Fracture Load Values Research Articles

Effect of the Ni coating thickness on laser welding-brazing of Mg/steel

Dissimilar AZ31B-Mg and Ni-coated Q235A-steel alloys were joined in a lap configuration using a laser welding-brazing process. The effects of the Ni coating thickness on the joint appearance, interfacial reactions and mechanical properties were investigated with the assistance of thermodynamic calculations. In the direct irradiation region, Ni atoms diffused and were accumulated near the interface due to the driving force of the chemical potential. Al atoms from the filler were attracted by the concentrated Ni atoms, which resulted in the formation of the FeAl phase. In the middle region and weld toe region, a Fe(Ni) solid solution formed at the interface. Heterogeneous reaction products were produced at the seam near the interface, varying from only the AlNi phase to the AlNi+(α-Mg+Mg2Ni) eutectic with the increase of the coating thickness. Additionally, a bulky Mg2Ni phase also formed in the matrix of Mg-Ni eutectic with a coating thickness of 40 μm. Strong mutual attraction occurred between the Al and Ni atoms at the seam, resulting in a higher driving force for generating the AlNi phase than for generating the Mg-Ni phase. All joints were finally fractured at the seam. The maximum value of the tensile-shear fracture load approached 230 N/mm with a Ni coating thickness of 20 μm, reaching an 88.5% joint efficiency relative to the Mg base metal.

Critical considerations on load-to-failure test for monolithic zirconia molar crowns

Application of monolithic zirconia crowns (MZCs) with reduced thickness to the molar region has been proposed, but potential complications have yet to be fully evaluated in laboratory tests. The present study aimed to develop a clinically relevant load-to-failure test in combination with fatigue treatments involving thermal and mechanical cycling (TC and MC) to evaluate the fracture resistance of molar MZCs. MZCs with a minimal thickness of 0.5 mm were bonded to dies made of resin-based composite (RBC), epoxy resin (EP), or polyoxymethylene-copolymer (POM-C). The samples were either untreated (UT) or subjected to TC (5–55 °C for 1 × 105 cycles) and MC (300 N for 2.4 × 106 cycles). The stress generated by TC and MC was simulated by finite element modeling. The load-to-failure test was performed using an inverse V-shaped two-plane indenter and was followed by fractographic analysis. The median values of fracture load for MZC/RBC and MZC/EP in the TC group were significantly lower than those in the UT group. MC also decreased the median value of fracture load for MZC/RBC significantly, but not that for MZC/EP and MZC/POM-C. Fractography revealed that the fracture started in the cervical area in all groups, which is similar to clinically failed crowns. The simulation confirmed stress concentration at the cervical area in both TC and MC groups. The present study suggests that the load-to-failure test using a two-plane indenter could induce clinically relevant fracture of MZCs, the vulnerability of the MZCs depends largely on the die material employed, and MZCs are more likely to be damaged by thermal fatigue than mechanical fatigue.

How does the piston material affect the in vitro mechanical behavior of dental ceramics?

Statement of problemVariables involving the indication, manufacturing, and clinical use of ceramic restorations make the standardization of in vitro studies a challenge and raise questions as to the clinical validity of the resulting data. PurposeThe purpose of this in vitro study was to assess the effect of piston material on the fracture behavior of ceramics tested under compressive load. Material and methodsTwo ceramics were evaluated: a lithium disilicate–based glass-ceramic (D) and a feldspathic porcelain (P). Plate-shaped ceramic specimens (1.5-mm thick) were adhesively cemented onto a dentin analog substrate. The specimens from each ceramic were divided into 4 groups according to the piston material (n=20): metal (M) (stainless steel), composite resin (R) (NEMA-G10, fiber-reinforced epoxy resin), ceramic (C) (lithium disilicate–based glass-ceramic), and human tooth (T) (canine). A gradual compressive load (0.5 mm/min) was applied to the center of the specimen with a universal testing machine. The test was performed in 37°C distilled water, and the initial crack was detected by using an acoustic system. The fracture load values (N) were statistically analyzed with the Kruskal-Wallis and Dunn tests (α=.05). A finite element analysis (FEA) was also performed. ResultsPiston material had no influence on fracture load and failure mode of ceramic D. Ceramic P showed higher fracture load values when loaded with the composite resin piston. Ceramic P showed more combined failures (cone crack and radial crack) than D. The FEA showed a distinct stress distribution for R piston on P. Pistons C and T resulted in similar stress distribution, fracture load, and failure mode for both ceramics. ConclusionsThe effect of piston material on the ceramic fracture behavior depends on the ceramic being evaluated.

