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OBJECTIVE: To evaluate the flexural strength and fatigue behavior of a novel 3D-printed composite resin for definitive restorations. MATERIALS AND METHODS: Fifty disc-shaped specimens were manufactured from each of a nanohybrid composite resin (NHC), polymer-infiltrated ceramic network (PICN), and 3D-printed composite resin (3D) with CAD-CAM technology. Biaxial flexural strength (σin ) (n = 30 per group) and biaxial flexural fatigue strength (σff ) (n = 20 per group) were measured using piston-on-three-balls method, employing a staircase approach of 105 cycles. Weibull statistics, relative-strength degradation calculations, and fractography were performed. The results were analyzed with 1-way ANOVA and Games-Howell post hoc test (α = 0.05). RESULTS: Significant differences in σin and σff among the groups (p < 0.001) were detected. The NHC group provided the highest mean ± standard deviation σin and σff (237.3 ± 31.6 MPa and 141.3 ± 3.8 MPa), followed by the PICN (140.3 ± 12.9 MPa and 73.5 ± 9.9 MPa) and the 3D (83.6 ± 18.5 MPa and 37.4 ± 23.8 MPa) groups. The 3D group exhibited significantly lower Weibull modulus (m = 4.7) and up to 15% higher relative strength degradation with areas of nonhomogeneous microstructure as possible fracture origins. CONCLUSIONS: The 3D-printed composite resin exhibited the lowest mechanical properties, where areas of nonhomogeneous microstructure developed during the mixing procedure served as potential fracture origins. CLINICAL SIGNIFICANCE: The clinical indications of the investigated novel 3D-printed composite resin should be limited to long-term provisional restorations. A cautious procedure for mixing the components is crucial before the 3D-printing process, since nonhomogeneous areas developed during the mixing could act as fracture origins.

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Many bridge structural components are subjected to repetitive vehicle load and temperature gradient action. The resulting cyclic tensile stresses within the structures could cause premature fatigue failure of concrete, dramatically impairing structural components' durability and sustainability. Although substantial knowledge of fatigue properties on low-strength pavement concrete and high-strength structural concrete has been obtained, research on the most widely used normal-grade ordinary concrete in bridge engineering is still ongoing. Therefore, a four-point bending fatigue test of 97 C50 concrete specimens under a constant amplitude sinusoidal wave was conducted in the laboratory, the flexural fatigue behavior of plain and reinforced concrete specimens was studied, and the cyclic deformation evolution of concrete under fatigue loading was obtained. The empirical fatigue S-N equations of concrete with a failure probability p of 0.1~0.5 were derived through statistical analysis of the test results. The fatigue life of the tested specimens exhibited a two-parameter Weibull distribution. In addition to the maximum stress level Smax, the stress ratio R is also a key factor affecting the flexural fatigue life of concrete N. The semi-logarithmic and logarithmic equations were almost identical at the tested stress levels, the latter predicting longer fatigue life for Smax < 0.70. The restraining effect from steel reinforcement slightly lengthened the concrete's fatigue cracking initiation life. The insight into concrete flexural fatigue properties from this study not only contributes to a better understanding of structural concrete, but also provides a basis for the practical evaluation of concrete or composite bridge decks.

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Additively manufactured wrist-hand orthoses (3DP-WHOs) offer several advantages over traditional splints and casts, but their development based on a patient's 3D scans currently requires advanced engineering skills, while also recording long manufacturing times as they are commonly built in a vertical position. A proposed alternative involves 3D printing the orthoses as a flat model base and then thermoforming them to fit the patient's forearm. This manufacturing approach is faster, cost-effective and allows easier integration of flexible sensors as an example. However, it is unknown whether these flat-shaped 3DP-WHOs offer similar mechanical resistance as the 3D-printed hand-shaped orthoses, with a lack of research in this area being revealed by the literature review. To evaluate the mechanical properties of 3DP-WHOs produced using the two approaches, three-point bending tests and flexural fatigue tests were conducted. The results showed that both types of orthoses had similar stiffness up to 50 N, but the vertically built orthoses failed at a maximum load of 120 N, while the thermoformed orthoses could withstand up to 300 N with no damages observed. The integrity of the thermoformed orthoses was maintained after 2000 cycles at 0.5 Hz and ±2.5 mm displacement. It was observed that the minimum force occurring during fatigue tests was approximately -95 N. After 1100-1200 cycles, it reached -110 N and remained constant. The outcomes of this study are expected to enhance the trust that hand therapists, orthopedists, and patients have in using thermoformable 3DP-WHOs.

