Disc Specimens Research Articles

Effect of layer orientation on behaviour of 3D-printed rock specimens in indirect tensile testing

The mineralogical composition, texture and fracture of natural rocks can be complicated due to the inherent heterogeneity of geomechanical properties, making it challenging to understand their behaviour. Due to the intrinsic layer by layer manufacturing process, 3D printing technology enables to create rock-like materials and specimens with known and controllable inhomogeneity. In this study, disc specimens of a rock-like material are fabricated using 3D printing of cement mortar for testing to understand fabric orientation effect on fracture behaviour better. For the first time, a novel technique, named AUSBIT (Adelaide University Snap-back Indirect Tensile test), for Brazilian disc tests is applied for obtaining both strength and snap-back post-peak behaviour of the 3D printed rock-like materials. Adopting Digital Image Correlation (DIC) technique, in conjunction with the use of AUSBIT, allows obtaining full-field strains and their evolutions, showing strong effects of fabric orientation on strain distributions and both pre-and post-peak responses.

Numerical investigations of the failure mechanism evolution of rock-like disc specimens containing unfilled or filled flaws

Numerical investigations of the failure mechanism evolution of rock-like disc specimens containing unfilled or filled flaws

Strain-rate-dependent tensile response of an alumina ceramic: Experiments and modeling

The strain-rate-dependent tensile response of a commercial alumina ceramic (CeramTec 98% alumina) is investigated by experimental and modeling methods. The experiments at different loading rates are carried out on a standard MTS machine and a modified split-Hopkinson pressure bar system with flattened Brazilian disk specimens. High-speed imaging coupled with digital image correlation (DIC) is used to measure the strain fields, and this enables us to capture the fracture process and the corresponding stress field based on theoretical considerations. In the dynamic tests, it is verified that multiple cracks appear simultaneously around the locations of maximum tensile stress and strain. Next, a matching approach based on theoretical models (i.e., the uniform and sinusoidal load models) is proposed to synchronize the stress and strain history curves in time, and the matching results show the tensile cracks are often generated prior to the peak stress as visualized in ultra-high-speed camera images. This peak stress corresponds to the failure of the sample structure, which is different from the material tensile strength as an inherent material property. In this study, we use the term “overloading” to describe the structural failure of the material. The difference between the peak stress and material tensile strength is associated with the time it takes for the crack to propagate, interact, and span the structure during the loading process, which is termed as “time-dependent structural failure”. The strain-rate-dependent tensile strength of the alumina ceramics is computed with a correction method, and the tensile strength is defined as the tensile stress when the central crack first appears in the ultra-high-speed camera images. Then, the fracture surfaces of the alumina fragments are examined by Scanning electron microscopy to explore the fracture mechanism in the failure process. Finally, a strain-rate-dependent tensile strength model is proposed to describe the tensile strength of the CeramTec 98% alumina and other alumina ceramics in the literature.

Experimental stress determination of blunt notches under combinations of modes I and II loading

An appropriate evaluation of the stress field for designing new or optimizing existing engineering structures is required. The primary goal of this paper is to use the digital image correlation (DIC) approach to ascertain the stress field around the root of various blunt notches. At first, extensive tests were performed using the Brazilian Disc (BD) specimen containing three distinct notches (U, VO, and Keyhole) under diverse loading conditions, including pure mode I, pure mode II, and mixed-mode I/II. The displacement fields output from the DIC experiments were employed as the input data for a computer program, and the coefficients of the stress and displacement fields were then determined utilizing the over-deterministic technique. Afterward, the computed coefficients were used to calculate the analytical stress field around the notch root by considering singular and higher-order terms for each notch type. Subsequently, the finite element (FE) method was employed to verify the accuracy of the stress field obtained from the DIC experiments. In addition, the influence of some major parameters such as notch opening angle, notch tip radius, and geometry on the stress distribution was investigated. This study validated the efficiency of the DIC approach in determining the unknown stress coefficients of rounded notches, especially the first non-singular term of the stress expansion, which is revealed to have a considerable impact on appraising the exact stress field around the notch curvature.

