Reduction In Fracture Resistance Research Articles

A numerical study of dehydration induced fracture toughness degradation in human cortical bone

A 2D plane strain extended finite element method (XFEM) model was developed to simulate three-point bending fracture toughness tests for human bone conducted in hydrated and dehydrated conditions. Bone microstructures and crack paths observed by micro-CT imaging were simulated using an XFEM damage model. Critical damage strains for the osteons, matrix, and cement lines were deduced for both hydrated and dehydrated conditions and it was found that dehydration decreases the critical damage strains by about 50%. Subsequent parametric studies using the various microstructural models were performed to understand the impact of individual critical damage strain variations on the fracture behavior. The study revealed the significant impact of the cement line critical damage strains on the crack paths and fracture toughness during the early stages of crack growth. Furthermore, a significant sensitivity of crack growth resistance and crack paths on critical strain values of the cement lines was found to exist for the hydrated environments where a small change in critical strain values of the cement lines can alter the crack path to give a significant reduction in fracture resistance. In contrast, in the dehydrated state where toughness is low, the sensitivity to changes in critical strain values of the cement lines is low. Overall, our XFEM model was able to provide new insights into how dehydration affects the micromechanisms of fracture in bone and this approach could be further extended to study the effects of aging, disease, and medical therapies on bone fracture.

Impact of Root Canal Taper on Fracture Resistance of Endodontically Treated Teeth Prepared by Hero Shaper and RaCe File Systems: An In-vitro Study

Introduction: Tooth fracture is one of the most undesirable phenomena in Endodontically Treated Teeth (ETT) and usually leads to tooth extraction. Basically, removal of any hard tissue from the canal walls raises the chance of root fracture. Aim: To evaluate the impact of root canal taper on fracture resistance of ETT prepared by two different file systems (Hero Shaper and RaCe file systems). Materials and Methods: This in-vitro study was conducted in the Department of Conservative Dentistry and Endodontics at Bharati Vidyapeeth (Deemed to be University) Dental College and Hospital, Sangli and Praj Metallurgical Laboratory Pune, Maharashtra, India, frome February 2021 to November 2021. The study included 44 freshly extracted mandibular premolar teeth were randomly divided into four groups. Group 1a had Hero Shaper 4%, group 1b had 6% Hero Shaper, group 2a had RaCe 4% and group 2b had RaCe 6% file system. After cleaning and shaping the root canals, obturation was completed using cold lateral compaction and root canals were embedded in standardised autopolymerising acrylic resin blocks, subjected to a vertical load in universal testing machine to cause vertical root fracture. The forces required to induce fractures were measured in Newtons. Data was analysed by using independent t-test and two-way Analysis of Variance (ANOVA) test for intergroup comparison. A p-value≤0.05 was considered as statistically significant. Results: Among instrumented groups, group 1a (Hero Shaper 4%) showed higher fracture resistance of 372.5 Newtons and group 2b (RaCe 6%) showed lowest fracture resistance of 314.56 Newtons as compared to other groups. Statistically significant difference (p-value=0.026) was seen in the mean fracture resistance among the group 1a and 1b, and group 2a and 2b. At pair wise comparison there was statistically significant difference in group 1a and group 2a (p-value=0.016). Conclusion: Amongst the instrumented groups, Hero Shaper file system showed the higher fracture resistance, than RaCe file system. Marked reduction in fracture resistance of ETT was seen with the use of greater taper instruments.

Effect of different access cavity designs on fracture toughness of endodontically treated teeth: a systematic review and network meta-analysis.

