Maximum Fracture Load Research Articles

Quasi-brittle fracture criterion of CFRP with shallow surface scratch based on boundary effect model

This study investigates the quasi-brittle fracture parameters of carbon fiber composites with shallow surface scratches by considering single-layer prepreg thickness as the characteristic composite microstructure. Three-point bending fracture tests were conducted on single-edge notched specimens of various-sized carbon fiber composites. The boundary effect model was employed to establish the relationship between the microstructure and macroscopic mechanical characteristics of carbon fiber composites. The maximum fracture load was used to determine the quasi-brittle fracture characteristics. By employing normal distribution analysis, the tensile strength and fracture toughness of each specimens were determined asft = 453.28 MPa andKIC = 22.2 MPa √ m, respectively. The analysis results exhibited an error of only 0.39 % compared to the least-squares fit, and encompassed almost all discrete points of the specimens within a 95 % reliability range. Using standard laboratory dimensions, fracture intervals for different notched depths at the same thickness can be predicted. Furthermore, the fracture parameters demonstrated an increasing trend within a certain range as the crack-thickness ratio increases, which aligns with the theoretical findings.

Modeling and evaluation of the fracture resistance of alkali-resistant glass fibre reinforced concrete under different loading rates

Alkali-resistant glass fibre reinforced concrete (AR-GFRC) has garnered significant interest due to its economic and environmental friendliness. Given the critical importance of evaluating the crack resistance of AR-GFRC under dynamic loading conditions and the size effect inherent in determined fracture parameters using traditional methods, this study aims to propose an analytical approach based on the boundary effect model. The goal is to predict the realistic dynamic tensile strength (ft) and fracture toughness (KIC) while elucidating the rate sensitivity of fracture resistance. Three-point bending tests were conducted on AR-GFRC with three different fibre volumes to investigate dynamic fracture behaviors under varying loading rates. By introducing discrete parameter and the microstructure characteristic parameter that account for material discontinuity and heterogeneity, the realistic dynamic ft and KIC of AR-GFRC can be directly obtained as closed-form solutions upon determining the maximum fracture load from the tests. The results revealed a substantial increase in ft and KIC with an improvement in the loading rate. However, due to relatively weakened fibre activities, the enhancement of dynamic fracture resistance decreases with the increase in fibre content and loading rate.

Influence of ceramic crown design (translucent monolithic zirconia vs. bilayered) of implant-supported single crowns after mechanical cycling.

This study aimed to assess the influence of translucent monolithic versus bilayered crowns and whether the use of a CoCr base abutments affects the fatigue and fracture resistance of screwed implant-supported single crowns with external connections under mechanical cycling. Fifty specimens were divided into groups: (1) metal-ceramic (MC) crown, (2) veneered zirconia crown (Zr), (3) veneered zirconia crown with a CoCr base abutment (ZrB), (4) monolithic translucent zirconia crown (MZr), and (5) monolithic translucent zirconia crown with a CoCr base abutment (MZrB). Specimens underwent mechanical cycling (5 × 106 cycles; 150 N) evaluating fatigue resistance (number of failures) and those that failed were subsequently subjected to fractographic analyses (stereomicroscope and scanning electron microscope) to evaluate failure location and area, and maximum fracture load was also measured. The failure-related survival rate (100%) and maximum fracture resistance of the MZrB were significantly higher than those of MC and Zr (50%; p < 0.05). There were no significant differences in the failure rate and fracture resistance when a CoCr base abutment was used or not in the translucent monolithic Zr groups (p > 0.05;MZrB vs. MZr). Failure location, with MC crowns' fractures, noted at the screw area (p = 0.043), while all-ceramic crowns were mostly in the cuspid and to failure area, the Zr group had the largest mean (15.55 ± 9.17 mm2) among the groups, significant difference only when compared with MC (1.62 ± 0.81 mm2) (p = 0.025). Translucent monolithic zirconia crowns exhibited significantly higher fatigue and fracture resistance compared with conventional MC and bilayered crowns. The appropriate choice of material and manufacturing technique is crucial for predicting the higher clinical performance of single crowns. Enhanced mechanical resistance in terms of fatigue and fracture resistance can be achieved by replacing MC and bilayered restorations with computer-aided design and computer-aided manufacturing monolithic zirconia.