Degree of conversion and bond strength of resin cements through different thicknesses of lithium disilicate ceramic using different curing modes

Objectives: The aim of this in vitro study was to assess the physical and mechanical properties of resin cements to ceramic substrate using different curing modes by evaluating the degree of conversion and bond strength of these cements. 
 Methods: Hundred twenty square lithium disilicate ceramic discs specimens of three thicknesses (0.5, 1 and 1.5 mm) and 10x10mm diameter were bonded to 120 bovine teeth using two resin cements (one light cure and one dual cure) and using two curing modes (ramp and continuous). As a result, 12 experimental groups were established for bond strength testing. degree of conversion of luting cements was evaluated using Fourier transform infrared spectroscopy (FTIR).
 Results: using the continuous mode compared to the ramp after controlling the type of resin and material thickness showed no significant difference(P>0.001) in SBS and DC. the effect of changing the material thickness between 0.5mm, 1mm and 1.5mm after controlling the resin type and mode of light curing, showed significant difference (p<0.001) from 1mm to 1.5mm using light-cured resin and continuous curing mode for SBS. while for the DC these correlations between different material thicknesses and both light cure type of resin and continuous light-curing mode was significant (p<0.001), it was also significant with dual cure cement. This study also evaluated the effect of using dual-cured resin cement compared to the light-cured resin cement on the SBS and DC, for the SBS the effect was significant with higher thicknesses (p<0.001 for the thickness of 1.5 mm) but not significant with thicknesses less than this such as 0.5mm and 1mm (P>0.05).
 Conclusion: There is no change on the effect of changing light curing mode on the DC and SBS increasing the thickness decrease SBS and DC, dual-cured resin performed better when used for thickness of 1.5mm.

The Investigation on Dimensional Stability in Some Sintered Powder Metallurgy Alloys

The subject of this research was to study the effect of sintering time on the dimensional characteristics of some powder metallurgy (PM) materials. The powders used in this study are prealloyed iron-based powders with Cu, Ni and Mo. The powders were single pressed at 600 MPa and the disc specimens have the dimensions of 8-6 mm. The green compacts were sintering in a laboratory furnace at 1150 ºC for 60, 75 and 90 minutes and air-cooled to room temperature. The density of green and sintered specimens, the porosity and the dimensional changes were evaluated.

The Efficacy and Durability of Three Desensitising Agents for Management of Hypersensitive Teeth: An In Vitro Study

Introduction: Dentine hypersensitivity (DH) is a brief and acute pain produced when dentine is subjected to various stimuli. The treatment for DH has been classified by mode of delivery as at- home and in-office therapy. It was proven that desensitising agents have the capacity for occluding dentinal tubules. The purpose of this study was to evaluate the efficacy and durability of desensitising agents for management of DH. Methods: Twelve non-carious extracted human permanent premolars were sectioned into dentine layer. The dentine disc specimens were divided into four groups. Each group were applied with 8% arginine-calcium toothpaste, 0.24% sodium fluoride toothpaste, 5% potassium nitrate toothpaste and distilled water respectively. Scanning Electron Microscope (SEM) is used to evaluate the magnitude changes of dentinal tubules post treatment with the desensitising toothpastes in terms of the efficacy and durability at day one, week one and week three after application. The data will be analyzed by analysis of variance (ANOVA) using the Statistical Package for the Social Sciences (SPSS) software (version 19.0). Results: All toothpastes demonstrated significant dentinal tubule occlusions. However, 8% arginine-calcium toothpaste showed the greatest percentage of occlusion at day one followed by potassium nitrate toothpaste and sodium fluoride toothpaste. After three weeks, 8% arginine-calcium toothpaste remained the greatest percentage of dentinal tubules occlusion Conclusion: The result revealed that 8% arginine-calcium toothpaste is the most efficient and has longer durability among the other toothpaste. Therefore, with this finding it could help the patient for selection of desensitising toothpaste in managing the DH.

Mixed Mode Fracture Investigation of Rock Specimens Containing Sharp V-Notches.