THE EFFECT OF THERMOCYCLING AND MECHANICAL LOADING ON THE FRACTURE RESISTANCE OF ALL-CERAMIC AND HIGH PERFORMANCE POLYMERS FIXED PARTIAL DENTURES

Objectives: The purpose of this study was to evaluate the fracture resistance of the heat pressed ceramic (IPS e-max press) and the high performance polymers BioHPP anterior FPDs.Materials and Methods A total number of 96 sound freshly extracted maxillary central incisors and canines were used in this study. The teeth were used to prepare a total of 48 three units FPD tested specimens. Specimens were used to test the fracture resistance of the materials under investigation without and after thermocycling and mechanical loading.Results : The fracture load values of the IPS e-max specimens without TCML were ranged from 271.7 N to 206.01 N. While, after TCML the values were ranged from 171.6 N to 191.2 N. The fracture load values of the BioHPP specimens without TCML were ranged from 789.7 N to 665.1 N. While, after TCML the values were between 789.7 N and 615.4 N.Conclusion: IPS e-max press FPDs had a lower fracture resistance than the BioHPP FPDs before and after TCML. The TCML decreased the resistance to fracture of both materials.

Assessment of marginal adaptation and fracture resistance of endocrown restorations utilizing different machinable blocks subjected to thermomechanical aging

This in vitro study was conducted to assess the marginal adaptation and fracture resistance of computer aided design/computer aided manufacturer (CAD-CAM) fabricated endocrowns restoring endodontically treated molars using different machinable blocks with thermomechanical loading protocols. Devitalized mandibular molars were prepared in a standardized way and divided into 4 groups (n = 10) to receive CAD/CAM fabricated endocrowns using four materials (Lithium disilicate ceramics, polymer infiltrated ceramics, zirconia-reinforced lithium silicate ceramics and resin nanoceramics. Marginal gaps (µm) were measured using stereomicroscope before cementation and after cementation. After thermomechanical aging, marginal gap measurements were repeated, and then fracture resistance test was performed. Two-way analysis of variance (ANOVA) and Tukey HSD multiple comparisons were used to assess the effect of material on the marginal gap before, after cementation, and after thermomechanical aging. One Way ANOVA was used to assess the effect of material on the fracture resistance. The difference between marginal gaps values of the tested materials was statistically insignificant but with significant increase after cementation and after thermomechanical aging. Cerasmart endocrowns showed the highest mean fracture load value (1508.5 ± 421.7N) with statistically significant difference than Vita Enamic endocrowns and Celtra Duo. The tested materials showed marginal vertical gap readings within the limits of clinically acceptable standards. Resin nanoceramics and lithium disilicate showed the highest values of fracture resistance followed by polymer infiltrated ceramics favoring their use for endocrown restorations. The mechanical behavior of ceramic materials varies with the variation of their structure and mechanical properties. Accordingly, further investigation is always needed to explore the biomechanical behavior of recent materials when used as endocrowns before clinical trials.

Experimental analysis of metal/plastic composites made by a new hybrid method

The purpose of this paper is to identify the key elements of a new hybrid process to produce high quality metal/plastic composites. The process is a combination of Fused Deposition Modelling (FDM), vacuum forming and CNC machining. The research aims to provide details of the proposed hybrid process, equipment used, and the experimental results of the composites produced. The research has been separated into three study areas. In the first, the hybrid process has been defined as a whole whereas the second area deals with the breakdown of steps to produce the metal/plastic composites. The third area explains the varied materials used for the production and testing of the composites. Composites have been made by joining copper (99.99% pure) mesh with ABS (acrylonitrile butadiene styrene). Strain measurement has been carried out on Cu/ABS sample to analyse the effect of metal mesh and to verify the effectiveness of the hybrid process. The resulting composites (Cu/ABS) have also been subjected to tensile loading with different layers of metal mesh, followed by microstructural analysis and comparative studies to serve as a proof of the methodology. The results show that the proposed hybrid process is very effective in producing metal/plastic composites with lower strain values compared to the parent plastic indicating a lower level of deformation due to interlocking of the metal and plastic layers. This effect has been reinforced by the tensile testing where the composites showed higher fracture load values compared to the parent plastic. Microstructural analysis shows the layer of metal mesh sandwiched between ABS layers indicating the existence of a bond holding the layers of metal and plastic together. These results demonstrate the capabilities and effectiveness of the proposed process that has shown promising results under tensile and static loading.