Prediction of crack initiation for notched concrete beam from FPZ at maximum load

The initial cracking load (Pi) of a notched concrete beam, difficult to measure, has been predicted from the maximum fracture load (Pmax) together with the notch-tip fracture process zone (FPZ) at Pmax. A simple analytical model is used to link Pi and Pmax together through the FPZ. The corresponding initiation and unstable fracture toughness, Ki and Kun, have also been determined. The model assumes that the lower limit of Pmax from a group of identical notched concrete beams corresponds to the lower limit of the FPZ length FPZL, so that the lowest limit Pmax = Pi is established for FPZL = 0. Detailed FPZ measurements before and at Pmax using a digital image correlation technique were analysed and used to predict the initial cracking load Pi. Six different specimens with various sizes (W = 40, 60 and 80 mm), initial notches (a0 = 12 to 48 mm) and FPZL (7 to 13 mm) at Pmax showed the average ratio of FPZL/(W – a0) was around 0.25, indicating the Pi/Pmax ratio was around 0.67 based on the analytical model. The crack growth resistance KR-curve between the crack initiation toughness Ki at Pi and the unstable fracture toughness Kun at Pmax was also established approximately by the simple model. Estimated intrinsic fracture toughness KIC was compared with Ki and Kun. The influence of average aggregate size dav on Pi and FPZL at Pmax was also discussed.

Fracture load in double keyhole notch PLA-Cu2O nanocomposites manufactured via compression molding and 3D printing: An experimental and numerical study

Polylactic acid (PLA) polymer has garnered significant attention due to its biocompatibility. The incorporation of copper oxide (Cu2O) nanoparticles into this polymer is expected to enhance its antibacterial, electrical, and thermal properties. This modification can potentially improve the performance of PLA in the fields of prosthetics manufacturing or printed circuit fabrication. However, the current research is rather focused on the mechanical properties of the PLA-Cu2O nanocomposites. This research is thus aimed to analyze PLA-Cu2O (97-3 wt%) nanocomposites with a double keyhole notch configuration both experimentally and numerically. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray mapping of elemental distribution(X-map), and thermogravimetric analysis (TGA) were employed to explore the morphology, crystallinity, homogeneity, purity, and thermal stability of the nanocomposite. The specimens were fabricated through two different processes: the classical method of compression molding and the innovative method of 3D printing. The results revealed the superior mechanical performance of the 3D-printed nanocomposite at a 0° raster angle, while the mechanical properties gradually decreased for raster angles of 45° and 90°. The experimental test also indicated a decline in the maximum fracture load of specimens with a double keyhole notch and constant notch inclination angle by raising the notch radius. This behavior was also observed by increasing the notch inclination angle at constant notch radius. The numerical results were similar to the experimental findings. Moreover, the nanocomposite manufactured through the classical method exhibited higher critical fracture load compared to their 3D-printed counterparts with the same geometry.

Closed-form fracture model for evaluating crack resistance of 3D printed fiber-reinforced alkali-activated slag/fly ash recycled sand concrete

To reduce resource consumption and greenhouse gas emissions, 3D printed alkali-activated slag/fly ash recycled sand (RS) concrete (3DP AARSC) was developed in this paper. The reasonable evaluation of its crack resistance is crucial to structural safety and stability since steel bars are still difficult to embed in 3DP samples automatically so far. The realistic fracture parameters can be responsible for the reasonable evaluation and are extremely difficult to predict accurately based on traditional methods unless the specimen sizes are large enough. Therefore, the purpose of this study is to determine the size-independent tensile strength (ft) and fracture toughness (KIC) through a predictive fracture model incorporating the heterogeneity and discontinuity of 3DP AARSC. First, fracture tests of small-sized specimens were conducted to clarify the fracture mechanisms of 3DP AARSC in different loading directions. Subsequently, based on the boundary effect model, the size-independent ft and KIC were explicitly correlated to the maximum fracture load (Fmax) in different loading directions and directly obtained if Fmax was measured from the tests. The results displayed that the predicted ft and KIC of 3DP AARSC loaded perpendicular to the printing direction were significantly higher than the counterparts of concrete loaded parallel to the printing direction by 108.7% at most. The greatest cracking resistance was demonstrated in the specimens with the RS replacement rate of 60% for natural river sand (NS), resulting in the maximum increase of 19.9% in ft and KIC compared with the 3DP concrete with 100% NS. The fiber addition and partial RS replacement increased the difference in the crack resistance along various loading directions. Moreover, the fiber incorporation had no significant improvement in the crack resistance of 3DP AARSC loaded parallel to the printing direction and apparently increased its ft and KIC along other loading directions.