This work aims to assess both experimentally and analytically the fracture behavior of rock specimens containing sharp V-notches (SV-notches) subjected to mixed mode I/II loading. To this end, firstly, several mixed mode fracture tests were conducted on Brazilian disk specimens weakened by an SV-notch (SVNBD sample), performed in their corresponding center and with various notch opening angles. Secondly, the fracture resistance of the tested samples was predicted using a criterion named MTS-FEM. This approach is based on the maximum tangential stress (MTS) criterion, in which the tangential stress is determined from the finite element method (FEM). Additionally, in the present research, the required critical distance is calculated directly from finite element analyses performed on cracked samples. Comparing the experimental results and the analytical predictions, it is shown that the fracture curves obtained from the MTS-FEM criterion are in agreement with the experimental results. These results are achieved without the need for the calculation of stress series expansion coefficients, as an additional advantage of the proposed approach.

Fluoroscopy-Guided Percutaneous Transpedicular Biopsy Versus Posterolateral Endoscopy for Infective Spondylodiskitis Diagnosis: A Comparative Study

We sought to determine the concordance in frequency of microbiologic isolation and species identification in specimens obtained by 2 methods. Intervertebral disk specimens were taken simultaneously from each patient using percutaneous needle and posterolateral endoscopic biopsies. The isolates were reported in frequencies and concordance using the chi square and Cohen kappa tests. Thirty patients were recruited. The average age was 58.1 years, and 15 patients were women. The clinical evolution time was 7 ± 4 months. The causative organism was identified in 12 (40%) specimens obtained by fluoroscopy-guided percutaneous transpedicular biopsy and in 14 (46.6%) obtained by posterolateral endoscopy. The most common organism isolated was Staphylococcus aureus in 3 patients with the percutaneous technique and in 5 with the endoscopic one; Escherichia coli was isolated in 3 patients with each method. The kappa test showed a high degree of agreement between both methods (kappa= 0.86); the agreement in bacterial species identification was 100%. Fluoroscopy-guided percutaneous biopsy and endoscopic sampling have a good degree of concordance for both, frequency of organism isolation and identification in patients with infectious spondylodiskitis.

An Evaluation Index of Fracability for Reservoir Rocks Based on Fracture Process Zone

A reliable evaluation method for the fracability (i.e., ability to generate abundant cracks) of reservoir rocks is a critical issue for maximum hydraulic fracturing efficiency. Most previous fracability indices lacked enough rationality and practicability, and thus could not consistently provide a reliable evaluation. We suggest a new fracability index called crack tolerance, which is represented by the maximum radius of the fracture process zone at the crack tip of a cracked chevron notched Brazilian disk specimen, which corresponds to the critical state for unstable propagation of the notched crack. In experiments and simulations based on the discrete element method, we showed quantitative methods to conveniently determine the value of the crack tolerance and showed that specimens with a greater crack tolerance generated more cracks before rupture and had complex morphologies, which would indicate stronger fracability. The crack tolerance can well characterize the effects of structural and loading conditions, including the grain size heterogeneity, bedding orientation, and environmental temperature, on fracability, and the inherent heterogeneity of rock is the physical basis for it as a fracability evaluation index. Our studies showed the rationality and practicability of this index and provide hints for how to produce abundant complex cracks in reservoirs.

Experimental study on acoustic emission and surface morphology characteristics of concrete under different fracture modes

In this work, experiments were employed to study on fracture behaviors of concrete under different fracture modes using cracked straight-through Brazilian disc (CSTBD) specimens, focusing on revealing the characteristics of acoustic emission (AE) and surface morphology. The results show the failure modes of CSTBD specimens under different fracture modes contain tensile coalescence, mixed coalescence, as well as shear coalescence. Based on the AE count, peak frequency and stress, the loading process can be dived into four stages (i.e., microcrack closure, linear elastic deformation, plastic deformation, post-peak failure). The b value can be considered as a precursor index of concrete structure instability. A novel method is proposed to distinguish the diving line of RA-AF distribution based on the splitting tensile test and direct shear test, and the evolution trends of the microcrack propagation modes with different curing regimes are consistent. Meanwhile, as the fracture mode moves from pure mode I towards mixed-mode I-II and pure mode II, the fracture surfaces become uneven and rough, the fractal dimension presents an increasing tendency, the standard deviation of asperity height and slope angle exhibits an increasing tendency, and the distribution of the asperity direction frequency becomes disorganized and fluctuates widely. In addition, according to the proposed 2D micro-element model, tangential deformation plays a crucial role in the fracture process, and a larger tangential deformation leads to a rougher fracture surface.