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NiTi endodontic instruments for glide path are the most susceptible to fractures inside the root canal, mainly as a result of high torsional stress. Objective: The present study investigated the resistance to torsion and angular deflection of instruments destined for the glide path: ProGlider #16.02; T-File #17.02 and the MK Life #16.02. Materials and methods: Thirty rotating NiTi glide path instruments (n=10) with 25mm lengths were selected. The torsion test was performed based on ISO 3630-01 (1992). Three millimeters from the tip of the instruments, it was attached to a small load cell by a lever arm connected to the torsion shaft. Torsional strength and angular deflection were evaluated. Fracture surfaces were examined by scanning electron microscopy with magnifications of 1000x and 5000x in the cross section, and 50x in the lateral section. Statistical analysis was performed using the Kruskal-Wallis H test, followed by the Down's post hoc test. Results: The ProGlider instrument showed greater torsional strength (p<0.05) compared to the T-File (p<0.05) and MK Life (p<0.05), respectively. However, the T-File showed greater angular deflection (p<0.05) than the other groups tested. Conclusion: It can be concluded that the ProGlider instrument presented greater torque for the fracture, while the T-File instrument presented greater angular deflection.

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To address flexural fractures and predict fatigue life, an ordinary state-based peridynamic (PD) fatigue model is proposed for the initiation and propagation of flexural fractures. The key to this model is to replace the traditional partial differential fracture model with a spatially integral peridynamic model. Based on the contact and slip theory, the nonlocal peridynamic contact algorithm is confirmed and the load transfer is through the contact area. With the 3D peridynamic J-integration and the energy-based bond failure criterion, the peridynamic fatigue model for flexural cracks' initiation and propagation is constructed. The peridynamic solid consists of a pair of gear contact surfaces and the formation and growth of flexural fatigue cracks evolved naturally over many loading cycles. The repeated load is transferred from the drive gear to the follower gear using the nonlocal peridynamic contact algorithm. The improved adaptive dynamic relaxation approach is used to determine the static solution for each load cycle. The fatigue bending crack angle errors are within 2.92% and the cycle number errors are within 10%. According to the experimental results, the proposed peridynamic fatigue model accurately predicts the location of the crack without the need for additional criteria and the fatigue life predicted by the simulation agrees quite well with the experimental results.

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Polyoxymethylene (POM) fiber is a new polymer fiber with the potential to improve the performance of airport pavement concrete. The effect of POM fiber on the flexural fatigue properties of concrete is an important issue in its application for airport pavement concrete. In this study, four-point flexural fatigue experiments were conducted using ordinary performance concrete (OPC) and POM fiber airport pavement concrete (PFAPC) with fiber volume contents of 0.6% and 1.2%, at four stress levels, to examine the flexural fatigue characteristics of these materials. A two-parameter Weibull distribution test of flexural fatigue life was performed, after examining the change in flexural fatigue deformation using the cycle ratio (n/N). A flexural fatigue life equation was then constructed considering various failure probabilities (survival rate). The results show that POM fiber had no discernible impact on the static load strength of airport pavement concrete, and the difference between PFAPC and OPC in terms of static load strength was less than 5%. POM fiber can substantially increase the flexural fatigue deformation capacity of airport pavement concrete by almost 100%, but POM fiber had a different degree of detrimental impact on the fatigue life of airport pavement concrete compared to OPC, with a maximum decrease of 85%. The fatigue lives of OPC and PFAPC adhered to the two-parameter Weibull distribution, the single- and double-log fatigue equations considering various failure probabilities had a high fitting degree based on the two-parameter Weibull distribution, and their R2 was essentially over 0.90. The ultimate fatigue strength of PFAPC was roughly 4% lower than that of OPC. This study on the flexural fatigue properties of POM fiber airport pavement concrete has apparent research value for the extension of POM fiber to the construction of long-life airport pavements.

