Disc Specimens Research Articles

Effect of Erosive Agents on Surface Characteristics of Nano-Fluorapatite Ceramic: An In-Vitro Study.

Erosive beverages cause dissolution of natural teeth and intra-oral restorations, resulting in surface characteristic changes, particularly roughness and degradation. The purpose of this study was to evaluate the surface roughness and topography of a dental ceramic following immersion in locally available erosive solutions. A total of 160 disc specimens of a nano-fluorapatite type ceramic (12 mm diameter and 2 mm thickness) were fabricated and equally distributed into two groups (n = 80) and then evenly distributed among the following five testing groups (n = 16): lemon juice, citrate buffer solution, 4% acetic acid, soft cola drink, and distilled water which served as a control. The surface roughness (Ra) and topography were evaluated using a profilometer and scanning electron microscope at baseline, 24 h, 96 h, and 168 h respectively. Data were analyzed using ANOVA and Tukey’s multiple comparisons (p ≤ 0.05). Surface changes were observed upon exposure to all acidic beverages except distilled water. Amongst all immersion media, 4% acetic acid produced the most severe surface roughness across all time periods (i.e., baseline, 24 h, 96 h, and 168 h). A statistically significant difference in the surface roughness values between all immersion media and across all four time intervals was observed. Erosive agents had a negative effect on the surface roughness and topography of the tested ceramic. The surface roughness increased with increased storage time intervals.

Corn Starch-Based Sandstone Sustainable Materials: Sand Type and Water Content Effect on Their Structure and Mechanical Properties

A new biodegradable, sustainable and environmentally friendly building material is introduced and studied in this work, which can be applied to lightweight architectural structures, aiming for the reduction of the greenhouse gas emissions and mitigation of the climate change effects. The focus was to investigate the effect of water concentration and different types of sand on the mechanical properties of corn starch-based artificial sandstone. A series of cubic, cylindrical and disk specimens were prepared by varying the concentration of water and using different sources of commercial quartz sand. The quasi-static and cyclic compressive properties of starch-based artificial sandstone samples were measured as a function of water concentration and sand type, while the structure of the artificial sandstone specimens was examined by scanning electron microscopy (SEM) and optical microscopy. Moreover, the Brazilian Test was employed as the indirect method to determine the tensile strength of the samples based on the type of the commercial sand they contained. The experimental results showed that the homogeneous grading of sand grains and the latter’s chemical composition have a significant effect on the mechanical properties of the sandstone samples. The highest compression values were obtained using the microwave heating method at a water concentration of about 12 wt%, while the cyclic compression and Brazilian Tests have shown that the granulometric grading of the sand particles and the chemical composition of the sand influence the compressive and tensile strength of the material.

Effect of different exposure conditions on the self‐healing capacity of engineered cementitious composites with crystalline admixture

AbstractIn this study, the effect of different exposure conditions of ambient air, tap water, and seawater on two levels of crack widths in engineered cementitious composites (ECC) were investigated. Crystalline admixture (CA) was implemented in the ECC mix to promote self‐healing capacity. Flexural testing on prism specimens (100 × 100 × 350 mm) was conducted to evaluate self‐healing by recovering the stiffness in the specimens with crack widths below 200 μm. Water permeability test was also carried out to assess the crack‐filling capability of ECC disk specimens (100‐mm diameter × 50‐mm thickness) with single crack widths over 1 mm. Digital image correlation technique was used to monitor crack propagation patterns and crack widths. To analyze the microstructure of the healing products, X‐ray diffraction test was conducted on groups of tap water and seawater exposures. Concluding results proved seawater to be a promising environmental condition for the self‐healing process in ECC specimens incorporating CA, in terms of recovery of mechanical and transport properties. Brucite was found to be formed as the additional healing agent that promoted self‐healing in this condition. These results can be applied for coastal concrete structures.