Mixed mode fracture parameters and fracture characteristics of diorite using cracked straight through Brazilian disc specimen

In this study, the cracked straight through Brazilian disc (CSTBD) specimen was applied to study the I/II mixed mode static fracture of diorite. The fracture load and fracture toughness for different prefabricated crack angles β were determined. The fracture and morphological characteristics were analyzed using digital image correlation (DIC), scanning electron microscope (SEM) and 3D topography scanning techniques. In addition, the key fracture parameters such as fracture energy, crack mouth opening displacement (CMOD) and fracture process zone (FPZ) were characterized, and a fracture energy calculation method considering the primary crack growth was proposed. The results show that the fracture mechanical behavior and fracture trajectories are controlled by the crack angle. At the millimeter measurement scale, the fractal dimensions of fracture surfaces do not vary with crack angle. Further SEM observation shows that intergranular fracture and transgranular fracture are the dominate micro fracture modes of fracture surfaces under mode I and mode II loading, respectively. With the increase of crack angle, the fracture energy first increases and then decreases, while the allowable CMOD decreases monotonically and even appears negative when β ≥ 45°. Based on the displacement gradient method, the calculated width of FPZ for the mode I fracture specimen at peak load is 0.5–1.4 mm and the length is 2.66 mm. Finally, the stress-based and strain-based fracture criteria were modified. It is found that the proposed EEMTSN criterion has the best prediction effect on the mixed mode fracture toughness of diorite.

Observation of fracture process zone and produced fracture surface roughness in granite under Brazilian splitting tests

In many rock engineering structures, the fracture network's complexity and the generated fractures' roughness are immensely important for understanding the hydraulic conductivity of the rock mass. Despite significant progress, the mechanisms governing nucleation and growth of fracture process zone (FPZ) are still debated. The present work investigates the characteristics of the FPZ and the surface roughness of the produced fracture in granite under tensile loading at two different loading rates. Acoustic emission (AE) and digital image correlation (DIC) techniques were simultaneously employed to monitor the development of the FPZ in disc specimens of granite subjected to Brazilian splitting tests. In addition, several roughness parameters of the produced fractures were measured, and it was tried to link the locations and the magnitudes of the AE events with the height distribution of the fracture surface. Our results indicated that the loading rate significantly influences the extent of the FPZ. The tensile strength of specimens also increased with the loading rate. The FPZ estimated from the AE and DIC data were in good agreement; however, AE provided a slightly wider process zone for all specimens, regardless of the loading rate. It was observed that reducing the loading rate led to a larger FPZ and generated macroscopic fractures with higher roughness characteristics. Moment tensor inversion of the AE signals revealed that in all tested specimens, macroscopic fractures, which are conventionally considered tensile fractures, are composed of three main cracking mechanisms at the microscale, including tensile, shear, and compressive sources. Focal mechanisms distribution showed that the cracks might initiate under tensile, shear, or a combination of both mechanisms. This study’s findings can help in better understanding the rock fracturing processes under tensile loading in granitic rocks for field applications.

Application of the Deformation Fracture Criterion to Cracking of Disc Specimens with a Central Narrow Slot

Abstract Using the method of singular integral equations, the elastic-plastic problem for cracked Brazilian disk was solved. Based on the Dugdale model and deformation fracture criterion, the relationships between critical load, notch tip opening displacement and length of the plastic strips were established. Also, the comparison between the present solution for the finite domain and the known solution obtained for the semi-infinite notch in the elastic plane was performed.