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The synergistic effect of applying hybrid nanoparticles in improving the fatigue property of fiber reinforced polymer composites has rarely been explored before. Hence the monotonic and fatigue flexure properties of the carbon fiber reinforced epoxy laminates with matrix modified by multiwalled carbon nanotubes and graphene nanoplatelets were experimentally studied herein. The nanofiller ratio applied in the matrix modification was considered as a variable in the experimental program to investigate the effect of nanofiller ratio on the studied mechanical properties. A synergistic index has been employed to evaluate the synergistic effect of hybrid nanoparticles on the studied properties successfully. Experimental results show that the laminates with matrix modified under a nanofiller ratio (multiwalled carbon nanotube: graphene nanoplatelet) of 9:1 have the higher monotonic and fatigue strengths than those modified under other nanofiller ratios. The monotonic flexural strength and fatigue limit of the specimens modified under a nanofiller ratio of 9:1 are higher than the neat laminate specimens by 9.3 and 11.0%, respectively. The fatigue limits of the studied nano-modified laminates increase with the static strengths. Adding hybrid nanoparticles under proper nanofiller ratios in the matrix can suppress the degradation of the stiffness, further increase the resistance to fatigue damage. Examining the fracture surfaces of fatigued specimens reveals that the pullout/bridging effects of carbon nanotubes and the crack deflection effect of graphene nanoplatelets are the main reinforcement mechanisms in enhancing the fatigue strength of the composites.

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AIM: To evaluate the influence of the design of endodontic access cavities on the percentage of unprepared areas of canal walls and flexural fatigue of instruments activated by reciprocating movement in oval-shaped straight root canals of extracted teeth. METHODOLOGY: Forty-two mandibular incisors with oval canals were scanned by a microcomputed tomography (micro-CT) device for homogeneous selection and distribution of the samples. Then, the teeth were divided into two groups (n = 21) according to the design of access cavity being tested: ultraconservative endodontic access cavity (UltraAC) and traditional access cavity. The canals were accessed with the aid of a surgical microscope, instrumented with the WaveOne Gold Medium system and irrigated with 2.5% NaOCl and 17% EDTA. The unprepared areas of the canal wall were analysed by overlaying images before and after instrumentation and expressed as percentages. micro-CT data were analysed using t-test, Mann-Whitney and Wilcoxon tests. The endodontic instruments used during instrumentation were subjected to static flexural fatigue testing using an artificial stainless steel canal with a 60° angle of curvature and a radius of 5 mm, located 5 mm from the tip of the instrument. The instruments were activated until fracture occurred, and the time in seconds for the fracture was recorded using a digital timer. The number of cycles to fracture was calculated and analysed statistically. For flexural fatigue data, an anova test complemented by a Tukey range test was used. The significance level of 5% was used for all analyses. RESULTS: There was no significant difference between the groups related to unprepared areas by the instrument during canal preparation (p > .05). The difference in flexural fatigue resistance between the groups was not significant. CONCLUSION: The use of UltraAC did not interfere with the canal instrumentation of extracted mandibular incisors with straight and oval canals. There was no difference in the flexural fatigue resistance of the instruments in relation to access cavity design.