Axisymmetric V-bending: A single experiment to determine the fracture strain and weakest sheet material direction for plane strain tension

Most metals exhibit a critical minimum in ductility for plane strain tension. The fracture strain for this particular state of loading is an important reference point for the calibration of ductile fracture models. Here, an axisymmetric V-bending technique is proposed to identify the strain to fracture for plane strain tension for the weakest in-plane direction of sheet metal. After clamping the center and outer boundary of a disc specimen onto an axisymmetric fixture, the specimen is drawn over a tubular knife until fracture initiates on the specimen surface. Throughout the experiment, the specimen surface is monitored with a digital camera for timely crack detection and strain measurement by planar Digital Image Correlation (DIC). It is shown through numerical simulations that the experimental setup is remarkably robust with the results exhibiting a low sensitivity to geometric imperfections. Experiments are performed on aluminum AA2024 and two dual phase steels (DP780 and DP 980) to demonstrate the validity of the proposed technique through comparison with conventional V-bending and dihedral mini-Nakazima experiments.

Experimental and Numerical Small Punch Tests of the 14Cr ODS Ferritic Steel

Abstract Nowadays, various small specimen test techniques have gained wide popularity and appreciation among researchers as they offer undoubtful benefits in terms of structural material characterisation. This paper focuses on small punch tests (SPTs) performed on small-sized disc specimens to assess the mechanical properties of 14Cr oxide dispersion strengthened (ODS) steel. A numerical model was established to support experimental data and gain deeper insight into complex strain states developing in a deformed specimen. Modern evaluation procedures were discussed for obtaining mechanical properties from the small punch force-deflection response and were compared with the literature. Applicability and universality of those relations at different test conditions were also studied. It appeared that different ball diameters used had negligible influence on yield point but strongly affected ultimate strength estimation. It was found that friction belongs to decisive factors determining strain distribution in samples, as dry conditions increase the peak strain and move its location farther from the punch pole.

Experimental pastes containing niobophosphate and 45S5 bioactive glasses for treatment of dentin hypersensitivity: dentin permeability and tubule obliteration.

This study tested the ability of bioactive pastes containing niobophosphate and 45S5 glasses to reduce dentin permeability and to obliterate dentinal tubules, as a mean of reducing human dentin hypersensitivity. Experimental pastes with concentrations of 10, 20, and 30 wt% of two bioactive glasses (45S5 or niobophosphate [NbG]) were formulated. A paste without bioactive glass (placebo) and a commercial paste (Nano P, FGM) were used as controls. Forty dentin disc specimens were obtained from caries-free extracted third human molars and divided in 8 groups (n = 5). Percentage of permeability (%Lp) was assessed in a dental permeability machine considering hydraulic conductance, immediately after pastes application and at day 7, day 14, and day 21. The precipitates formed on the surface of the dentin discs (and dentinal tubules) were analyzed by SEM/EDS and micro-Raman spectra. Data of dentin permeability (%) 2-way repeated-measures (ANOVA) and Holm-Sidak post-tests (α = 0.05). Dentinal tubule obliteration was visually (and elemental) evaluated and descriptively reported. The experimental bioactive glass pastes containing NbG and 45S5, regardless of the concentration, reduced dentin permeability in comparison with pastes without bioactive glasses (P < 0.05). The formulated placebo and commercial paste did not reduce permeability over time (P < 0.05). SEM/EDS and micro-Raman analyses showed that both type of bioactive pastes (NbG or 45S5-based) presented mineral precipitates obliterating the dentinal tubules at day 21. NbG seems to offer a better initial effect than 45S5, while at 21 days there is no difference between both glasses. Experimental bioactive pastes containing NbG and 45S5 (at concentrations of 10%, 20%, or 30%) have potential to reduce dentin permeability (over time) in comparison with pastes without bioactive glasses; and this occurs on behalf of obliteration of dentinal tubules by microparticle and precipitate formation. Bioactive pastes containing NbG and 45S5 may benefit patients presenting dentin hypersensitivity, because these pastes can start acting fast after application and maintain their action up to 21 days.

A Novel Microshear Geometry for Exploring the Influence of Void Swelling on the Mechanical Properties Induced by MeV Heavy Ion Irradiation.