The Effect of Die Material on the Crown Fracture Strength of Zirconia Crowns.

Determination of the eligibility of several tooth analog materials for use in crown fracture testing. A standardized premolar crown preparation was replicated into three types of resin dies (C&B, low modulus 3D printed resin; OnX, high modulus 3D printed resin composite; and highest modulus milled resin composite). 0.8 mm zirconia crowns were bonded to the dies and the maximum fracture load of the crowns was tested. Twelve extracted human premolars were prepared to a standardized crown preparation, and duplicate dies of the prepared teeth were 3D printed out of C&B. Zirconia crowns were bonded to both the dies and natural teeth, and their fracture load was tested. There was no statistical difference between the fracture load of zirconia crowns bonded to standardized dies of C&B (1084.5 ± 134.2 N), OnX (1112.7 ± 109.8 N) or Lava Ultimate (1137.5 ± 88.7 N) (p = 0.580). There was no statistical difference between the fracture load of crowns bonded to dentin dies (1313 ± 240 N) and a 3D-printed resin die (C&B, 1156 ± 163 N) (p = 0.618). There was no difference in the static fracture load of zirconia crowns bonded to standardized resin dies with different moduli or between a low modulus resin die and natural dentin die.

Maximum Fracture Load of Interim Crowns Fabricated by Additive and Subtractive Techniques.

To evaluate fracture load values of five types of interim CAD/CAM crowns with and without thermocycling. A complete coverage crown was designed on a mandibular first molar with a uniform 1.5-mm axial and occlusal reduction, and the STL file was exported to manufacture 100 crowns using five materials (20 crowns per material): ZCAD Temp Esthetic (SM-ZCAD; Harvest Dental); Telio CAD (SM-TCAD); P pro Crown and Bridge (AM-PPRO); E-Dent 400 C&B MHF (AM-EDENT); and DENTCA Crown & Bridge (AM-DENTCA). Each group was then divided into two subgroups: before and after thermocycling (10 cornws per subgroup). The STL file of the mandibular first molar die was used to manufacture 100 resin dies. Each die was assigned to one interim crown. Interim crowns were then luted to their assigned die using a temporary luting agent. The fracture strength of crowns was then assessed using a universal testing machine at a crosshead speed of 2 mm/minute. Two-way ANOVA followed by Tukey multiple comparations post-hoc tests were used to assess the effect of material choice and thermocycling process on the fracture strength of interim crowns (α = .05). Material choice and the thermocycling process exerted a significant (P < .001) impact on the fracture strength of interim crowns. However, the interaction between these two factors did not yield a statistically significant effect (P = .176). Within the limitations of this study, the type of interim materials and thermocycling process have a significant impact on the fracture strength of interim crowns.

Maximum Fracture Load of Interim Crowns Fabricated by Additive and Subtractive Techniques

Maximum Fracture Load of Interim Crowns Fabricated by Additive and Subtractive Techniques

Comparison of color changes, fracture strengths, and failure modes of conventional endocrowns and endocrowns with different design modifications.