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Some types of fiber-reinforced concrete (FRC) such as steel fiber-reinforced concrete (SFRC) or polyolefin fiber-reinforced concrete (PFRC) are suitable for structural uses but there is still scarce knowledge regarding their flexural fatigue behavior. This study aimed to provide some insight into the matter by carrying out flexural fatigue tests in pre-cracked notched specimens that previously reached the Service Limit State (SLS) or the Ultimate Limit State (ULS). The fatigue cycles applied between 30% and 70% of the pre-crack load at 5 Hz until the collapse of the material or until 1,000,000 cycles were reached. The results showed that the fatigue life of PFRC both at SLS or ULS was remarkably higher than the correspondent of SFRC. The fracture surface analysis carried out found a linear relation between the fibers present in the fracture surface and the number of cycles that both SFRC and PFRC could bear.

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This study evaluated the effect of distinct surface treatments on the fatigue behavior (biaxial flexural fatigue testing) and surface characteristics (topography and roughness) of a 5% mol yttria partially stabilized zirconia ceramic (5Y-PSZ). Disc-shaped specimens of 5Y-PSZ (IPS e.max ZirCAD MT Multi) were manufactured (ISO 6872-2015) and allocated into six groups (n = 15) considering the following surface treatments: Ctrl - no-treatment; GLZ - low-fusing porcelain glaze application; SNF - 5 nm SiO2 nanofilm; AlOx - aluminum oxide particle air-abrasion; SiC - silica-coated aluminum oxide particles (silica-coating); and 7%Si - 7% silica-coated aluminum oxide particles (silica-coating). The biaxial flexural fatigue tests were performed by the step-stress method (20Hz for 10,000 cycles) with a step increment of 50N starting at 100N and proceeding until failure detection. The samples were tested with the treated surface facing down (tensile stress side). Topography, fractography, roughness, and phase content assessments of treated specimens were performed. GLZ group presented the highest fatigue behavior, while AlOx presented the lowest performance, and was only similar to SiC and 7%Si. Ctrl and SNF presented intermediary fatigue behavior, and were also similar to SiC and 7%Si. GLZ promoted a rougher surface, Ctrl and SNF had the lowest roughness, while the air-abrasion groups presented intermediary roughness. No m-phase content was detected (only t and c phases were detected). In conclusion, the application of a thin-layer of low-fusing porcelain glaze, the deposition of silica nanofilms and the air-abrasion with silica-coated alumina particles had no detrimental effect on the fatigue behavior of the 5Y-PSZ, while the air-abrasion with alumina particles damaged the fatigue outcomes.

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The fracture of endodontic instruments inside the canal represents a problem that is not always easy to solve. The reutilization of endodontic instruments after sterilization procedures raises the question of how these processes affect their physical and mechanical properties. Alterations can involve the surface of the instruments, as well as their cutting effectiveness, shape and resistance to torsional and cyclic fatigue. The methodology adopted for this systematic review followed the PRISMA guidelines for systematic reviews. The following search terms were used in PubMed and Scopus: "endodontic sterilization", "endodontic autoclave", "cyclic fatigue", "torsional", "cutting efficiency", "sterilization", "surface characteristics" and "corrosion". After the screening phase, the application of exclusion criteria and the removal of duplicates, 51 studies were identified and divided into four outcomes: cyclic fatigue; deformation and torsional fatigue; corrosion or surface alterations; and cutting efficiency. Our study of the scientific literature highlights disagreements between studies on these effects. After autoclaving, instruments exhibit a reduction in the cutting efficiency, but NiTi alloy instruments have an improved resistance to cyclic and torsional fatigue.