Objective To compare the effect of different endodontic access cavities on fracture toughness of extracted endodontically treated human teeth.Data/sources An electronic literature search was performed in seven databases as well as hand search until September 2020. Risk-of-bias tool was used to evaluate the quality of included studies. Random effects frequentist network meta-analysis was performed, with mean difference (MD) and 95% confidence interval (CI) as the effect measure. Confidence in the documented evidence was assessed through the newly fuelled Confidence in Network Meta-analysis (CINeMA) framework based on the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach.Study selection A total of 844 articles were obtained in the electronic and hand search. After the application of the eligibility criteria and duplicate removal, 14 studies were included in this systematic review. All included studies were in vitro that evaluated the influence of conservative endodontic cavities (CECs) on fracture toughness in extracted endodontically treated human teeth and compared to traditional endodontic cavities (TECs). In total, ten studies (n = 456) were included in the network meta-analysis for molars. The overall risk of bias was moderate.Results The results showed that when compared to intact teeth, the greatest reduction in fracture resistance was reported for TEC (MD: -927.52; 95% CI [-1304.80; -550.24]) and CEC showed least reduction in fracture resistance (MD: -365.59; 95% CI [-759.02; 27.83]). The surface under the cumulative ranking curve (SUCRA) value for intact teeth was highest (85.4% probability of being ranked as first), followed by CEC (51.4% probability of being ranked as second), with CEC presenting the highest probabilities to be the most effective access cavity design, according to the RANK (receptor activator of NF-kappaB) table. Level of confidence varied from low to moderate across all formulated comparisons.Conclusion Overall, based on the included in vitro studies in this systematic review, CEC was the most favourable access cavity design when compared to other (truss access cavities, TEC and ninja cavities) and TEC was the least favourable for fracture resistance; however, the level of evidence was moderate.

Performance of thin blast-loaded steel plates after ballistic impact from small-arms projectiles

It is known from real blast-load events that the combined effect of fragment impact and blast loading is more severe than the effect of the blast loading alone. The present study investigates this effect by comparing the blast response of thin steel plates pre-damaged by either ballistic impacts or idealized pre-cut circular holes under similar blast loading conditions. The ballistic impacts were performed with a rifle, firing 7.62 mm APM2 projectiles, and the subsequent blast loading was applied in a shock tube facility. During the blast tests, pressure measurements and two high-speed cameras were used to record the dynamic response of the target plates and allowed for a reliable experimental procedure. To investigate the strength-ductility trade-off dilemma of the material during extreme loading, three different steels with different levels of strength and ductility were used in the tests. Numerical simulations were conducted in an attempt to obtain more insight into the fracture characteristics of the plates.For similar loading conditions and material, the target plates exposed to ballistic impact showed a reduced resistance to fracture during blast loading compared with target plates containing pre-formed circular holes. As for the effect of material strength and ductility, the global deformation decreased, while the localization of plasticity increased, with an increased material strength. The fracture resistance was also observed to decrease with increased material strength. The numerical models were able to capture the main trends observed in the physical tests, i.e., a decrease in fracture resistance with initial ballistic impact, and a reduction in fracture resistance with increased material strength. The numerically predicted crack paths also showed a strong dependence upon the initial cracks caused by the petals from the ballistic impacts.

On the interplay between microstructure, residual stress and fracture toughness of (Hf-Nb-Ta-Zr)C multi-metal carbide hard coatings

The development of sputtered coatings with improved hardness-toughness property combination is widely sought after. Multi-element ceramic carbide (Hf-Nb-Ta-Zr)C coatings with excess carbon, synthesized by DC co-sputtering is presented in this study as a promising candidate to achieve this objective. The specific roles of microstructure and residual stress are decoupled in order to understand their influence on the mechanical properties. Extensive mechanical characterization through in situ testing of focused ion beam fabricated microcantilevers and nanoindentation based approaches are adopted to quantitatively separate the effect of residual stresses on the fracture toughness of the (Hf-Nb-Ta-Zr)C coatings. Residual stress free, microcantilever testing in notched and unnotched conditions, in combination with microstructural characterization unambiguously reveals the intrinsic mechanical behavior of coatings, which solely depend on the microstructure. On the other hand, nanoindentation based testing techniques probe the influence of residual stress and microstructure on the measured mechanical properties. The segregation and thickening of carbon-rich clusters, especially to the grain boundaries with increasing deposition temperatures is speculated to lead to substantial degradation in all mechanical properties measured. An easier fracture path through grain boundaries leads to a reduction in fracture resistance, which is possibly related to carbon enrichment.

The effect of calcium hydroxide on dentine composition and root fracture resistance of human teeth: An in vitro study.