Endocrowns (ECs) are alternatives for rebuilding severely damaged teeth and show superior efficacy in molars over premolars. The objective of this in vitro study is to evaluate the effects of different preparation designs with short pulp chambers on the mean color change (Δ⁢E), fracture resistance, and failure types of mandibular molar ECs. A total of 40 extracted mandibular molars were treated endodontically and divided into four groups. Samples in groups 1, 2, 3, and 4 had occlusal preparation depths of 5 mm, 3 mm, 3 mm with ferrule, and 3 mm with boxes, respectively. The samples were immersed in coffee and their Δ⁢E values were measured by using the Commission Internationale de l'Eclairage color system. They were also subjected to a fracture test. Next, all specimens were examined visually under a stereomicroscope to evaluate their failure modes and identify their fracture origins. Data were entered and analyzed by using Statistical Package for Social Sciences. Among all groups, group 4 (3 mm + boxes) presented the highest Δ⁢E (4.15) after immersion in coffee. Moreover, ANOVA revealed that the Δ⁢E of group 4 (occlusal preparation depth of 3 mm with boxes) was significantly different (p< 0.05) from that of group 2 (3 mm + ferrule, 3.07). The EC with a 3 mm chamfer and ferrule showed the highest maximum load of 2847.68 ± 693.27 N, whereas that with a 5 mm chamfer finish line had a marginally reduced load at fracture of 2831.52 ± 881.83 N. The EC with a 3 mm chamber and boxes had a slightly increased maximum load of 2700.75 ± 436.40 N, whereas that with the 3 mm chamber had the lowest maximum load at fracture of 2385.97 ± 465.61 N. One-way ANOVA showed that different EC preparation designs had no effect on maximum fracture load (F [3,16] = 0.550, p= 0.6). The recorded Δ⁢E values of ECs in all groups were equal or marginally higher than the acceptable values. The EC with a 3 mm chamfer and ferrule displayed the highest mean maximum load. The EC with a 5 mm chamfer finish line had a marginally lower maximum load at fracture than other ECs. Failures, such as ceramic fracture, split fracture, and ceramic and tooth splitting above the cemento-enamel junction (CEJ) or vertically were predominant in samples with occlusal preparation depths of 3 mm with ferrule and 5 mm.

A comparative study of the effect of various surface treatments on the fracture behavior of aluminum adhesive joints under mode I loading

This study investigates the effect of different surface treatments on the fracture behavior of epoxy-aluminum joints under mode-I loading. Six surface treatments including degreasing, abrasion with varying grit sizes, alkaline etching, acid etching, a combination of alkaline and acid etching, and a combination of abrasion and acid etching were applied to aluminum surfaces before bonding. Surface morphology, roughness parameters, total surface free energy, contact angle, and elemental composition were analyzed. Results showed that the ultimate fracture load and fracture energy initially increased and then decreased with increasing surface roughness. The most effective treatments were found to be acidic etching in combination with alkaline etching and abrasion due to their synergetic effects. These treatments removed the natural oxide layer and created a porous oxide layer, enhancing surface roughness, increasing adhesive-substrate contact areas, and providing more sites for mechanical interlocking. Compared to the degreased specimens, significant improvements were observed, including a 40% and 31% increase in maximum fracture load, a 74% and 53% increase in initiation fracture energy, and a 65% and 47% increase in propagation fracture energy, respectively. Etching treatments demonstrated superior effects on the fracture behavior of aluminum adhesive joints compared to abrasion methods, leading to cohesive failure after etching treatments.

Impact of Element Layout and Notching Technique on the Fracture Toughness of FFF-Processed Thermoplastics

This work explores the effects of notching method and element layout on the fracture loading properties of thermoplastic materials processed using fused filament fabrication (FFF). Three common thermoplastic materials were used (acrylonitrile butadiene styrene, polylatide, and polycarbonate). Four different notching methods were used, with printed and machined notches and with and without pre-cracking on ASTM D5045 compact tension specimens (n = 36). It was concluded that the notching method has a statistically significant impact on the sample preparation and that pre-cracking is necessary in all cases. Using this information to prepare specimens, a designed experiment using four different element layout strategies and two different nozzle sizes was completed with a total of 72 tests. The layout pattern was shown to have a very strong effect on the maximum fracture load, with the nozzle size showing a smaller but still statistically significant impact. With the exception of one layout using polycarbonate with likely design-driven printing defects, the results were very consistent through several replications. The results of this study are useful for making design decisions with FFF-processed materials, for better understanding the impact of the process design, and for working toward standardized printing and testing methods for additive manufacturing.

Effect of surface texture spacing on interface heat transfer and tensile property of laser‐welded steel/CFRTP joint