Fracture load of metal-ceramic, monolithic, and bi-layered zirconia-based posterior fixed dental prostheses after thermo-mechanical cycling

ObjectivesThis study aims to evaluate the fracture load of differently fabricated 3-unit posterior fixed dental prostheses (FDPs) with an intermediate pontic. MethodsFifty sets of two stainless-steel abutments were randomly assigned to five groups (n = 10 each) depending on the material and technique used for manufacturing the FDPs: (1) Metal-ceramic (MC, control); (2) Lava Zirconia (LZ, bi-layered); (3) Lava Plus (LM, monolithic); (4) VITA In-Ceram YZ (YZ, bi-layered); and (5) IPS e-max ZirCAD (ZZ, bi-layered). After being luted to the dies, all FDPs were submitted to thermo-mechanical cycling (120,000 masticatory cycles, 50 N; plus 774 thermal cycles of 5 °C/55 °C, dwell time: 30 s). Samples were then subjected to a three-point bending test until fracture in a universal testing machine (cross-head speed: 1 mm/min). Fracture load of the veneering ceramic (VF) and total fracture load (TF) were recorded. Microstructure and failure patterns were assessed. Data was analysed using one-way ANOVA and Tukey HSD post-hoc tests (α = 0.05). ResultsMC restorations recorded higher VF and TF values than did zirconia FDPs (p = 0.0001), which showed no between-group differences. Within the bi-layered groups, TF was significantly higher than VF. LM pieces registered lower average grain size than did LZ specimens (p = 0.001). Overall, the connector was the weakest part. ConclusionsAll of the groups tested could withstand clinical chewing forces in terms of average fracture load. Zirconia-based samples performed similarly to each other, but showed lower mean fracture load values than did metal-ceramic ones. Clinical significanceMonolithic zirconia may be recommendable for solving the chipping problem.

Fracture Resistance of CAD/CAM-Fabricated Lithium Disilicate MOD Inlays and Onlays with Various Cavity Preparation Designs.

To examine the fracture resistance of premolars restored with CAD/CAM lithium disilicate mesio-occlusal-distal (MOD) inlays and onlays of different cavity designs. Two widths of occlusal isthmus (75%, 100% of intercuspal distance) and three designs of cuspal coverage (none, palatal, complete) were used for the preparation of MOD inlays and onlays in the extracted maxillary premolars. Sixty lithium disilicate restorations were milled and bonded into the cavities. After 24 hours of water storage, the specimens were loaded until fracture, and the fracture loads (N) were measured. Any evidence of cracks and fractures on the tested specimens were examined to classify failure patterns. Mean fracture load values for the tested groups were as follows: 664.4 ± 214.7 N (group A), 659.3 ± 391.2 N (B), 681.9 ± 258.1 N (C), 938.1 ± 862.0 N (D), 841.7 ± 375.4 N (E), and 994.2 ± 486.3 N (F). The width of occlusal isthmus did not significantly affect the fracture loads among all the groups. Within groups with identical isthmus width, the fracture loads showed no significant difference depending on the designs of cuspal coverage. The majority of specimens showed either type III or IV fracture mode. Within limitations of this study, the bonded restorations of premolars with CAD/CAM-generated lithium disilicate were reliable, regardless of cavity preparation design.

Fracture Toughness Evaluation of S355 Steel Using Circumferentially Notched Round Bars

In engineering applications, steels are commonly used in various areas. The mechanical members are exposed to different loading conditions and this subject can be investigated in fracture mechanics. Fracture toughness (KIC) is the important material property for fracture mechanics. Determination of this properties is possible using a compact tension specimen, a single edge notched bend or three-point loaded bend specimen, which are standardized by different institutions. Researchers underline that these standardized methods are complex, the manufacturing process is difficult, they require special fixtures for loading during the experiment and the test procedures are time consuming. Alternative methods are always being sought by researchers. In this work, two different approaches are investigated for S355 steels. In the first method, a circumferentially cracked round bar was loaded in tensile mode and pulled till failure. Using suitable equations, fracture toughness can be calculated. In the second method, a circumferentially notched bar specimen without fatigue pre-cracking was loaded in a tensile machine. By means of fracture load values, fracture toughness was determined by the proposed equations. It can be stated that these two different approaches for calculating fracture toughness are simple, fast and economical.