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O objetivo deste trabalho foi avaliar a resistência à fadiga e a resistência à corrosão em instrumentos endodônticos de NiTi dos sistemas HyFlex EDM, Hyflex CM, e RaCe, além de relacionar estes resultados às características físicas, químicas e estruturais destes instrumentos. Para tal, instrumentos 25/.~ do sistema HyFlex EDM (EDM), instrumentos 30/.06 do sistema HyFlex CM (CM) e RaCe (RC) foram utilizados. A composição química foi determinada por espectroscopia dispersiva de raios-X, as fases presentes nos materiais foram determinadas por difratometria de raios-X (DRX) e as temperaturas de transformação de fases por calorimetria exploratória diferencial (DSC). A análise da rugosidade superficial dos instrumentos foi realizada por microscopia de força atômica (AFM) e a caracterização dimensional dos instrumentos foi realizada através da mensuração do diâmetro a 3mm da ponta. Ensaios de fadiga até a ruptura foram realizados nos três grupos (n=10) e o tempo médio até a fratura determinado. Instrumentos novos foram subdivididos em grupos controle e experimental para cada sistema. Os grupos experimentais foram submetidos à ensaios de fadiga, até 3/4 do tempo médio para a ruptura e posteriormente a testes de corrosão (potencial de circuito aberto e polarização). Os grupos controle foram diretamente submetidos aos mesmos testes de corrosão. A análise de Weibull foi aplicada para avaliação da probabilidade de falha em relação ao tempo nos instrumentos testados em fadiga até a ruptura. Análise de variância com índice de significância de 0,05% foi adotada para os resultados de composição química, temperatura de transformação de fases, diâmetro, rugosidade e testes de corrosão. A composição química foi praticamente equiatomica em instrumentos RaCe, com presença de Níquel e Titânio. Já instrumentos EDM e CM apresentaram, além destes elementos, Oxigênio na composição. A caracterização estrutural evidenciou maiores temperaturas de transformação de fase Af (austenite finish) para os grupos tratados termicamente (CM e EDM), este dado foi comprovado pelos resultados da difratometria de raios-X com a presença de fase-R e austenita (fase ß) à temperatura ambiente para ambos. O grupo RC, apresentou menores temperaturas Af (p<.05), além de apenas austenita na análise por DRX. As medidas de diâmetro apresentaram-se sem diferenças estatisticamente significativas entre os grupos CM e RC, ambos significativamente maiores que EDM. Os testes de fadiga flexural apresentaram tempo até a fratura sem diferença entre EDM e CM (p>0,05) e ambos foram superiores em relação à RC (p<0.05). Por outro lado, a análise de Weibull apresentou melhores parâmetros de confiabilidade para RC e CM. Os ensaios de corrosão apresentaram diferenças significativas entre os sistemas analisados, mas não entre instrumentos novos e ciclados. Os grupos CM apresentaram maior resistência à corrosão, seguido de EDM e RC. A rugosidade superficial foi maior em instrumentos CM, seguidos de EDM e RC. De acordo com os resultados a microestrutura dos instrumentos tratados termicamente melhora a resistência à fadiga e resistência à corrosão, independentemente do tipo de usinagem.

The aim of this study was to assess the fatigue and corrosion resistance of NiTi endodontic instruments. HyFlex EDM 25 /. ~, HyFlex CM 30/.06 and RaCe 30/.06 were used. Chemical composition was determined by energy-dispersive X-ray spectroscopy, phase constitution by X-ray diffraction and transformation temperatures by differential scanning calorimetry. Surface roughness was analyzed by atomic microscopic force (AFM) and the geometric characterization of the instruments was performed by measuring the diameter at 3mm from the tip. Initially, 10 instruments from each group were tested until rupture in the flexural fatigue bench to obtain the mean time until fracture. Then, new instruments were subdivided into control and experimental groups for each system. Experimental groups were subjected to fatigue tests, up to 3/4 of the mean time to rupture and subsequently to corrosion tests. Control groups were directly subjected to the same corrosion tests. Weibull analysis was applied to assess the reliability in the tested instruments to failure. Analysis of variance with a significance of 0.05% was adopted for the results of chemical composition, phase transformation temperature, diameter, roughness, and corrosion tests. The chemical composition was near equiatomic in RaCe instruments, with the presence of Nickel and Titanium. On the other hand, EDM and CM instruments presented, in addition to these elements, Oxygen in the composition. The structural characterization showed higher Af phase transformation temperatures (austenite finish) for the heat-treated groups (CM and EDM), this data was confirmed by the results of X-ray diffractometry with the presence of R-phase and austenite (phase ß) at room temperature for both. The RC group had lower temperatures Af (p<.05), in addition to only austenite in the XRD analysis. Diameter measurements showed no statistically significant differences between the CM and RC groups, both significantly larger than EDM. Flexural fatigue tests showed time to fracture with no difference between EDM and CM (p>0.05) and both were superior to RC (p<0.05). However, the Weibull analysis showed better reliability parameters for RC and CM. The corrosion tests showed significant differences between the analyzed systems, but not between new and cycled instruments. The CM groups showed greater resistance to corrosion, followed by EDM and RC. Surface roughness was higher in CM instruments, followed by EDM and RC. According to the results, the microstructure of the thermally treated instruments improves fatigue resistance and corrosion resistance, regardless of the type of machining.