Small disks are often the specimen of choice for exposure in nuclear reactor environments, and this geometry invariably limits the types of mechanical testing that can be performed on the specimen. Recently, shear punch testing has been utilized to evaluate changes arising from neutron irradiation in test reactor environments on these small disk specimens. As part of a broader effort to link accelerated testing using ion irradiation and conventional neutron irradiation techniques, a novel microshear specimen geometry was developed for use with heavy-ion irradiated specimens. The technique was demonstrated in pure Cu irradiated to 11 and 110 peak dpa with 10 MeV Cu ions. At 11 peak dpa, the Cu specimen had a high density of small voids in the irradiated region, while at 110 peak dpa, larger voids with an average void swelling of ~20% were observed. Micropillar and microshear specimens both exhibited hardening at 11 dpa, followed by softening at 110 dpa. The close alignment of the new microshear technique and more conventional micropillar testing, and the fact that both follow intuition, is a good first step towards applying microshear testing to a wider range of irradiated materials.

A novel non-destructive testing method for turbine disks using dual array ultrasonic transducer.

A novel dual array inspection method for detecting the diffusion bonding defects of superalloy turbine disk has been proposed in this study. The influence of relative position between the planar defect and acoustic source has been analysed, and based on which, the transmission and reception algorithm for the dual array method has been proposed. The time delay law of the dual array transducer for the complex turbine disk structure has been investigated. Finite-difference time-domain theory has been used to establish the numerical model of the dual array method. In the numerical simulations, the novel method has been applied for the superalloy turbine disk specimen with prefabricated defects at the depth of 18.3 m and 28.3 mm. Furthermore, the corresponding experiment has been conducted and verifies the reliability of the simulation. The novel method shows advantages in detecting small diffusion bonding defects in complex structure, assisting the manufacture of superalloy turbine disks, and ensuring the safety of aircrafts.

Experimental study on the quasi-static loading rate dependency of mixed-mode I/II fractures for marble rocks

To explore the quasi-static loading rate effect on the mixed-mode I/II fractures of rock materials, the centrally cracked Brazilian disc (CCBD) specimens prepared from a marble rock were tested under different mixed-mode loading conditions (Me = 0, 1/3, 2/3, and 1) and quasi-static loading rates (0.05 mm·min−1, 0.1 mm·min−1, 0.5 mm·min−1, and 5 mm·min−1), and the generalized maximum tangential strain energy density (GMTSED) criterion was also derived to discuss the development of mixed-mode fracture toughness and crack initiation angle. Based on the experimental and analytical results, several conclusions were obtained. (1) The mixed-mode fracture toughness gradually increases with increasing the quasi-static loading rate, while the crack initiation angle and fracture geometry of the tested specimens change barely; the specimen might have a higher rate dependency of the fracture toughness under mode I dominant loading. (2) The modified criteria (GMTSED, EMTSN, and GMTS) match well with the experimental results; the theoretical predictions based on GMTSED criterion is between the predictions of the EMTSN and GMTS criteria. (3) The critical distance of the tested rock is not sensitive to the quasi-static loading rate, which means that the fracture behavior, i.e. crack initiation angle and mixed-mode fracture resistance, under quasi-static high loading rate can be well predicted by only taking into account the critical distance obtained at quasi-static low loading rate.

Correlation between the fatty infiltration of paraspinal muscles and disc degeneration and the underlying mechanism

