

RESULTS

Mean fracture load and standard deviation was 4263.8±1110.8 N for Group LT, 5070.8±1016.4 for Group HT and 6242.0±1759.5 N for Group ST. The values of Group ST were significantly higher than those of the other groups.

CONCLUSION

Zirconia crowns veneered with CAD/CAM generated glass ceramics by the sintering technique are superior to those veneered with feldspathic porcelain by the layering technique or veneered with glass ceramics by the heat-pressing technique in terms of fracture strength.