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Concerning the structural applications of steel fiber reinforced expanded-shales lightweight concrete (SFRELC), the present study focuses on the flexural fatigue performance of SFRELC superposed beams with initial static-load cracks. Nine SFRELC superposed beams were fabricated with the SFRELC depth varying from 50% to 70% of the whole sectional depth, and the volume fraction of steel fiber ranged from 0.8% to 1.6%. The fatigue load exerted on the beams was a constant amplitude sinusoid with a frequency of 10 Hz and a fatigue characteristic value of 0.10; the upper limit was taken as the load corresponded to the maximum crack width of 0.20 mm at the barycenter of the longitudinal rebars. The results showed that with the increase of SFRELC depth and the volume fraction of steel fiber, the fatigue life of the test beams was prolonged with three altered failure modes due to the crush of conventional concrete in the compression zone and/or the fracture of the tensile rebar; the failure pattern could be more ductile by the prevention of fatigue fracture by the longitudinal tensile rebar when the volume fraction of steel fiber was 1.6% and the reduction of crack growth and concrete strain in the compression zone; the fatigue life of test beams was sensitive to the upper-limit of the fatigue load, a short fatigue life appeared from the higher stress level and larger stress amplitude of the longitudinal rebar due to the higher upper-limit of the fatigue load. The methods for predicting the stress level, the stress amplitude of the longitudinal tensile rebar, and the degenerated flexural stiffness of SFRELC superposed beams with fatigue life are proposed. With the optimal composites of the SFRELC depth ratio and the volume fraction of steel fiber, the controllable failure of reinforced SFRELC superposed beams could be a good prospect with the trend curves of fatigue flexural stiffness.

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Sterilization is a fundamental step in the reuse of endodontic instruments. The sterilization procedure involves disinfection, cleaning, washing, drying, packaging, and sterilization by heat. Heat sterilization can lead to changes in the physical and mechanical properties of dental instruments. These changes can affect the external surfaces via micropitting, corrosion, a reduction in cutting capacity, and/or an influence on the resistance to cyclic fatigue or to torsional fatigue. In this study, we examined the modification of the torsional properties of endodontic instruments after hot sterilization, and compared the properties with instruments not subjected to hot sterilization cycles in terms of resistance to torsional fatigue and deflection angle in NiTi and steel instruments. The following work was performed based on the PRISMA indications. Studies were identified through bibliographic research using electronic databases. A total of 725 records were identified in the PubMed and Scopus databases. A total of 685 records remained after exclusion by year of publication (1979 to 2019). With the application of the eligibility criteria (all articles pertaining to the issue of sterilization in endodontics), we found 146 articles, which decreased to 130 articles after elimination of duplications. There were 45 articles that studied the influences of sterilization procedures on the physical and mechanical characteristics of the instruments, and 12 that measured parameters related to resistance to torsional fatigue. Applying the inclusion and exclusion criteria resulted in a total of eight articles for quantitative analysis. The meta-analysis results show a pejorative effect of torsional fatigue for NiTi instruments subjected to heat sterilization compared to the non-sterilized control.