BackgroundLow back pain (LBP) is associated with lumbar disc degeneration (LDD) and fatty infiltration of paraspinal muscles. However, there are some controversies about the relationship between LDD and fatty infiltration of paraspinal muscles, and the causation of them is also not clear. Thus, we investigated whether the degree of LDD was associated with fatty infiltration of paraspinal muscles and preliminarily explored the underlying mechanism.MethodsA retrospective study was conducted on 109 patients with chronic LBP. The degree of LDD was assessed by the Pfirrmann classification. Total muscle cross-sectional area, L4 vertebral body endplate area, and fat cross-sectional area at axial T2-weighted MRI were measured. Multifidus and lumbar disc specimens were taken from eight individuals undergoing discectomy for disc herniation. Gene and protein expression levels of TNF were quantified through qPCR assays and ELISA, respectively.ResultsThe relative cross-sectional area, total muscle cross-sectional area, and muscle cross-sectional area asymmetry were not related to LDD. Pfirrmann grades correlated strongly with fatty infiltration of the multifidus and moderately with fatty infiltration of the erector spinae and the psoas muscles. Linear regression analysis suggested that Pfirrmann grades were most associated with fatty infiltration of the multifidus. Compared with II-degree degeneration discs (mild-degeneration group), fatty infiltration of the multifidus in IV-degree degeneration discs (severe-degeneration group) significantly increased, accompanied by increased mRNA expression of TNF. Meanwhile, the protein expression levels of TNF (pg/g protein) in discs (16.62 ± 4.33) and multifidus (13.10 ± 2.76) of the severe-degeneration group were higher than those in the mild-degeneration group (disc: 9.75 ± 2.18; multifidus: 7.84 ± 2.43). However, the mRNA expression of TNF in the multifidus was not significantly different between the two groups.ConclusionsThe results suggest that LDD is associated with fatty infiltration of the multifidus. The possible underlying mechanism is that LDD induces fatty infiltration by inflammation. Furthermore, compared with the erector spinae and the psoas muscles, fatty infiltration of the multifidus shows an optimal correlation with LDD, which may contribute to further understanding of LDD pathology.

Effect of steel fiber type and content on the dynamic tensile properties of ultra-high performance cementitious composites (UHPCC)

As one of the most promising construction and building materials in the past 30 years, ultra-high performance concrete (UHPC) has attracted a great deal of attention and the studies on its improvement and dynamic tensile behaviors become more important for its wider use in the related fields. In this contribution, a novel ultra-high performance cementitious composites (UHPCC) was developed with lower cement content, 20% of cement is replaced with fly ash and blast furnace slag, to reduce the raw material cost and carbon emission in production. Uniaxial compression, quasi-static and dynamic splitting tests were carried out to investigate the influences of steel fiber type (straight fiber and waved fiber) and content (volume fraction (0%, 1%, 2%)) on the uniaxial compressive strength, static tensile strength and dynamic tensile behaviors for current UHPCC material. The dynamic splitting tests were conducted on 100 disc specimens (75 mm in diameter and 37.5 mm in thickness) under five different impact pressure at a strain-rate range of 20–110 s−1 to study the strain rate effect by using a Split Hopkinson pressure bar (SHPB) system and the samples’ failure processes were captured by a high-speed camera. Based on the test results, the variations of energy absorption with the fiber type and content under static and dynamic tensile conditions are also analyzed, and combined with the experimental data from previous studies, an improved empirical tensile strength rate sensitivity (DIFft) model is proposed, which agrees well with the experiment results and can be used in the numerical simulation. At last, the failure process and pattern of different types of UHPCCs in the dynamic splitting test are also studied.

Dynamic splitting tensile behavior of rock–concrete bimaterial disc with multiple material types under different interface inclination angle

AbstractThe experimental investigation on the dynamic mechanical properties and failure behavior of rock–concrete composite is of great significance for understanding the mechanical response of rock–shotcrete supporting structures. In the paper, impact splitting tests were performed on Brazilian disc specimens of the rock–concrete bimaterial with different interface inclination angles. Effects of interface inclination angle and material type on dynamic response and failure behavior under dynamic impact were analyzed experimentally. The results show that the dynamic splitting strength and energy dissipation ratio increase with the increase of interface inclination angle. However, the incorporation of fibers will reduce the splitting tensile strength. The crack‐resisting effect of sandstone–fiber concrete is more obvious, and the hysteresis plateau phenomenon appears in the post‐peak section of the stress‐time curve. The interface failure behavior is related to the stress state of the interface and can be divided into three patterns: tensile fracture, shear fracture, and interface non‐ fracture.