AbstractMetal‐polymer hybrid joints can contribute to weight reduction and have potential use in transportation industries. In this work, laser texturing was used to enhance bonding strength of laser‐welded 304 stainless steel/carbon fiber reinforced thermoplastics (CFRTP) joint. The tight relationship between surface texture spacing and porosity characteristic as well as tensile property of joint was systematically investigated. The temperature field distribution during welding process was simulated and the mechanism of porosity suppression depending on grid spacing was discussed. The result showed that the number of pores at cross‐section decreased first and then rose with the increase of micro‐structure spacing, which corresponded to the variation trend of bonding strength and was inconsistent with the evolution of connecting area. The joint achieved the minimum porosity defect and reached maximum fracture load of 2594 N when the grid spacing was 1.2 mm, while the maximum joint width of 9.28 mm was obtained when 0.4 mm spacing was adopted, indicating that the joint quality rather than mechanical interlocking governed the mechanical property of joint. Appropriate interface contact conductance (TCC) related to texture spacing could optimize the interface heat dissipation rate, and was the key factor to efficiently control the shrinkage porosity. The TCC of small or large grid spacing was too high or too small, resulting in failure of the heat transfer path through the interface to metal during cooling process. Moreover, the proper texture spacing improved the uniformity of joint strain distribution in tensile‐shear testing, which benefited joint strength.Highlights Laser‐textured grid pattern spacing affected number and distribution of pores. Heat dissipation rate varied with various spacing at joint interface. 1.2 mm spacing texture reduced stress concentration during tensile process.

The effect of bedding planes on the bending strength of rock-like material and evaluation of the crack propagation mechanism

Bedding planes affects various material properties, including the bending strength and the crack propagation mechanism of rocks. In this paper, laboratory studies on rock-like material specimens were conducted to investigate the effect of bedding planes on the failure and crack propagation mechanism and to reduce the effect of secondary factors on the results. To this end, semi-circular bend (SCB) specimens were prepared with two concrete mixing designs using aggregate and geopolymer concretes as materials. Next, they were subjected to compressive and three-point bending tests. The crack propagation mechanism, changes in the bending strength, maximum fracture load, relative midspan displacement, fracture energy, and fracture toughness were analyzed for the anisotropic and isotropic specimens. Also, different bedding angles were examined for the anisotropic specimens. The results revealed that bedding planes generally affected the crack propagation mechanism and changed the fracture mode from pure opening mode for the isotropic specimens to mixed modes for different bedding angles. Hence, the strength and fracture parameters were generally smaller in the anisotropic specimens than in the isotropic ones. With an increase in the bedding angle in the specimens, the maximum fracture load and the fracture surface energy increased, and the midspan displacement behaved differently. Moreover, the fracture toughness displayed an increasing trend with increasing the bedding angle in the anisotropic specimens. The rate of changes in the parameters were different for different bedding angles. This may be attributed to the changes in the stress distribution and to the fracture and crack propagation modes in the anisotropic specimens compared to the isotropic ones.

An analytical model for determining realistic tensile strength and fracture toughness of fired clay bricks

The crack resistance of fired clay bricks is crucial for the safety design of brick masonry structures and needs to be evaluated reasonably. However, the realistic fracture parameters are extremely difficult to predict owing to the porous and defective characteristics of bricks. Thus, this paper aims to determine the tensile strength (ft) and fracture toughness (KIC) independent of size for fired clay bricks, which are rarely reported in previous studies. The fracture mechanisms of the bricks were elucidated using the fracture tests on brick specimens having varied sizes and notch lengths. Considering the material heterogeneity and discontinuity, the maximum fracture load (Fmax) correlates linearly with ft and KIC according to boundary effect model. ft and KIC can be predicted directly once only Fmax is determined from the test. The results showed that the predicted ft and KIC were generally independent of notch length and specimen size. Using statistical analysis, the means and upper and lower limits of ft and KIC were determined. Moreover, the predicted ft and KIC of bricks proved to be suitable for recycled brick aggregates with particle sizes of 5–40 mm. Thus, the crack resistance of the brick aggregates was clarified based on the proposed analytical model.

Study on fracture performance and failure mechanism of hydraulic basalt fiber asphalt concrete by considering temperature effect