The purpose of this study was to evaluate the effect of calcium hydroxide [Ca(OH)2 ] on dentine composition and root fracture resistance. Root canals of fifty extracted mandibular premolars were prepared and divided into five groups (N=10). Canals were filled with non-setting Ca(OH)2 or left empty and were subsequently stored in phosphate buffered saline for 7, 14, 30, or 90days, respectively. Ca(OH)2 was then washed out, and roots were separated from the crowns and subjected to fracture strength test. Fractured fragments were examined by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The results showed a significant and progressive reduction in fracture resistance the longer the duration of exposure to Ca(OH)2 , and the fracture resistance in teeth exposed for 90days was significantly lower than in any other group. SEM revealed cracks in the dentine surfaces that had been exposed to Ca(OH)2 . FTIR showed a significantly lower mineral to matrix ratio in roots exposed to Ca(OH)2 for 30 and 90days than seen in roots exposed for a shorter duration. XRD analysis revealed a reduction in dentine crystallinity. It was concluded that Ca(OH)2 significantly reduced the fracture resistance of the roots overtime. Exposure to Ca(OH)2 reduced the mineral to matrix ratio and crystallinity of the dentine.

Impact of Coping Veneering Techniques on the Survival of Implant-Supported Zirconia-Based-Crowns Cemented to Hybrid-Abutments: An-In-Vitro Study.

The objective of this study is to investigate the influence of veneering technique (hand-layering vs. milling) on the fracture resistance of bi-layer implant-supported zirconia-based hybrid-abutment crowns. Mandibular molar copings were anatomically designed and milled. Copings were then veneered by hand-layering (HL) (n = 20) and milling using the Cad-On technique (LD) (n = 20). Crowns were cemented to zirconia hybrid-abutments. Ten samples of each group acted as a control while the remaining ten samples were subjected to fatigue in a chewing simulator. Crowns were loaded between 50 and 100 N for 1.2 million cycles under simultaneous temperature fluctuation between 5 and 55 °C. Crowns were then subjected to static load a to fracture test. Data were statistically analysed using the one-way ANOVA. Randomly selected crowns from each group were observed under scanning electron microscopy to view fractured surfaces. Study results indicate that during fatigue, LD crowns had a 100% survival rate; while HL crowns had a 50% failure rate. Fracture resistance of LD crowns was statistically significantly higher than that of HL crowns at the baseline and after fatigue (p ≤ 0.05). However, fatigue did not cause a statistically significant reduction in fracture resistance in both LD and HL groups (p > 0.05). Copings fractured in the LD crowns only and the fracture path was different in both LD and HL groups. According to the results, it was concluded that milled veneer implant-supported hybrid-abutment crowns exhibit significantly higher fracture resistance, and better withstand clinical masticatory loads in the posterior region compared to the hand-layered technique. Also, fatigue application and artificial aging caused no significant strength reduction in both techniques. Clinical significance: Different veneering techniques and materials (hand-layering or milling) act differently to clinical forces and environment and may be prone to early chipping during service. Therefore, practitioners are urged to consider the appropriate veneering protocol for posterior implant-supported hybrid-abutment restorations.

Influence of fatigue loading on fracture resistance of endodontically treated teeth restored with endocrowns.

This in vitro study aimed to evaluate the influence of fatigue loading on fracture resistance of endodontically treated molars restored with endocrowns using different machinable blocks. Endodontically treated mandibular first molars were prepared using a standardized method. Specimens were divided into 4 groups (n = 10).Anatomically shaped endocrowns groups were manufactured using VITA ENAMIC (VE) and KATANA Zirconia (KZ). Layered endocrowns groups were manufactured using IPS e.max CAD (EM) and BioHPP (BH). Half of the specimens of each group were subjected to fracture resistance test, while the other half were subjected to thermocycling and chewing simulation. After fatigue loading, specimens were loaded until failure. Specimens were examined using stereomicroscopy. Data were analyzed using ANOVA analysis of variance and Bonferroni post hoc test (α=0.05). KZ group had the highest initial fracture resistance value (1810.20± 119.56 N) and BH had the lowest value (579.50± 76.15 N). The reduction of fracture resistance after fatigue loading was significant for KZ group (1588.30±216.25 N) and BH group (502.60±11.53 N) and non-significant to VE group (1101.70±77.05 N) and EM group (1112.10±74.12 N). Failure modes of KZ and EM groups showed high percentage of non-restorable fractures, while VE and BH groups showed high percentage of restorable fractures. Within the limitations of this study, the following can be concluded: Poly infiltrated ceramics should be considered as a proper material to be used as an endocrown material because of its ability to be restorable if failure occurred.