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This study assessed the fatigue performance (biaxial flexure fatigue strength), surface characteristics (topography and roughness) and structural stability (t-m phase transformation) of a Y-TZP ceramic subjected to air-abrasion using new powders (7% and 20% silica-coated aluminum oxide particles) in comparison to commercially available powders. Disc-shaped specimens were manufactured (ISO 6872-2015) and randomly allocated into four groups considering the air-abrasion materials: SiC: commercially available silica-coated aluminum oxide; AlOx: commercially available aluminum oxide; 7%Si and 20%Si: experimentally produced materials consisting of 7% and 20% silica-coated AlOx, respectively. Air-abrasion was executed by a blinded researcher (1 cm distance from the tip to the specimen surface, under 2.8 bar pressure for 10 s). The fatigue tests (n = 15) were performed by the staircase method under a piston-on-three-balls assembly. Topography and roughness assessments (n = 30) of abraded samples and fractography of failed discs were performed. The highest fatigue strength (MPa) was observed for 7%Si (887.20 ±â€¯50.54) and SiC (878.16 ±â€¯29.81), while the lowest fatigue strength for 20%Si (773.89 ±â€¯46.44) and AlOx (796.70 ±â€¯46.48). Topography analysis depicted similar surface morphology for all conditions. However, roughness (µm) was only statistically different between 7%Si (Ra = 0.30 ±â€¯0.09; Rz = 2.31 ±â€¯0.63) and SiC (Ra = 0.26 ±â€¯0.04; Rz = 1.99 ±â€¯0.34). Monoclinic phase grains appeared on Y-TZP surface in a similar content (≈11-12%) for the protocols. Fractography showed all failures starting on air-abraded surface/sub-surface defects from the tensile side. In terms of roughness, phase transformation and fatigue, the new 7% silica-coated aluminum oxide presented similar behavior to the commercially available powder. Increasing silica-coating concentration to 20% did not lead to a gentle air-abrasion protocol.

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AIM: This in-vitro study evaluated and compared the flexural fatigue resistance for Hero Shapers, Hyflex CM, One Shape, Profile Vortex and Protaper Next rotary NiTi files. METHOD AND MATERIALS: Total 25 rotary NiTi files allocated to each experimental group were tested in a simulated constructed apparatus with an angle of curvature 60° & radius of curvature 5 mm. Each experimental file was coated with EDTA gel and was placed in endodomotor handpiece with rubber stopper at support steel cylinder and its end between two shaping steel cylinders on the simulated apparatus. File was then rotated at 400 rpm, 2.5 N/cm torque and simultaneously digital stop watch was started. Time taken (in seconds) until the file got fractured was recorded. RESULTS: Time taken to fracture ranged from 7 to 58 s in different groups. Analysis of variance show a statistically significant intergroup difference (p < 0.001). Tukey HSD test showed the significant differences. The Weibull modulus values ranged from 7.31 to 24.19. CONCLUSION: Cyclic flexural fatigue resistance was observed highest for Grp IV (Hyflex CM) and lowest for Grp I (Hero Shapers). CLINICAL SIGNIFICANCE: Hyflex CM rotary NiTi files can be used in curved root canals as they had superior resistance and long survival time which will be helpful in eliminating one of the reasons for file fracture (due to cyclic flexural fatigue) during the root canal treatment clinically where root canal possesses a sharp bend or curve.

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In recent years, the application of engineered cementitious composites (ECCs) in structures subjected to cyclic fatigue loading, such as highway bridges, has gained widespread attention. However, most existing ECCs do not have sufficient strength and ductility, which limits their applications, especially in highway bridge structures under high-stress. In this work, an ultra-high performance engineered cementitious composite (UHP-ECC) was configured, which had a compressive strength of approximately 120 MPa, a tensile strength of up to 12 MPa, and a tensile strain capacity of more than 8%. This paper presents a study of the fatigue performance of UHP-ECC at four different fatigue stress levels through the four-point bending test. The mid-span deflection of the specimen was monitored along with the crack opening displacement (COD) of the pure bending section at the bottom of the specimen, and the crack width. In addition, the dissipated energy was calculated at various stress levels. The progressive formation of cracks under static loading was monitored using the digital image correlation (DIC) technique. The fibers at the fractured surface of the specimens were observed and analyzed by environmental scanning electron microscopy, and the morphology of the fibers was obtained at different fatigue stress levels. Eventually, the fatigue life under different stress levels was obtained, and the relationship between the fatigue life and the stress level was established.