An experimental investigation of windage and oil churning power losses of gears and discs

This study presents comparable sets of churning and windage power loss measurements of a gear. Measurements are performed under windage (air only) and churning (immersion in oil up to a certain level) to quantify load-independent power losses of a family of spur and helical gears along with representative discs. Aiming at quantifying the impact of gear geometry on churning or windage losses to establish a baseline for the case of no enclosures around the gear, the test matrix is designed to investigate three key items: (a) contribution of losses along the periphery and faces, (b) the influences of gear teeth versus a toothless replacement disc as well as key design parameters (gear module and helix angle), and (c) a comparison of churning and windage power losses measured from the same exact set-up. The churning power losses are shown to increase with the static oil levels due to increased solid-fluid interactions. The impact of oil bath temperature (oil viscosity) on churning loss is observed to be more pronounced for higher oil levels. The presence of gear teeth on the rotating specimen (spur or helical) cause churning loss to increase modestly in comparison to the disc specimens. Furthermore, a larger tooth size results in higher churning power losses. The influence of gear teeth (gear vs. disc) on windage loss is shown to be significant, indicating that modeling a gear in a high-speed windage condition as a cylindrical disc might not be an acceptable practice.

Influence of irradiation distance on the mechanical performances of resin composites polymerized with high-irradiance light curing units

BackgroundThe aim of this study was to evaluate the influence of increased irradiation distance on the flexural strength (FS), dentin micro-shear bond strength (μSBS), and the degree of conversion (DC) of bulk-fill flowable, conventional flowable, and packable resin composites.MethodsThe resin composites tested were Surefil® SDR™ (SDR), Filtek Z350 XT Flowable Restorative A2 shade (Z3F), and Filtek Z350 XT Universal Restorative A2 shade (Z3P). Specimens were cured at four irradiation distances (0, 2, 4, and 8 mm) with an Elipar DeepCure-S LED curing light for 20 s. FS tests were performed (n = 15) using bar-shaped specimens (8 mm × 2 mm × 2 mm) of the resin composites. μSBS tests were performed on the occlusal surfaces of extracted third molars from humans that were ground to expose dentin (n = 15). DC was measured by using Raman spectroscopy on the top and bottom surfaces of disk specimens (2-mm thick) (n = 3). To further investigate whether extended irradiation times could compensate for reduced irradiance, additional Z3P specimens were prepared, which were light-cured at 8-mm distances for 40 and 60 s and subjected to FS tests, μSBS tests, and Raman spectroscopy. Both two-way and one-way ANOVA were used for statistical analyses.ResultsBoth FS and DC of Z3P specimens cured at an 8-mm distance were significantly lower than those cured at shorter distances (p < 0.05), whereas the FS and DC of the Z3F and SDR specimens were not significantly influenced by increasing distances. The μSBSs of the three types of resin composites reduced with increasing irradiation distances. The FS, μSBS, and DC of the Z3P specimen light-cured at 8 mm for 40 s were comparable to those of the Z3P specimen cured at 0 mm for 20 s.ConclusionsIncreasing the irradiation distance to 8 mm can have a deleterious influence on mechanical performances, including the FS, DC, and dentin μSBS, of the resin composites polymerized with high-irradiance light curing units.

Stress intensity factors and T-stress for an edge cracked Brazilian disk specimen under diametrically distributed load

Analytical formulae of stress intensity factors (SIFs) and T-stress in an edge cracked Brazilian disc (ECBD) subjected to diametrically uniformly and parabolic distributed pressure were firstly derived with the weight function method (WFM). Meanwhile, these analytical expressions of SIFs and T-stress were verified to be very accurate through the comparative analysis with the existing results, which could be expediently used to calculate the SIFs and T-stress in ECBD specimens with normalized crack length less than 0.50. Furthermore, load distribution form and load distribution angle affecting SIFs and T-stress was also investigated, which indicated that their influences were closely related to the crack length and crack inclination angle. The values of YI and T* decreased with increasing load distribution angle, however, YII was increased for θ = 30°, a/D < 0.25. It was also found that load distribution form and distribution angle had almost no effect on SIFs for a/D > 0.35, but their effects on T-stress should be considered for large crack inclination angles. Moreover, the influence of uniformly distributed pressure on SIFs and T-stress was more significant than that of parabolic distributed pressure for a larger load distribution angle.