Environmental degradation of fracture resistance in high-temperature water environments of low-alloy reactor pressure vessel steels with high sulphur or phosphorus contents

The fracture behaviour of low-alloy reactor pressure vessel steels with various sulphur and phosphorus contents, different environmentally-assisted cracking (EAC) and temper embrittlement (TE) susceptibilities was evaluated by elastic plastic fracture mechanics tests in air and various simulated light water reactors environments. A moderate but clear reduction of fracture initiation resistance occurred in a) high-sulphur steel with high EAC susceptibility in oxygenated high-temperature water with aggressive occluded crevice environment and preceding EAC growth, and in b) high-phosphorus steel with high TE susceptibility, where the reduction of fracture resistance was most pronounced in hydrogenated high-temperature water.

Evaluation of the thermal ageing of austenitic stainless steel welds with 10 % of δ-ferrites

Three austenitic stainless steel welds (ASSW), such as E308, ER316L and ER347, with about 10% of δ-ferrite were thermally aged at 400 °C up to 10000 h to accelerate the thermal ageing expected in pressurized water reactor (PWR) environment. Fluctuation of Fe and Cr due to spinodal decomposition is observed with width of 5–12 nm within the ferrite phase. For all 3 welds, increase in strength and decrease in tensile elongation is observed after thermal aging at room temperature and 320 °C. The reduction in fracture resistance, measured as J-R curves, is most significant for ER316L weld with decrease by about 50%. However, it is quite small (about 10% decrease) for ER347 weld even after ageing at 400 °C for 10000 h despite the large reduction in tensile elongation. Finally, the double loop-electrochemical potentiodynamic reactivation (DL-EPR) tests show the degree of sensitization (DOS) values are negligible even after the accelerated thermal ageing at 400 °C for 10000 h.

Reduction in Fracture Resistance of the Root with Aging

IntroductionThe incidence of vertical root fracture in endodontically treated teeth increases with patient age. This study evaluated the microstructure, chemical composition, and mechanical properties of radicular dentin as a function of aging. MethodsSingle-rooted teeth were obtained from adult donors ranging from age 15 to older than 80 years. Beams were extracted from the middle third of the root and subjected to 4-point flexure to failure to evaluate strength of dentin in terms of donor age. Based on the strength distribution, the fatigue strength of root tissue from young (≤30 years) and old (≥55 years) donor teeth was evaluated via cyclic flexure loading. The microstructure and chemical composition of radicular dentin from the 2 groups were evaluated as a function of distance from the root apex using microscopy and Raman spectroscopy, respectively. ResultsThe strength decreased with age by approximately 25 MPa per decade, which resulted in a significant difference (P ≤ .0001) between the young (199 ± 36 MPa) and old (122 ± 11 MPa) groups. There was also a significant difference (P ≤ .0001) in fatigue strength between the young and old age groups. Both the mineral-to-collagen ratio and degree of cross-linking increased with age, with the largest changes identified in the apical and middle thirds, respectively. ConclusionsThere is a reduction in the damage tolerance of radicular dentin with increasing age. This degradation appears to result from changes in the microstructure, corresponding chemical composition, and increase in collagen cross-linking with age, and is most severe near the root apex.

Reinforcing Effects of Calcium Silicate-based Cement and Dual Cure Composite Resin in Simulated Immature Teeth with an Open Apex: An in vitro Study.