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In this paper, comparative assessment of failure fatigue lives of thin laminated cementitious composites (LCCs) modeled by two modeling approaches-double-parameter Weibull distribution model and triple-parameter distribution model-was carried out. LCCs were fabricated of ordinary Portland cement (OPC), fly ash cenosphere (FAC), quartz sand, and reinforcing meshes and fibers. The failure fatigue life assessment at various probabilities by the two-parameter model was based on numerical calculations whereas the three-parameter model was applied by an open source program-ProFatigue®. Respective parameters, shape and scale parameters in the two-parameter Weibull distribution model while shape, scale, and location parameters in three-parameter model were determined, and the corresponding probabilistic fatigue lives at various failure probabilities were calculated. It is concluded that the two-parameter model is more accurate in probabilistic fatigue life assessment of double-layer mesh-reinforced LCCs, whereas for single-layer reinforced LCCs, both models could be used at a fair confidence level.

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The objective of this work is two-fold. First, we attempt to fit the experimental data on the flexural fatigue of plain and fiber-reinforced concrete with a probabilistic model (Saucedo, Yu, Medeiros, Zhang and Ruiz, Int. J. Fatigue, 2013, 48, 308-318). This model was validated for compressive fatigue at various loading frequencies, but not for flexural fatigue. Since the model is probabilistic, it is not necessarily related to the specific mechanism of fatigue damage, but rather generically explains the fatigue distribution in concrete (plain or reinforced with fibers) for damage under compression, tension or flexion. In this work, more than 100 series of flexural fatigue tests in the literature are fit with excellent results. Since the distribution of monotonic tests was not available in the majority of cases, a two-step procedure is established to estimate the model parameters based solely on fatigue tests. The coefficient of regression was more than 0.90 except for particular cases where not all tests were strictly performed under the same loading conditions, which confirms the applicability of the model to flexural fatigue data analysis. Moreover, the model parameters are closely related to fatigue performance, which demonstrates the predictive capacity of the model. For instance, the scale parameter is related to flexural strength, which improves with the addition of fibers. Similarly, fiber increases the scattering of fatigue life, which is reflected by the decreasing shape parameter.

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Twisted files (TFs) are rotary nickel-titanium (NiTi) instruments that are produced with a newly developed manufacturing process that is supposed to improve their properties, especially flexibility and resistance to cyclic flexural fatigue failure. The aim of this study was to study the cyclic flexural fatigue failure resistance of tip size International Standards Organization 25 TFs with two tapers 0.04 and 0.06 and to compare them with the Profile (PF) rotary NiTi files of similar tip size and taper. Four groups of fifteen files were used in this study. TF (25/0.04), TF (25/0.06), PF (25/0.04) and PF (25/0.06) were tested using the cyclic flexural fatigue testing device. The time to failure during cyclic flexural fatigue testing was recorded. The mean time required for the instrument to fail under cyclic flexural fatigue testing was 235.5 ± 68 s for 25/0.04 TF, 188.5 ± 75 s for 25/0.06 TF, 180.3 ± 102 s for 25/0.04 PF and 156.3 ± 17 s for 25/0.06 PF. The difference between the time to failure of 25/0.04 TF and 25/0.06 TF was not statistically significant. The difference between the time to failure of TF and PF of similar tip size and taper was not statistically significant. The findings of this study indicate that size 25/0.04 and 25/0.06 TFs had similar resistance to cyclic flexural fatigue failure. In addition, TFs were not superior, in terms of resistance to cyclic flexural fatigue failure, to PF of similar tip size and taper.

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