Lower Leg Morphology and Stretch-Shortening Cycle Performance of Dancers.

Greater levels of bone ultimate fracture load, bone stress-strain index, muscle cross-sectional area, and maximal voluntary isometric plantarflexion (MVIP) strength of the lower leg may be adaptations from chronic exposure to stretch-shortening cycle (SSC) actions. Dancers, a population that habitually performs SSC movements primarily about the ankle joint, may serve as a novel population to gain broader understanding of SSC function. A total of 10 female collegiate dancers and 10 untrained controls underwent peripheral quantitative computed tomography scans of both lower legs and performed MVIPs, countermovement hops, and drop hops at 20, 30, and 40cm on a custom-made inclined sled. Dancers had greater right and left ultimate fracture load values and significantly (P ≤ .05) greater left leg stress-strain index than controls. Dancers had significantly larger right and left muscle cross-sectional area and MVIP values and hopped significantly higher during all hopping conditions in comparison with controls. Average force-time and power-time curves revealed significantly greater relative force and power measurements during the concentric phase for all hopping conditions in dancers when compared with controls. This investigation provides evidence that dance may be a stimulus for positive muscle and bone adaptations, strength levels, and enhanced SSC capabilities.

Direct or Indirect Restoration of Endodontically Treated Maxillary Central Incisors with Class III Defects? Composite vs Veneer or Crown Restoration.

The aim of this ex-vivo study was to evaluate the load capacity of direct or indirect endodontically restored maxillary central incisors with Class III defects, with or without glass-fiber posts. Seventy-two extracted human maxillary central incisors were endodontically treated and bi-proximal Class III cavities were prepared. Specimens were randomly allocated to six groups (n = 12): direct restoration with composite (C); direct restoration with composite and additional glass-fiber post (CP); ceramic veneer restoration (V), ceramic veneer restoration and additional glass-fiber post (VP), ceramic crown restoration (Cr), ceramic crown restoration and additional glass-fiber post (CrP). Specimens were exposed to thermomechanical loading (TML: 1.2 million cycles, 1 to 50 N; 6000 thermal cycles between 5°C and 55°C for 1 min each), and subsequently linearly loaded until failure (Fmax [N]) at an angle of 135 degrees 3 mm below the incisal edge on the palatal side. Statistical tests were performed using the Kruskall-Wallis and Mann-Whitney U-Test. During dynamic loading by TML, one early failure occurred in group C, CP, and CrP. Subsequent linear loading resulted in mean fracture load values [N] of C = 483 ± 219, CP = 536 ± 281, V = 908 ± 293, VP = 775 ± 333, Cr = 549 ± 258, CrP = 593 ± 259. The Kruskal-Wallis test showed significant differences of load capacity between groups (p < 0.05). Mann-Whitney U-test revealed significantly lower maximum fracture load values of group C compared to group V (p = 0.014), after Bonferroni-Holm correction. Non-restorable root fracture was the most frequent type of failure. Endodontically treated maxillary central incisors with Class III defects directly restored with composite are as loadable as indirect crown restorations. Compared to full-coverage restorations, less invasive veneers appear to be more beneficial. Additional placement of glass-fiber posts shows no positive effect.

