Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio.

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m1/2) initiated from cone cracks developed during cyclic loading for 103 to 105 cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (Kc = 2.1 MPa m1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 103 to 105 cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 105 cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.

Fracture toughness and fractal analysis of ceramic benchmark materials.

Previous studies have reported various methods of measuring the fracture toughness of brittle ceramics. The purpose of the present research was to use a new method of fractal dimension measurement on benchmark materials (silica glass, Viosil SX, Shin-Etsu, n = 13, and silicon nitride standard reference material, SRM2100, NIST, n = 10), to compare the fracture toughness calculated using different methods, and to study the effect of noise filtering on the fractal dimension and fracture surface roughness. Fracture toughness was determined using surface crack in flexure method according to ASTM C1421 and fractal analysis method. Fractal dimension was determined using the Minkowski Cover algorithm on atomic force microscope scans of epoxy replicas of fracture surfaces. The mean ± standard deviation of fracture toughness using surface crack in flexure method and fractals method were 0.97 ± 0.18 MPa·m1/2 and 1.03 ± 0.07 MPa·m1/2 for silica glass and 4.62 ± 0.14 MPa·m1/2 and 2.54 ± 0.07 MPa·m1/2 for silicon nitride, respectively. The mean ± standard deviation of fractal dimension was 2.17 ± 0.03 for silica glass and 2.13 ± 0.01 for silicon nitride. The mean ten-point roughness (Rz) before and after noise filtering was 89 ± 102 nm and 87 ± 101 nm for silica glass and 355 ± 132 nm and 357 ± 134 nm for silicon nitride, respectively. Noise filtering had no significance on the fracture surface roughness of the two materials. The newly developed fractal analysis method can be used to predict the baseline fracture toughness of specimens with unknown failure stress.

Fractographic analysis of separated endodontic file designs.

Endodontic rotary files are cutting instruments used to perform root canal procedures within a tooth interior. Focusing on quantitative fractographic analysis increases necessary, clinical performance understanding of file separation failure. This research employed controlled, dynamic testing to failure of commercial rotary files, analyzing the fractographic, forensic characteristics in relation to Weibull reliability determination, considering: (1) design analysis; (2) stress concentrations; (3) times to failure; (4) number of cycles to failure (NCF). Ex vivo testing included three file designs, each having constant tip size (0.035 mm), taper (0.06 mm/mm), and length (25 mm). Files were individually tested using an electric, torque-controlled handpiece, rotating within a standardized, simulated canal until fracture separation occurred. Fractographic analysis, including critical measurements, was conducted using the scanning electron microscope (SEM) (PhenomProX, PhenomWorld, NL). Weibull statistical analysis established reliability factors per design group. Fractographic analysis identified separation fractures, processing inclusions, flexural-fatigue striations, and stress concentrations at flute pitches. Calculated NCF median values (1277-EE; 899-VB; 713-PI) demonstrated significant statistical differences among groups (p < 0.001). Separated apical fragments yielded statistically significant differences (p ≤ 0.05) for varying file design groups. Weibull moduli among groups were statistically equivalent. Fractographic analysis exposed a presence of multiple failure factors in addition to defect distribution, governing cyclic fatigue failure originating at stress concentration points irrespective of file design. Fractographic analysis indicated that a change in file design, specifically at the working edges, in addition to improved surface finish, has the potential of reducing failures by lowering points of stress concentration and reducing fracture initiating surface cracks.

Factors influencing the survival of implant-supported ceramic-ceramic prostheses: A randomized, controlled clinical trial.