AimTo evaluate the fracture resistance of simulated immature teeth with an apical plug of biodentine followed by composite resin vs total obturation with biodentine tested immediately and after 3 months of aging and also to find out the chemical composition of dentin in contact with these materials.Materials and methodsExtracted human maxillary central incisors with simulated immature apex with radicular dentin thickness (RDT) of 1 to 1.5 mm selected and divided into three groups of 20 each. Group I (control)—4 mm biodentine apically and thermoplasticized gutta-percha. Group II—4 mm biodentine apically and composite resin. Group III—complete obturation with biodentine. About 10 samples from each group were tested immediately and remaining 10 stored in phosphate buffered solution (PBS) and tested after 3 months for fracture resistance and chemical analysis of dentin.ResultsNo significant difference in fracture resistance between the groups was observed when tested immediately. After 3 months of aging, only biodentine group showed a significant reduction in fracture resistance with increased Ca/P ratio of root dentine.ConclusionBiodentine group has shown drastic reduction in fracture resistance after 3 months of aging, and hence, cannot be recommended as a reinforcement material in immature teeth with thin dentin walls.How to cite this article: Zhabuawala MS, Nadig RR, Pai VS, Gowda Y, Aswathanarayana RM. Reinforcing Effects of Calcium Silicate-based Cement and Dual Cure Composite Resin in Simulated Immature Teeth with an Open Apex: An in vitro Study. Int J Clin Pediatr Dent 2017;10(4):351-357.

Comparison of fracture resistance of simulated immature teeth with an open apex using Biodentine and composite resin: An in vitro study.

To evaluate the fracture resistance in simulated immature teeth that had been backfilled using composite resin and Biodentine after using Biodentine as an apical plug material immediately and after 3 months of aging. Sixty extracted human maxillary central incisors were simulated in an immature open apex. The roots of all the specimens were then standardized to a length of 10 mm and canals were instrumented to obtain the radicular dentin thickness around 1.5 mm. All the specimens were then randomly divided into three groups of twenty teeth each. Group I (control) - 4 mm apical plug of Biodentine backfilled with thermoplasticized gutta-percha. Group II - 4 mm apical plug of Biodentine and then backfilled with ParaCore. Group III - completely filled with Biodentine. Ten samples from each group were randomly divided into two subgroups. In subgroup A: Specimens were stored for 1 week. In subgroup B: Specimens were stored in phosphate-buffered saline solution for 3 months and were subjected to universal testing machine. Statistical analysis was done using one-way analysis. No significant difference in fracture resistance between the groups was observed when tested immediately. After 3 months of aging, only Biodentine group showed a significant reduction in fracture resistance without significant reduction with other two groups. Biodentine group has shown a drastic reduction in fracture resistance after 3 months of aging, and hence cannot be recommended as a reinforcement material in immature teeth with thin dentin walls.

In Vitro Comparative Analysis of Fracture Resistance in Inlay Restoration Prepared with CAD-CAM and Different Systems in the Primary Teeth.

Objective. The aim of this study was to compare to fracture resistance test of inlay restorations prepared using direct inlay technique (Gradia® Direct Composite) and Indirect Restoration System® (Gradia Indirect Composite) and CAD/CAD system (Vita Enamic® Block). Study Design. 48 noncarious extracted maxillary second primary molars were randomly divided into 4 groups with 12 in each group. All the teeth were prepared based on inlay class II preparations except for the control group. Other groups were restored with Gradia Direct Composite, Gradia Indirect Composite, and Vita Enamic Block, respectively. All restorations were cemented self-adhesive dual cure resin (3M Espe, RelyX™ Unicem Aplicap). A fracture test was performed using a compressive load. Results were analyzed using one-way analysis of variance and Duncan's post hoc multiple comparison tests (α = 0.05). Results. Vita Enamic Block and Gradia Indirect Composite showed significantly higher fracture resistance than Gradia Direct Composite (p < 0.05). There was no significant difference fracture resistance between Vita Enamic Block and Gradia Indirect Composite (p > 0.05). All restorations tested led to a significant reduction in fracture resistance (p < 0.05). Conclusion. In inlay restorations, Indirect Restoration Systems and CAD/CAM systems were applied successfully together with the self-adhesive dual cure resin cements in primary molars.