Fracture resistance and failure modes of polymer infiltrated ceramic endocrown restorations with variations in margin design and occlusal thickness

PurposeThe purpose of this in vitro study was to assess the effect of varying the margin designs and the occlusal thicknesses on the fracture resistance and mode of failures of endodontically treated teeth restored with polymer infiltrated ceramic endocrown restorations. MethodsRoot canal treated mandibular molars were divided into four groups (n=8) and were prepared to receive Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) fabricated polymer infiltrated ceramic endocrowns (ENAMIC blocks). Group B2 represents teeth prepared with a butt joint design receiving endocrowns with 2mm occlusal thickness and the same for group B3.5 but with 3.5mm occlusal thickness. Group S2 represents teeth prepared with 1mm shoulder finish line receiving endocrowns with 2mm occlusal thickness and the same for group S3.5 but with 3.5mm occlusal thickness. After cementation and thermal aging, fracture resistance test was performed and failure modes were observed. ResultsGroup S3.5 showed the highest mean fracture load value (1.27±0.31kN). Endocrowns with shoulder finish line had significantly higher mean fracture resistance values than endocrowns with butt margin (p<0.05). However, the results were not statistically significant regarding the restoration thickness. Evaluation of the fracture modes revealed no statistically significant difference between the modes of failure of tested groups. ConclusionsFor the restoration of endodontically treated teeth, adding a short axial wall and shoulder finish line can increase the fracture resistance. However, further investigations, especially the fatigue behavior, are needed to ensure this effect applies with small increases of restoration thickness.

Correlations between fracture load of zirconia implant supported single crowns and mechanical properties of restorative material and cement.

Zirconia implants that were restored with veneered zirconia displayed severe chipping rates of the restorations in clinical studies. Purpose of this study was to evaluate the fracture load of different zirconia implant supported monolithic crown materials (zirconia, alumina, lithium disilicate, feldspar ceramic and polymer-infiltrated ceramic) cemented with various cements (Harvard LuteCem SE, Harvard Implant Semi-permanent, Multilink Automix, VITA Adiva F-Cem). Flexural strength and fracture toughness of crown materials and compressive strength of the cements were measured. Fracture load values of crowns fabricated from lithium disilicate, feldspar ceramic and polymer-infiltrated ceramic were increased when cement with high compressive strength was used. Fracture loads for zirconia and alumina crowns were not influenced by the cement. Flexural strength and fracture toughness of the ceramics correlated linearly with the respective fracture load when using adhesive cement with high compressive strength. To achieve sufficient fracture load values, cementation with adhesive cement is essential for feldspar and polymer-infiltrated ceramic.

Experimental and theoretical investigation of mixed mode I/III brittle fracture of U-notched polystyrene components

Brittle fracture of components made of the general-purpose polystyrene and weakened by an edge U-notch under combined tension/out-of-plane shear loading conditions (mixed mode I/III) has not been studied yet experimentally or theoretically. In this research, a recently developed loading fixture is employed for experimentally investigating the fracture of U-notched general-purpose polystyrene samples with various notch tip radii of 0.5, 1, 2 and 4 mm when they are subjected to different combinations of tension/out-of-plane shear. The samples are fabricated with four different notch tip radii with the purpose of assessing the influence of this geometrical parameter. The experimental values of fracture load and out-of-plane fracture angle are theoretically predicted by the two stress-based criteria of point stress and mean stress lately extended to general loading case of mixed mode I/II/III. It is shown that both the point stress and mean stress criteria provide acceptable predictions to fracture behavior of U-notched general-purpose polystyrene specimens. The critical distances needed for the point stress and mean stress criteria are determined based on the experimental results of the U-notched samples tested under pure mode I loading. No meaningful difference is found between the fracture loads and fracture initiation angles predicted by the point stress and mean stress criteria. It is also observed that as the mode III contribution in the applied mixed mode I/III loading increases, a larger total external load is needed for the fracture of U-notched general-purpose polystyrene specimens to occur.

Evaluating mode I fracture resistance in asphalt mixtures using edge notched disc bend ENDB specimen with different geometrical and environmental conditions

Till now, several test specimens have been used for obtaining the value of fracture toughness (KIc) in asphalt concretes. The majority of proposed test configurations in this regard are in the shape of cylindrical, disc or semi-disc specimens. However, despite many engineering materials like metals, polymers, ceramics and rocks, there is no popular test procedure and test specimen for determining a valid and representative KIc value for the asphalt concretes. Recently, a new disc shape specimen, called “edge notched disc bend – ENDB” specimen has been suggested by the authors and coworkers that can be successfully used for conducting pure mode I fracture toughness experiments in hot mix asphalt mixtures. The practical ability of this specimen has already been demonstrated experimentally via conducting very limited fracture toughness tests on a typical HMA mixture. In this paper, to extend the application of ENDB specimen for wider geometries and environmental conditions, a large number of mode I fracture toughness tests are conducted on HMA mixtures containing different air void percentages and at three low temperature test conditions (0, −12 and −24 °C). Furthermore, the effect of ENDB size (i.e. the thickness of disc) is investigated experimentally on the fracture load and fracture toughness values. Based on these new and comprehensive test results, it was concluded that the mode I fracture toughness of HMA mixtures becomes more by decreasing the air void content and test temperature or increasing the size of ENDB specimen. Meanwhile, the scatter of results was studied thorough a regression analysis and a linear regression model was extracted for predicting the KIc value in terms of void content, ENDB size and temperature. Based on this model, the size of specimen had the highest effect on the value of fracture toughness. Consequently, due to some primary advantages, the ENDB specimen can be used as a suitable candidate specimen for measuring the fracture toughness in asphalt mixtures.