An asphalt concrete anti-seepage core wall may be faced with the risk of cracking due to problems like uneven settlement of the foundation during the service period. Considering that hydraulic asphalt concrete (HAC) has a high asphalt content while basalt fibers have excellent tensile properties and good compatibility with asphalt, adding basalt fibers into HAC may potentially form a useful material for hydraulic engineering. Therefore, it is of great significance to comprehensively examine the fracture properties of this new composite material from both macroscopic and mesoscopic perspectives. To explore the effect of basalt fibers on the fracture performance of HAC, an experimental study was carried out on hydraulic basalt fiber asphalt concrete (HBFAC) under different temperatures by considering 3 fiber lengths and 4 fiber contents. The results showed that, when the temperature remained constant, the fracture load, fracture toughness and fracture energy of HAC all exhibited an upward trend with the increase of fiber content, and reached the maximum values at the fiber content of 4‰. Of different fiber lengths, the maximum fracture load, fracture toughness and fracture energy were observed at the fiber length of 6 mm. With the growth of temperature, the fracture load and fracture toughness exhibited a decreasing trend, while the fracture energy showed an increasing trend. At the same time, the increase in fracture load and fracture toughness due to the fiber reinforcing effect was gradually enlarged, but the increase in fracture energy was gradually reduced. Overall, the optimum fiber length and fiber content were determined to be 6 mm and 4‰, respectively. Subsequently, the fiber properties were further investigated with the discrete element method, and the results of internal mesoscopic mechanical behavior, crack development and particle displacement indicate that basalt fibers play a positive role in dispersing stress and inhibiting crack development so as to improve the toughness and cracking resistance of HAC. Finally, the failure mechanism of HBFAC at different temperatures was revealed from a mesoscopic level. The outcome of this paper provides a theoretical basis for the research and application of basalt fibers in HAC.

Prediction of interfacial tensile bond strength in 3D printed concrete based on a closed-form fracture model

Interfacial bond strengths in 3D printed concrete may affect the deformation compatibilities among the extruded filaments and the anisotropy of concrete. However, the previous predictions were size dependent inevitably based on the assumption of interfacial continuity and homogeneity. This work aims to propose a fracture model capable of predicting the realistic tensile bond strength (ft) and fracture toughness (KIC) of interfaces between filaments and layers in 3D printed concrete. To reflect the discontinuity and heterogeneity of interfaces, a discrete number (β) and a microstructure characteristic parameter (G) were proposed. A simple linear relationship of the maximum fracture load (Fmax) related to ft or KIC was established, and thus, the realistic interfacial ft and KIC can be determined after obtaining Fmax from the tests on the specimens with any sizes. Subsequently, three-point bending tests were performed to study the interfacial fracture properties of printed concrete with various contents of manufactured sand. The results show that the addition of manufactured sand improved the interfacial ft and KIC between filaments but had little effect on the interfacial fracture parameters between layers. The anisotropic behavior of the 3D printed concrete was clarified by evaluating the predicted ft and KIC along different directions. This research brings novel insights into the interfacial behaviors in 3D printed concrete.

Fracture Load of Mesio–Occluso–Distal Composite Restorations Performed with Different Reinforcement Techniques: An In Vitro Study

Mesio-occluso-distal (MOD) cavity preparations are often fragile due to the amount of tooth and carious structure removed. MOD cavities can often fracture if left unsupported. The study investigated the maximum fracture load of mesi-occluso-distal cavities restored using direct composite resin restorations with various reinforcement techniques. Seventy-two freshly extracted, intact human posterior teeth were disinfected, checked, and prepared according to predetermined standards for mesio-occluso-distal cavity design (MOD). The teeth were assigned randomly into six groups. The first group was the control group restored conventionally with a nanohybrid composite resin (Group I). The other five groups were restored with a nanohybrid composite resin reinforced with different techniques: the ACTIVA BioACTIVE-Restorative and -Liner as a dentin substitute and layered with a nanohybrid composite (Group II); the everX Posterior composite resin layered with a nanohybrid composite (Group III); polyethylene fibers called "Ribbond" placed on both axial walls and the floor of the cavity, and layered with a nanohybrid composite (Group IV); polyethylene fibers placed on both axial walls and the floor of the cavity, and layered with the ACTIVA BioACTIVE-Restorative and -Liner as a dentin substitute and nanohybrid composite (Group V); and polyethylene fibers placed on both axial walls and the floor of the cavity and layered with the everX posterior composite resin and nanohybrid composite (Group VI). All teeth were subjected to thermocycling to simulate the oral environment. The maximum load was measured using a universal testing machine. The highest maximum load was exhibited by Group III with the everX posterior composite resin, followed by Group IV, Group VI, Group I, Group II, and Group V. A statistically significant difference was demonstrated between groups (p = 0.0023). When adjusting for multiple comparisons, there were statistical differences specific to comparisons between Group III versus I, Group III versus II, Group IV versus II, and Group V versus III. Within the limitations of the current study, it can be concluded that a higher maximum load resistance can be achieved (statistically significant) when reinforcing nanohybrid composite resin MOD restorations with everX Posterior.