OBJECTIVE: The goals of this research are: (1) to determine the clinical survival of ceramic-ceramic 3-unit implant supported fixed dental prostheses (FDPs) compared with control metal-ceramic and; (2) to analyze the effects of design parameters such as connector height, radius of curvature of gingival embrasure, and occlusal veneer thickness. MATERIALS AND METHODS: This randomized, controlled clinical trial enrolled 96 participants with 129 3-unit implant-supported FDPs. Participants were randomized to receive different design combinations to include FDP material, thickness of occlusal veneer ceramic, radius of curvature of gingival embrasure and connector height. Participants were recalled for 6 months, 1year and yearly thereafter for the next 5 years. FDPs were examined for evidence of fracture and radiographs were made to assess viability of implants. Fractographic analyses and Kaplan Meier survival analysis was used to analyze the data. RESULTS: 27 FDPs, representing 21%, exhibited chipping fractures of the veneer during the 5-year observation period. There was no statistically significant effect of type of material, veneer thickness, radius of curvature of gingival embrasure and connector height on occurrence of fracture. Fractographic and occlusal analyses reveal that fractures originated from the occlusal surface and that occlusion was the most important factor in determining survival. Stresses calculated at failure demonstrated lower values compared with in vitro data. CONCLUSION: Implant-supported ceramic-ceramic prosthesis is a viable alternative to metal-ceramic. Survival analysis for both materials were comparable and design parameters employed in this study did not affect survival as long as zirconia was used as the core material.

Fractography and fracture toughness of human dentin.

Dentin, the mineralized tissue forming the bulk of the tooth, serves as an energy-absorbing cushion for the hard, wear-resistant enamel and protects the inner soft tissues. Several studies used fracture mechanics methods to study the fracture toughness of dentin. However, all of them utilized precracks and cannot be used to estimate the intrinsic critical flaw size of dentin. We applied quantitative fractography to study the fracture pattern and fracture toughness of human dentin. Sixteen specimens were prepared from the coronal dentin and fractured in three-point flexure. Fracture surfaces were examined using a scanning electron microscope and the fracture toughness was calculated using a fracture mechanics equation. It was found that human dentin has a fracture surface similar to those of brittle materials. Twist hackle markings were observed and were used to identify the fracture origins. Average fracture toughness of all specimens was found to be 2.3 MPa m(1/2) and the average critical flaw size was estimated to 120 mum. It is suggested that fractography is a promising technique in analyzing the fracture of dentin under catastrophic failure.

Apparent interfacial fracture toughness of resin/ceramic systems.

We suggest that the apparent interfacial fracture toughness (K(A)) may be estimated by fracture mechanics and fractography. This study tested the hypothesis that the K(A) of the adhesion zone of resin/ceramic systems is affected by the ceramic microstructure. Lithia disilicate-based (Empress2-E2) and leucite-based (Empress-E1) ceramics were surface-treated with hydrofluoric acid (HF) and/or silane (S), followed by an adhesive resin. Microtensile test specimens (n = 30; area of 1 +/- 0.01 mm(2)) were indented (9.8 N) at the interface and loaded to failure in tension. We used tensile strength (sigma) and the critical crack size (c) to calculate K(A) (K(A) = Ysigmac(1/2)) (Y = 1.65). ANOVA and Weibull analyses were used for statistical analyses. Mean K(A) (MPa.m(1/2)) values were: (E1HF) 0.26 +/- 0.06; (E1S) 0.23 +/- 0.06; (E1HFS) 0.30 +/- 0.06; (E2HF) 0.31 +/- 0.06; (E2S) 0.13 +/- 0.05; and (E2HFS) 0.41 +/- 0.07. All fractures originated from indentation sites. Estimation of interfacial toughness was feasible by fracture mechanics and fractography. The K(A) for the systems tested was affected by the ceramic microstructure and surface treatment.

Fracture surface analysis of clinically failed fixed partial dentures.

Ceramic systems have limited long-term fracture resistance, especially when they are used in posterior areas or for fixed partial dentures. The objective of this study was to determine the site of crack initiation and the causes of fracture of clinically failed ceramic fixed partial dentures. Six Empress 2 lithia-disilicate (Li(2)O x 2SiO(2))-based veneered bridges and 7 experimental lithia-disilicate-based non-veneered ceramic bridges were retrieved and analyzed. Fractography and fracture mechanics methods were used to estimate the stresses at failure in 6 bridges (50%) whose fracture initiated from the occlusal surface of the connectors. Fracture of 1 non-veneered bridge (8%) initiated within the gingival surface of the connector. Three veneered bridges fractured within the veneer layers. Failure stresses of the all-core fixed partial dentures ranged from 107 to 161 MPa. Failure stresses of the veneered fixed partial dentures ranged from 19 to 68 MPa. We conclude that fracture initiation sites are controlled primarily by contact damage.