EFFECT OF FILLING PULP CHAMBER ON MARGINAL ADAPTATION AND FRACTURE RESISTANCE OF ZIRCONIA REINFORCED GLASS CERAMIC ENDOCROWN RESTORATIONS

Purpose: The purpose of this in vitro study was to evaluate the marginal adaptation and the fracture resistance of endodontically treated molars restored with zirconia reinforced glass ceramic endocrown restoration with or without incorporating a fiber-reinforced composite (FRC) layer into the pulp chamber.Materials and methods: Mandibular first molars (n= 30) were prepared by computerized numerical control (CNC) in a standardized way to receive standardized CAD/CAM fabricated zirconia reinforced glass ceramic endocrowns after root canal treatments. The selected teeth were divided into two groups (n=15) according to the presence of fiber reinforced composite (FRC) in the pulp chamber. Group NF (Non filled) represented teeth with the pulp chamber without filling and Group F (Filled)represented teeth with the pulp chamber filled with FRCs. Marginal gaps (µm) were measured using stereomicroscope (32x) before cementation and after cementation. Thermal aging (3000 cycles) was performed then marginal gaps measurements were repeated. Then, fracture resistance test was performed. ANOVA test was used to study the interactions while Independent t-test was used to study the effect of using FRCs in the pulp on marginal adaptation in each stage and on the fracture resistance (N).Results: The results of independent t test showed that there was no statistically significant effect of incorporating FRCs into the pulp chamber on the marginal adaptation during all stages before (p= 0.844) or after cementation (p=0.884) or after thermocyclying (p=0.875). Regarding the fracture resistance, the test showed that Group NF had higher mean fracture load value than Group F and the difference was statistically significant (p=0.004)Conclusions: Within the limitation of this study, the incorporation of FRCs into the pulp chamber of endodontically treated teeth restored with zirconia reinforced glass ceramic endocrowns had no influential effect on the marginal adaptation of the restoration before or after cementation or after thermocycling. However, the presence of the FRCs unexpectedly decreased the mean values of the fracture resistance of the restorations. Further investigations are needed to confirm these findings before clinical application.

Fracture Resistance and Retention of Metal-Free Inlay Retained Fixed Partial Dentures

Statement of the problem: Conservative esthetic restorative dentistry has become an essential component in modern clinical practice. However, the inlay retained fixed partial dentures (IRFPDs) have a higher risk of fracture and dislodgement due to minimal preparations. Purpose: to compare fracture resistance and retention of Lithium disilicate (IPS e.max) and Polyetheretherketone (BioHPP) IRFPDs fabricated with heat-press technique and CAD/CAM system. Materials and Methods: Forty models, each including extracted mandibular 2nd premolar and 2nd molar embedded in an epoxyresin block with 10 mm intra-abutment distance representing missing mandibular 1st molar were constructed. The premolar and molar of each model received standardized class II inlay preparations. The constructed models were randomly divided into four groups; Group (1): IPS e.max Press IRFPDs, Group (2): IPS e.max CAD IRFPDs, Group (3): BioHPP Granulates IRFPDs, and Group (4): breCAM.BioHPP IRFPDs. All IRFPDs were cemented using RelyX Ultimate adhesive resin cement. Samples were stored for 24 hours in distilled water at 37°C, and then thermocycled for 5000cycles. Universal testing machine was used to measure fracture and dislodging loads. Samples were examined for determination of failure mode using a magnifying lens. Results: The statistically significant highest mean dislodging load value was obtained in IPS e.max Press IRFPDs, followed by IPS e.max CAD, with the least value recorded in breCAM.BioHPP. In contrast, breCAM.BioHPP IRFPDs revealed the statistically significant highest mean fracture load value, followed by BioHPP Granulates, while the least mean fracture value was recorded in IPS e.max Press. Most failure occurred in IPS e.max groups was connector fracture, while in BioHPP groups was plastic deformation. Conclusions: Retention and fracture resistance of metal-free inlayretained bridges are significantly affected by both; material type and construction method. BioHPP has significant advantages for dental applications because of higher fracture resistance and better stress distribution. The bond strength at IPS e.max/ resin cement interface is considered a reliable bond presenting better retention to IRFPDs. Being a brittle material, connector fracture is the predominant failure mode of IPS e.max IRFPDs; however, BioHPP has low elastic modulus showing plastic deformation as a predominant failure mode.