On the importance of aging to the crack growth resistance of human enamel

With improvements in oral health and an overall increase in quality of life, the percentage of fully or largely dentate seniors is increasing. Understanding the effects of aging on the mechanical properties of teeth is essential to the maintenance of lifelong oral health. In this investigation the effects of aging on the fracture toughness of human enamel were evaluated from incremental crack growth experiments performed on tissue of donor teeth representing “young” (17⩽age⩽25) and “old” (age⩾55) age groups. Results showed that the old enamel exhibited significantly lower resistance to fracture than that of the young tissue in two orthogonal directions of crack growth. For crack growth transverse to the enamel rods, the fracture toughness of the old enamel (0.37±0.15MPam0.5) was nearly 70% lower than that of tissue from the young teeth (1.23±0.20MPam0.5). Based on results from a mechanistic analysis of crack growth, the reduction in fracture resistance is attributed to a decrease in the degree of extrinsic toughening. The practice of restorative dentistry should account for these changes in tooth tissues in the treatment of senior patients. Statement of SignificanceThe mechanical behavior of enamel has been studied for over 3 decades. Due to the limited volume of tissue available for evaluation, past work has been largely based on indentation methods. In this investigation we have evaluated the resistance to fracture of human enamel using a conventional fracture mechanics approach and incremental crack growth. We compared the fracture resistance of cuspal enamel obtained from the teeth of representative “young” and “old” donor groups. Our results show that there is a substantial reduction in the resistance to fracture with age, that it is anisotropic, and that the degradation is more severe than that which occurs to dentin. As such, we feel this work is a significant contribution to the field.

Incremental cyclic fracture behaviour of 20MnMoNi55 steel at room temperature

Fracture behavior of low alloy steel 20MnMoNi55 has been evaluated under cyclic loading condition. Effect of unloading extent and degree of cycling on fracture resistance has been evaluated by varying load ratio \((R)\) and incremental plastic displacement \((\varDelta V)\). It is observed that the initiation fracture toughness and tearing modulus decreased significantly with more negative \(R\) ratio and decrease in incremental plastic displacement. Examination of interrupted cyclic fracture test specimens revealed considerable crack tip re-sharpening and suppression of void growth and this is thought to be the reason for the reduction in fracture resistance under cyclic loading.

Effect of Dowel Material and Design on the Fracture Resistance of Premolars

Statement of problem: A dowel is essential to retain the core in case of severe tooth destruction. However, the placement of a dowel and core may actually weaken teeth and affect their failure pattern depending on their material properties and stress transfer. Purpose: The present study compared the fracture resistance and failure pattern of endodontically treated premolars with different amounts of tooth destruction restored with dowels of different materials: a heat pressable ceramic (IPS e.max) and glass fiber reinforced dowels with 3 dowel space designs. Materials and methods: Ninety single rooted premolars of similar dimensions were selected and divided at random into 9 groups (n=10) Group 1: Sound teeth (Control group), Group 2: Reduced sound teeth with 60 taper and a 1 mm shoulder FL. Groups (3-9) were endodontically treated and divided as follows: Group 3: Restored teeth with minimal endodontic access. Groups 4, 5 and 6 restored using a pressable ceramic dowel and core while groups 7, 8 and 9 were restored using fiber dowels and composite cores. The samples were loaded to fracture and the mode of fracture for each group was examined. The recorded values were tabulated and statistically analyzed. Results: Group 5 (tapered ceramic) showed the highest fracture mean values followed by group 6 (parallel ceramic). Group 4 (parallel tapered ceramic) showed statistically similar values to group 1 (sound teeth).Group 2 (Reduced sound teeth) registered 20% reduction in fracture resistance when compared to group 1 (sound teeth). Group 3 (minimal endodontic access) showed statistically similar values to group 2. Fiber groups 7, 8 and 9 displayed statistically similar mean fracture resistance values, which were 30% lower than those of groups 2 and 3. Conclusions: Reduction of sound teeth resulted in 20% decrease in their fracture resistance. Endodontically treated premolars with minimal access, restored with composite, maintained the same values as sound reduced teeth. Pressed ceramic bonded dowel and cores with three dowel designs displayed higher resistance means than sound reduced teeth; however, most failures were unfavorable. Fiber dowels and composite core groups registered approximately 70% of the values of sound reduced teeth but all failure patterns were favorable.