Experimental investigation of the cementation and tensile properties of cement to formation interface considering surface irregularity and drilling mud residue

Cement sheath is an annular structure situated between casing and formation, which is designed to seal the formation and reinforce the borehole. However, owing to the surface irregularity and drilling mud residue, the interface between the cement sheath and the formation is one of the weakest parts for fluid leakage causing well integrity issues, which would also affect the reliability of the cement bond logs. To evaluate the cementation state and tensile properties of the cement-formation interface, rock-cement composite disc specimens are prepared, considering key factors such as lithology, interface irregularity, treatment of oil-based drilling mud contamination, and flushing fluid washing. Flushing efficiency of the drilling mud contaminated rock surface, interface cementation characteristic, referenced tensile strength, and tensile fracture morphology are acquired and analysed comparatively. Results show that (1) the flushing efficiencies of shale and sandstone are 74.1 ~ 61.9% and 52.4 ~ 28.4%, respectively, decreasing with the increase in surface irregularity. (2) The rock-cement interface gradually becomes poorly cemented as the surface cleanliness decreasing and the irregularity increasing. (3) Under the condition of drilling fluid contamination, there exists a transitional area at the cemented interface, where microcracks and pores are developed. (4) With the influence of drilling mud contamination, the referenced tensile strengths of shale-cement and sandstone-cement interfaces drop from 2.716 to 0.586 MPa and from 2.840 to 0.007 MPa, respectively. The effects of underground temperature and pressure environment are not particularly considered in this study and would be investigated systematically in future research.

Fracture characteristics of sustainable crumb rubber concrete under a wide range of loading rates

The dynamic mechanical properties of concrete materials significantly influences the safety of construction under impact loads. In this study, a comprehensive experiment on the crack straight-through flattened Brazilian disc specimens of crumb rubber concrete (CRC) is undertaken. Three specific experiments were performed to obtain the wide loading-rate ranges of fracture characteristics, i.e., the quasi-static test, drop-hammer impact test, and split Hopkinson pressure bar test. The measured and calculated results are also discussed. The results reveal that the fracture toughness and energy are both positively correlated with loading rate. Moreover, a transition loading rate (around 1 GPa × m1/2/s) exists between the high and low loading rates, and the enhancing effect of high loading rate on fracture toughness cannot be neglected. The analysis of the dynamic increased factor (DIF) on the fracture toughness (DIFk) and fracture energy (DIFg) of CRC shows that DIFk and DIFg are related to the rubber content. Furthermore, the analysis demonstrates that crumb rubber particles can enhance the crack resistance of concrete under dynamic loads. Accordingly, empirical models for the DIFk and DIFg of CRC are proposed based on the experimental data. In addition, the mechanisms by which the loading-rate affects the fracture characteristics were preliminarily identified by observing the failure modes under various loading-rate loads. This study is intended to help spread the application of crumb rubber in concrete materials.

Characteristics of Acoustic Emission Response during Granite Splitting after High Temperature-Water Cooling Cycles

In order to investigate the effect of a high temperature-water cooling cycle on the acoustic emission characteristics of the granite splitting process, Brazilian splitting tests were conducted on granite disc specimens treated with high temperature–water cooling (cycle times 1, 5, 10, 15, 20) from 250 to 650 °C. The relationship between the acoustic emission count, cumulative acoustic emission number, amplitude distribution, and the maximum energy of the specimens and temperature as well as the number of hot and cold cycles were investigated, and the relationship between the acoustic emission changes and specimen damage during the splitting of the granite specimens after the high temperature-water cooling cycle was discussed and analyzed. The test results show that the acoustic emission changes in the splitting process of granite disc specimens have obvious hot and cold shock effects, and that the acoustic emission α value and amplitude density of the specimens at the initial stage of splitting show an increasing trend with an increasing number of hot and cold cycles, and the amplitude distribution is more obviously affected by temperature. When the temperature is low and the number of hot and cold cycles is small, the maximum energy value at the peak stress point is larger, and the maximum energy value tends to decrease gradually as the temperature increases and the number of cycles increases.