FRACTURE RESISTANCE, FINITE ELEMENT ANALYSIS AND WEIBULL RISK OF FAILURE FOR ENDODONTICALLY TREATED MOLARS RESTORED WITH LITHIUM DISILICATE AND HYBRID CERAMIC ENDOCROWNS WITH TWO PREPARATION DESIGNS

Objective: The present study aimed at evaluating the fracture resistance of endodontically treated molars restored using IPS e.max CAD (e.max CAD) and Enamic endocrowns using two preparations designs after thermomechanical fatigue.Materials and methods: 40 intact mandibular molars were selected and endodontically treated following a standardized procedure. Molars were decapitated 3.5mm coronal to the cervical line then embedded in epoxy resin blocks. Samples received standardized preparations for endocrowns including the preparation of the pulp chamber, then they were divided into two groups; group 1 where No-Ferrule was included in the preparation and group 2 with 2.5mm Ferrule extracoronally. Each group was further divided into two subgroups according to the material used for constructing endocrowns; e.max CAD endocrowns and Vita Enamic endocrowns. Endocrowns were constructed using Cerec CAD/CAM machine. Constructed endocrowns were adhesively bonded to their respective teeth then subjected to a thermomechanical fatigue procedure (49N, 1.6Hz, 120,000cycle, 5o-55o C) in a chewing simulator. The fracture resistance of each sample was determined by subjecting the samples to a static compressive load until failure. Failure loads were recorded as an indication of fracture resistance. Stresses at failure were analyzed using finite element analysis (FEA) and failure probability percentage was evaluated using Weibull risk of failure. Data were collected, tabulated and statistically analyzed.Results: For fracture resistance, the highest fracture load values were recorded with samples of Ferrule with e.max CAD endocrowns, followed by samples of No-Ferrule with e.max CAD endocrowns and then samples of Ferrule with Enamic endocrowns. The lowest fracture load values were recorded with samples of No-Ferrule with Enamic endocrowns. The magnitude of stresses generated in different parts of the samples as reveled by FEA generally followed a different order than in the fracture resistance. Weibull Risk of failure showed that samples of No-Ferrule with Enamic endocrowns had the highest failure probability especially under high loads. Conclusions: Ferrule design would improve the fracture resistance of endodontically treated teeth especially when restored with lithium disilicate based (e.max CAD) endocrowns. Under low magnitude of forces, both materials would work safely with either design.

FRACTURE RESISTANCE AND FAILURE MODE OF TWO RESTORATION DESIGNS MADE OF MONOLITHIC HYBRID AND GLASS MACHINABLE CERAMICS ; IN VITRO STUDY