Fracture Load of Molars Restored with Bulk-fill, Flowable Bulk-fill, and Conventional Resin Composite After Simulated Chewing.

This study aimed to compare the fracture toughness of molars with wide mesio-occlusal-distal (MOD) cavities restored with regular and flowable bulk-fill resin composite and a conventional resin composite after 250,000 mechanical cycles of chewing simulation. Thirty-two extracted mandibular third molars were selected and class II MOD cavities involving 2/3 of the intercuspal width and 4 mm depth were prepared. Teeth were divided into four groups based on resin composite type and insertion technique (n=8): (1) CT, unprepared teeth (control); (2) CV, conventional resin composite (Tetric N-Ceram, Ivoclar Vivadent) with incremental technique; (3) R-BF, regular bulk-fill resin composite (Tetric N-Ceram Bulk Fill, Ivoclar Vivadent) with a single increment; and (4) F-BF, flowable bulk-fill resin composite (Tetric N-Flow Bulk Fill, Ivoclar Vivadent) with a single increment, except for a 1-mm-thick layer at the occlusal surface, restored with conventional resin composite (Tetric N-Ceram). All specimens were evaluated to detect the presence and propagation of enamel cracks using a LED transilluminator before and after 250,000 mechanical cycles (SD Mechatronic GmbH). After a chewing simulation, they were subjected to a compressive force in a universal testing machine (DL-2000, EMIC) until fracture. The maximum fracture load of the specimens was measured (N) and the fracture patterns were classified based on the fracture site (above or below the cementoenamel junction [CEJ]). Data were statistically analyzed with one-way ANOVA. All specimens survived after 250,000 mechanical cycles, and no statistically significant differences among groups were observed regarding the fracture toughness (p<0.05). The fracture analysis demonstrated that failures below the CEJ were more common in CV (75%), while CT, R-BF, and F-BF showed this type of failure in 38%, 63%, and 63% of the specimens, respectively. The results of the crack analysis showed that the occurrence of new cracks and crack propagation was also higher in CV (33.3%), followed by R-BF, F-BF, and CT (14%, 14%, and 11% of the specimens, respectively). Teeth restored with regular and flowable bulk-fill composites showed similar fracture toughness after the chewing simulation compared to those restored with the conventional resin composite and unprepared teeth. Furthermore, teeth restored with both regular and flowable bulk-fill composites showed a lower incidence of enamel cracks and fractures below the CEJ compared to those restored with the conventional resin composite.

Fracture model for predicting tensile strength and fracture toughness of concrete under different loading rates

For the sake of safety evaluation of concrete structures under dynamic loading conditions, the rate dependences of concrete fracture parameters are necessary to be elucidated rationally first. Therefore, the aim of this study is to develop a fracture model for determining the size-independent tensile strength (ft), the fracture toughness (KIC) and the fracture energy (GF) under different loading rates. The fracture mechanism was analyzed comprehensively using three-point bending tests on concrete beams with varied depths (h) and ratios of the initial crack length to the beam depth (a0/h). As the loading rate increases, the number of fractured coarse aggregates is increased, and the shape of the load–displacement curve around the maximum fracture load (Fmax) becomes sharper. Subsequently, a fracture model was proposed to predict the size-independent parameters ft, KIC and GF by incorporating the average aggregate size (davg) and a discrete coefficient (β) to indicate material heterogeneity and discontinuity, respectively. The critical effective crack length (Δac) was discretized and quantified as davg multiplied by β. Closed-form solutions of ft, KIC and GF were obtained using Fmax based on the boundary effect model. ft, KIC and GF can be explicitly predicted after determining Fmax from the fracture tests, and were generally irrelevant to h and a0/h under individual loading rates, indicating their size-independence. The scatters in the ft, KIC and GF values were determined using a statistical analysis. The rate dependences of the fracture parameters were clarified quantitatively through elucidating the variations in ft, KIC and GF with respect to the loading rate. Moreover, the predicted fracture parameters based on the proposed model were insensitive to the possible variations of davg and β. This study brings a new insight into the fracture behaviors of concrete under different loading rates and provides scientific guidance for the disaster prevention of concrete structures.