The effect of spreader size on fracture resistance of maxillary incisor roots

To assess the effect of spreader size used during cold lateral compaction of gutta-percha on fracture resistance of roots in maxillary incisor teeth. The crowns of 50 human maxillary incisor teeth having no carious lesions, no fracture or crazing were resected 2 mm coronal to the cemento-enamel junction. The root canals of the teeth were prepared as follows: Group 1: No canal preparation. Group 2: Preparation using the stepback technique to a size 40 master apical file. Group 3: Canal preparation to size 40 and filling with laterally compacted gutta-percha; the first spreader used was equal to size 40. Group 4: Same as group 3 except the first spreader was equal to size 35. Group 5: Same as group 3 except the first spreader was equal to size 25. For each root, a simulated periodontal ligament was prepared. The roots were than mounted in polyester resin and fractured vertically on a universal testing machine (Shimadzu, Tokyo, Japan). The fracture values of teeth were analysed using Kruskal-Wallis and Mann-Whitney U-tests (P = 0.05). The uninstrumented group had the highest fracture resistance; instrumented, but unfilled roots, demonstrated the lowest resistance values (P = 0.009). There were no differences between the uninstrumented group and group 5 in which a size 25 spreader was used during filling. Use of spreaders larger than size 25 caused a significant reduction in fracture resistance of roots (P < 0.05). Spreader size used during lateral compaction of gutta-percha can affect the fracture resistance of roots in extracted teeth.

Fracture toughness evaluation of polytetrafluoroethylene

AbstractThe objective of this preliminary work was to explore the fracture resistance of polytetrafluoroethylene (PTFE) (DuPont tradename Teflon) as part of materials characterization work related to the development of “reactive” material projectiles. Little mechanical property data is available on this material since it is commonly used only as a coating material with the dominant properties being its low friction coefficient and high application temperature. Additional end products of the “7C” derivative, however, includes sheet, gaskets, bearing pads, piston rings, and diaphragms. In this work, standard ASTM E1820 fracture toughness specimens were machined from a 14‐mm‐thick sheet of this material obtained from NSWC Dahlgren Laboratory. These specimens were tested at three test temperatures and four test rates to determine if fracture would occur in this material, and if so, how the fracture toughness depends on the test temperature and specimen loading rate. Standard axial tensile specimens were also tested at quasi‐static and elevated loading rates at temperatures from ambient to −73°C. The major results are that while crack extension is difficult at ambient (20°C) temperature, for temperatures slightly below ambient, a rapid degradation of fracture resistance occurs. This reduction in fracture resistance is enhanced by rapid loading, and the material loses approximately 75% of its toughness (fracture energy absorption ability) at −18°C if the crack opening loading rate of the C(T) specimen approaches 0.25 m/s. Further reductions in temperature or increases in the loading rate appear to result in a reduced rate of degradation of fracture toughness.

A comparison of the mechanical behavior of posterior teeth with amalgam and composite MOD restorations

Objective: To compare the mechanical behavior, and infer differences in fracture resistance, of mandibular molars with amalgam and composite MOD restorations to that of an unrestored molar. Method: Finite element models were developed for an unrestored molar and molars with MOD amalgam and composite restorations. The location and magnitude of maximum principal stress resulting from simultaneous mechanical and thermal loads were determined for each molar using a series of designed experiments. An analysis of variance was conducted with the components of stress to distinguish the relative influence of oral parameters and restoration on the stress distribution in each molar. Results: The maximum principal stress in the unrestored molar was the largest of all three molars examined and occurred within the dentin along the pulpal wall. Maximum principal stresses in the molars with amalgam and composite restorations both occurred along the cavosurface margin. Maximum principal stresses in the molar with amalgam restoration occurred at the pulpal floor and lingual wall junction and resulted from large occlusal loads. Although occlusal loading had minimal effects on the stress distribution within the molar with composite restoration, low oral temperatures were responsible for the maximum principal stresses, which were found at the lingual margin and occlusal surface junction. Conclusion: There was no significant difference in the magnitude of maximum stress that occurred in the molars with amalgam and light curing composite restorations. However, the location and orientation of maximum stress in the restored molars were largely dependent on the restorative material. Although clinical studies report that tooth fracture occurs predominately to restored molars, the unrestored molar experienced the highest stress in this investigation. Therefore, the reduction in fracture resistance of restored posterior teeth appears to result from changes in the location of maximum stress resulting from mastication and temperature changes.