Statement of the problem: Monolithic hybrid and glass ceramics are indicated for intracoronal and extracoronal ceramic restorations. Available data regarding selection of the appropriate ceramic type for each preparation design and its influence on fracture resistance of tooth restoration complex is still scarce.Purpose: The purpose of this study was to evaluate in vitro the fracture resistance and investigate failure mode of two restoration designs (crowns and inlays) made from lithium disilicate glass ceramic and hybrid ceramic constructed by the CAD /CAM technique.Materials and Methods: A total of twenty ceramic restorations restoring lower molars were designed and fabricated in this study. The restorations were divided into two groups; Group 1: Ten full coverage crowns and Group 2: Ten inlays. Each of the previous groups was further subdivided into two equal subgroups; Subgroup 1: Five restorations constructed from lithium disilicate glass ceramics (e.max CAD) and Subgroup 2: Five restorations constructed from hybrid ceramic (VITA ENAMIC) by the CAD/CAM technology. Two ivorine teeth were prepared following the guidelines for full coverage and inlay ceramic restorations, they were duplicated into stone dies followed by scanning, designing and milling of restorations. The stone dies were duplicated into epoxy dies where the restorations were adhesively cemented on them. All cemented samples were exposed to 2000 thermal cycles (5°C and 55°C in water) with a dwell time of 25 seconds in a computer controlled thermocycler. All specimens were loaded in a universal testing machine where fracture test was done by compressive mode of load applied occlusally using a metallic rod with round tip at cross-head speed of 1mm/min until fracture occurred. Fracture resistance values were recorded in Newtons then they were statistically evaluated and failure modes were analyzed. Results: For group 1 representing crown restorations, a statistically significant difference was found between (e.max CAD) and (VITA ENAMIC) subgroups at (p=0.05) where the highest mean value of maximum load was found in (VITA ENAMIC) (951.77 N ± 81.05) and the lowest mean value of maximum load was found in (e.max CAD) (709.35 N± 49.51). While for the second group representing inlay restorations, no statistically significant difference was found between (e.max CAD) and (VITA ENAMIC) subgroups at (p=0.307) where the highest mean value of maximum load was found in (e.max CAD) (627.71 N ± 80.01) and the lowest mean value of maximum load was found in (VITA ENAMIC) (581.26 N± 51.74). For the e.max CAD subgroup, a statistically significant difference was found between (Crown) and (Inlay) groups at (p=0.005) where the highest mean value of maximum load was found in the crown group (790.35 N± 49.51) and the lowest mean value of maximum load was found in the inlay group (627.71N ± 80.01).Regarding the VITA ENAMIC subgroup, a statistically significant difference was found between (Crown) and (Inlay) groups at (p<0.001) where the highest mean value of maximum load was found in the crown group (951.77 N ± 81.05) and the lowest mean value of maximum load was found in the inlay group (581.26N±51.74).Conclusions: 1. All inlays and crowns constructed from hybrid and glass ceramics in this study revealed fracture load values falling in the range of physiologic masticatory forces 2. When considering restoration of molars with full coverage crowns, VITA ENAMIC is more preferred than e.max CAD ceramic in terms of fracture resistance as it showed a higher statistical significant mean values.3. When planning to restore molars with minimally invasive intracoronal restorations, both ceramics can be used as there was insignificant statistical difference between them , however e.max CAD can be recommended due to its more favorable mode of fracture compared to VITA ENAMIC.4. Irrespective of preparation designs, both ceramics showed insignificant difference regarding fracture resistance mean values.

Influence of Core-Veneer Thickness Ratio on the Fracture Load and Failure Mode of Zirconia Crowns.

To evaluate the influence of different core-veneer thickness (CVT) ratios on fracture loads and failure modes of zirconia crowns with or without thermomechanical cycling (TC). Forty yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) cores were veneered with a pressable ceramic and divided into four groups (n = 10): group 1 - 1:3 CVT ratio (0.5-mm core, 1.5-mm veneer); group 2 - 1:1 CVT ratio (1.0-mm core, 1.0-mm veneer); groups 3 and 4 - identical to groups 1 and 2, respectively, except that they underwent TC (1,000,000 cycles with an 84-N load applied at a 4-Hz frequency, temperature cycling at 4°C, 37°C, and 55°C - 30 seconds each). Loads were applied axially to the central fossa of the crowns with a 1 mm/min crosshead speed at 200 N until failure. Two-way ANOVA showed that the crowns with a 1:3 CVT ratio had a lower mean fracture load than crowns with a 1:1 CVT ratio (group 1 = 1666.4 ± 466.4 N; group 2 = 2156.6 ± 472.3 N). TC had no effect on the fracture load for crowns with the same CVT ratio. The failures observed for the crowns in groups 1, 3, and 4 were fractures within the veneering ceramic, which exposed the core. The failure severity of fractures for group 2 was less extensive, characterized by chipping within the ceramic veneer. Crowns with a 1:1 CVT ratio had higher fracture load values and more conservative failures. TC had no effect on the fracture load of the ceramic crowns.