Effect of aqueous environment on wear resistance of dental glass\u2013ceramics

BackgroundWear resistance affects dental ceramics longevity and the functions of the opposing teeth. However, data for the effect of aqueous environment on wear resistance of dental ceramics are lacking. This study evaluated the effect of aqueous environment on wear resistance of typical dental glass–ceramics.MethodsDisk specimens were prepared from lithium disilicate glass–ceramics (LD) and leucite reinforced glass–ceramics (LEU). The disk specimens paired with steatite antagonists were tested in a pin-on-disk tribometer under both wet and dry conditions with 10 N up to 500,000 wear cycles. The wear analysis of glass–ceramics was performed using a 3D profilometer after 100,000, 300,000 and 500,000 wear cycles. Wear morphologies were analyzed by employing scanning electron microscopy (SEM). The crystalline compositions of specimens stored in a dry environment and subsequently immersed in distilled water for 40 h were separately determined using X-ray diffraction (XRD). The chemical states of the wear surfaces for LD were analyzed by X-ray photoelectron spectroscopy (XPS). The data analysis and multiple pair-wise comparisons of means were performed by using one-way analysis of variance (ANOVA) and Tukey’s post-hoc test.ResultsLEU in a wet environment exhibited less wear volume loss than that in a dry environment (p < 0.05). The volume loss of LD in a wet environment was higher than that in a dry environment (p < 0.05). The wear volumes of steatite antagonists paired with two glass–ceramics under dry conditions were higher than under wet conditions.ConclusionsXPS spectra of LD under wet conditions indicated that high wear loss might result from the effect of stress corrosion by water and reaction of water with the ionic-covalent bonds at the crack tip. XPS spectra and SEM images of LD under dry conditions showed a possible formation of tribofilm. Within the limitations of this in vitro study, water was wear-friendly to LEU and all opposing steatites but aggravated wear for LD.

Experimental study on the deformation localisation and acoustic emission characteristics of coal in Brazilian splitting tests

This paper presents an experimental study to assess the behaviour of coal samples under tensile loadings to better understand the failure mechanisms and the interactions with the coal characteristics. A set of Brazilian splitting tests were carried out using disk specimens obtained from Tashan Coal Mine in China. The digital speckle correlation method and acoustic emission (AE) were used to capture the deformation localisation and AE characteristics of each specimen during the loading process. The precursor characteristics of AE and the failure mechanism are discussed. It was found that the entire loading process mainly consists of compaction, elastic and post-peak dropping stage without an obvious yielding stage. Two kinds of deformation localisation were observed: central symmetry and axis symmetry. The corresponding AE evolution patterns have different phases, including gradual rise, step rise, transient rise and steady rise. During the subcritical failure stage, AE counts demonstrate a “rapidly increasing + flatten” intermittent feature. The results provide a reference for a better understanding of the damage process of the brittle coal material and its application in ground control design.

An In Vitro Study to Compare the Release of Fluoride from Glass Ionomer Cement (Fuji IX) and Zirconomer.