Sintering temperature effects on the in vitro bioactive response of tape cast and sintered bioactive glass-ceramic in Tris buffer.

Tape casting procedures were used to form thin polymeric sheets (100 microm thickness) loaded with bioactive glass particulate. Blanks were punched from the sheets, stacked, laminated, and heated in air to 500 degrees C to remove the organic phase. The resulting bioactive glass discs were sintered at 800 degrees C, 900 degrees C, or 1000 degrees C. Because the material is built up in layers and can be machined in the green state, such a processing technique can be used to form complex-shaped materials. The in vitro bioactivity of the tape cast sintered (TCS) bioactive glass-ceramic discs was then assessed in Tris buffer. The sample surface area to volume buffer (SA/V) ratio was approximately 0.1 cm(2)/mL. Tape cast bioactive glass-ceramic sintered at 900 degrees C and 1000 degrees C formed crystalline hydroxyapatite layers after 24 h in Tris buffer as indicated by FTIR, SEM, and EDS analysis. Decreasing the SA/V ratio to 0.013 cm(2)/mL allowed for the formation of crystalline hydroxyapatite layers on the surface of 800C TCS bioactive glass-ceramic. Given the dependence of the bioactive response as a function of the processing schedule and SA/V ratio, it may be possible to tailor the response to that desired in vivo or in vitro for tissue engineering studies. Biaxial flexural strength of TCS bioactive glass-ceramic increased with increasing sintering temperature. Strength of samples sintered at 1000 degrees C for 3 h increased from 87 to 120 MPa after 2 weeks' immersion in Tris buffer.

Fracture mechanics principles.

The principles of linear elastic fracture mechanics (LEFM) were developed in the 1950s by George Irwin (1957). This work was based on previous investigations of Griffith (1920) and Orowan (1944). Irwin (1957) demonstrated that a crack shape in a particular location with respect to the loading geometry had a stress intensity associated with it. He also demonstrated the equivalence between the stress intensity concept and the familiar Griffith criterion of failure. More importantly, he described the systematic and controlled evaluation of the toughness of a material. Toughness is defined as the resistance of a material to rapid crack propagation and can be characterized by one parameter, Kic. In contrast, the strength of a material is dependent on the size of the initiating crack present in that particular sample or component. The fracture toughness of a material is generally independent of the size of the initiating crack. The strength of any product is limited by the size of the cracks or defects during processing, production and handling. Thus, the application of fracture mechanics principles to dental biomaterials is invaluable in new material development, production control and failure analysis. This paper describes the most useful equations of fracture mechanics to be used in the failure analysis of dental biomaterials.

Fractography: determining the sites of fracture initiation.

Fractography is the analysis of fracture surfaces. Here, it refers to quantitative fracture surface analysis (FSA) in the context of applying the principles of fracture mechanics to the topography observed on the fracture surface of brittle materials. The application of FSA is based on the principle that encoded on the fracture surface of brittle materials is the entire history of the fracture process. It is our task to develop the skills and knowledge to decode this information. There are several motivating factors for applying our knowledge of FSA. The first and foremost is that there is specific, quantitative information to be obtained from the fracture surface. This information includes the identification of the size and location of the fracture initiating crack or defect, the stress state at failure, the existence, or not, of local or global residual stress, the existence, or not, of stress corrosion and a knowledge of local processing anomalies which affect the fracture process. The second motivating factor is that the information is free. Once a material is tested to failure, the encoded information becomes available. If we decide to observe the features produced during fracture then we are rewarded with much information. If we decide to ignore the fracture surface, then we are left to guess and/or reason as to the cause of the failure without the benefit of all of the possible information available. This paper addresses the application of quantitative fracture surface analysis to basic research, material and product development, and "trouble-shooting" of in-service failures. First, the basic principles involved will be presented. Next, the methodology necessary to apply the principles will be presented. Finally, a summary of the presentation will be made showing the applicability to design and reliability.