Evaluation the Effect of Trichloroethylene on the Metal-Ceramic Bonding

The aim of the study is to evaluate the effect of trichloroethylene (TCE) on the metal-ceramic bond of ‎additively manufactured and casting Co-Cr alloys. For this purpose, 84 disc-shaped (r=12 mm, h=1 mm) ‎specimens were prepared for two experimental groups (n=42) with different fabrication techniques; casting ‎and additively manufactured with Co-Cr. Before ceramic application, the disc specimens divided into two ‎subgroups (n=21): Casting and cleaned with TCE (CT), casting and not cleaned with TCE (C), additively ‎manufactured and cleaned with TCE (AT), additively manufactured and not cleaned with TCE (A). Ceramic ‎‎(h=4 mm, r=6 mm) was applied to the disc specimens (n=20) and their shear bond strength (SBS) was ‎measured. The surface morphology of disc (n=1; for each subgroup) specimens before and after TCE ‎application was analyzed with a scanning electron microscope (SEM). Results were statistically analyzed with ‎‎1-way ANOVA and the Bonferroni multiple comparisons tests (α=0.05). Significant differences were found in ‎SBS of the CT, C, AT, and A groups. CT group (21.74±1.66 MPa) showed a significantly higher SBS value ‎than the C (18.65±2.11 MPa), AT (19.07±1.75 MPa) and A (18.52±1.94 MPa) groups (p<0.001). In ‎conclusion the application of the TCE increased the metal-ceramic bond of casting Co-Cr alloy. Although the ‎results were not statistically significant the TCE also increased the bond of additively manufactured Co-Cr ‎alloy.

Antimicrobial Activity of a 3D-Printed Polymethylmethacrylate Dental Resin Enhanced with Graphene.

The present study aimed to test, in vitro, the antimicrobial activity against Candida albicans and Streptococcus mutans and the surface roughness of a 3D-printed polymethylmethacrylate dental resin enhanced with graphene. A 3D-printed polymethylmethacrylate dental resin was reinforced with four different concentrations of graphene: 0.01, 0.1, 0.25 and 0.5 wt%. Neat resin was used as a control. The specimens were printed in a liquid crystal display printer. Disc specimens were used in antimicrobial evaluation, and bar-shaped specimens were used to measure surface roughness. The study of antimicrobial activity included the inhibition of the growth of C. albicans and S. mutans and their adhesion to the resin’s surface. Surface roughness increased with the increase in the graphene concentration. The growth inhibition of C. albicans was observed in the different concentrations of graphene after 24 h, with no recovery after 48 h. The specimens doped with graphene were capable of inactivating S. mutans after 48 h. The surface-adhesion studies showed that the density of microbial biofilms decreases in the case of specimens doped with graphene. Graphene, despite increasing the resin’s surface roughness, was effective in inhibiting the growth and the adhesion to the resin’s surface of the main inducers of prosthetic stomatitis.

Investigation of Impact Resistance of High-Strength Portland Cement Concrete Containing Steel Fibers.

Impact resistance of Portland cement concrete (PCC) is an essential property in various applications of PCC, such as industrial floors, hydraulic structures, and explosion-proof structures. Steel-fiber-fortified high-strength concrete testing was completed using a drop-weight impact assessment for impact strength. One mix was used to manufacture 320 concrete disc specimens cured in both humid and dry conditions. In addition, 30 cubic and 30 cylindrical specimens were used to evaluate the compressive and indirect tensile strengths. Steel fibers with hooked ends of lengths of 20, 30, and 50 mm were used in the concrete mixtures. Data on material strength were collected from impact testing, including the number of post-first-crack blows (INPBs), first-crack strength, and failure strength. Findings from the results concluded that all the steel fibers improved the mechanical properties of concrete. However, hooked steel fibers were more effective than crimped steel fibers in increasing impact strength, even with a smaller length-to-diameter ratio. Concrete samples containing hybrid fibers (hooked + crimped) also had lower compressive strength than the other fibers. Comparisons and analogies drawn between the test results and the static analyses (Kolmogorov–Smirnov and Kruskal–Wallis) show that the p-value of the analyses indicates a more normal distribution for curing in a humid environment. A significant difference was also observed between the energy absorptions of the reinforced mixtures into steel fibers.