Mercury toxicity from amalgam dental fillings and their potential for creating problems in the environment and for human health have prompted the development of new restorative materials. The leading alternatives among these are glass ionomer cements. According to current understanding, restorative materials that slowly release fluoride exert a local cariostatic effect. For this purpose, glass ionomer cements have desirable properties in that they help prevent recurrence of caries by releasing fluoride over a long period. Thus, they function in accord with the major cariostatic mechanism of fluoride, which is believed to be its action to promote remineralization and to influence the morphology of teeth by reducing enamel solubility and by suppressing oral cariogenic bacteria. Although the minimum local concentration of fluoride release required to inhibit demineralization has not been determined, it is reported that the cariostatic ability of fluoride releasing restorative materials is significant. Zirconomer defines a new class of restorative that promises the strength and durability of amalgam with the protective benefits of glass ionomer while completely eliminating the hazards of mercury. The inclusion of specially micronized zirconia fillers in the glass component of zirconomer reinforces the structural integrity of the restoration and imparts superior mechanical properties for the restoration of load-bearing permanent teeth. Combination of outstanding strength, durability, and sustained fluoride protection deems it ideal for multiple applications. The aim of the present study was to determine the fluoride release from glass ionomer cements and compare it with new material zirconomer.Materials and methodsSample preparationTablets of glass-ionomer cements and zirconomer were prepared. A dental floss was incorporated into the tablets during fabrication to allow suspension into the test medium. Each disk specimen was immersed in airtight polyethylene bottle containing 20 mL of deionized water and incubated at 37°C and stored for 24 hours.Determination of fluoride ion releaseFluoride ion measurement was performed after 6 hours, 24 hours, 48 hours, 7 days, and 14 days under normal atmospheric conditions by fluoride ion selective electrode connected to an ion selective electrode meter.Result and conclusionBoth the material tested in the study had the ability to release fluoride but higher fluoride release was observed by zirconomer as compared to GIC at all time intervals.Clinical significanceFrom a clinical point of view, both the restorative materials release fluoride at all time intervals; however, addition of zirconia particles in zirconomer increases its strength and provides superior mechanical properties. Therefore, due to the combination of both good structural integrity and fluoride releasing properties, zirconomer can be used for restoration of load bearing teeth.How to cite this articleKukreja R, Singla S, Bhadoria N, et al. An In Vitro Study to Compare the Release of Fluoride from Glass Ionomer Cement (Fuji IX) and Zirconomer. Int J Clin Pediatr Dent 2022;15(1):35-37.

Investigation of the compression–shear fracture propagation for rocks accounting for confining pressure and crack surface friction

A series of fracture experiments were carried out to study the coupling effects of confining pressure and crack surface friction on the propagation of compression-shear fracture in a self-developed triaxial compartment. Meanwhile, a fracture propagation code (FPC) that can effectively simulate the initiation and propagation of closed cracks was developed for the corresponding numerical research. The results show that the crack of the centrally cracked Brazilian disc (CCBD) specimen is still mainly of tension-shear fracture in a certain range of crack inclination angle under a high confining pressure coefficient. Compared with confining pressure, the influence of crack surface friction on fracture propagation is more prominent after crack closure. In addition, the fracture mode for closed crack is determined by confining pressure, crack surface friction, relative crack length and crack inclination angle.

Coverage error of three resin composite systems to vital unrestored maxillary anterior teeth.

Define the color of anterior teeth of a selected population and correlate it (by using coverage error [CE] and the frequency of best match) with the final color of all possible enamel-dentine combinations of three different resin composite systems. Color of 636 vital unrestored anterior teeth (central incisors, lateral incisors and canines; n=212) and disk specimens (12 mm diameter, varying thickness) corresponding to enamel-dentin combinations of all available enamel (0.5 mm and 1.0 mm thickness) and dentin shades (3.0 mm thickness) of Essentia, Enamel Plus HRi and IPS Empress Direct composite systems was measured using a clinical dental spectrophotometer (Spectroshade Micro). CE and frequency of best match for all composite systems were calculated for the measured in-vivo teeth color space. Natural in-vivo teeth exhibit higher lightness when compared to enamel-dentin composite combinations, independently of the enamel thickness used. The best (lowest) CE was found for IPS Empress, while the highest values were found for Enamel Plus Hri independently of tooth type and enamel thickness (p < 0.001). The use of 0.5 mm instead of 1.0 mm enamel thickness within enamel-dentin composite combinations resulted in a lower CE for in-vivo tooth color (p < 0.001). The color space defined by all possible enamel-dentin combinations of the studied resin composite systems does not fully match the color range of anterior teeth. All composite systems examined lack combinations with lightness values as high as the population's. IPS Empress Direct composite system represented better the in-vivo teeth color. The use of 0.5 mm enamel shade thickness is suggested when building layered restorations, as it provided better color coverage than using 1.0 mm thickness.