Circular Disc Specimens Research Articles

Fracture toughness determination for epoxy-based polymer concrete mixtures: Applicability of different rectangular beam and circular disc specimens

The purpose of this study was to examine the potential impact of the testing procedure, the shape of the test sample, loading method and sample size on the KIc value of polymer concrete (PC) materials. The research involved experimental investigations using five different testing techniques and specimen types, namely the single edge notched beam (SENB), short bend beam (SBB), semi-circular bend (SCB), edge notch disc bend (ENDB), and center cracked Brazilian disc (CCBD). A typical PC mixture made of mineral silicious aggregate, ML506 epoxy resin, chopped E-glass, and foundry sand filler. Despite the difference in the shape and loading type of the tested samples, the KIc data obtained from all groups of specimens are in good agreement with together and with the SENB proposed by RILEM. Depending on the test type, the KIc value varied from 1.43 to 1.74 MPa.m0.5 and the discrepancy between the data was mainly attributed to the type of loading (compression or bending) and the crack type (center crack or edge crack). The T-stress affects the fracture toughness for different testing samples and configurations. The lowest fracture toughness corresponds to the CCBD specimen (the center cracked disc loaded diametrically). The other test samples with edge cracks and loaded by a three-point bend setup showed KIc = 1.7 - 1.74 MPa.m0.5. Moreover, the fracture toughness data for PC mixtures can be achieved by utilizing sub-sized samples like SBB (for smaller amounts of PC material) instead of larger beam samples (i.e., SENB).

Study on fracture characteristics and mechanisms of red sandstone under high-voltage pulse discharge

To investigate the influences of geometrical size and discharge voltage of the pulse discharge equipment on the fracture characteristics and mechanisms of sandstone under high-voltage pulses, a series of experiments was conducted using a high-voltage pulse discharge device on sandstone circular disc specimens of sandstone with a thickness of 10 mm. These experiments covered a range of disc diameters ranging from 50 mm to 142 mm and discharge voltages from 15 kV to 40 kV. Through these experiments, the fracture characteristics of sandstone at both macroscopic and microscopic levels were investigated. In the experiments, a quantitative analysis of surface fracture was undertaken based on fracture density and fractal damage. Additionally, using the principle of energy equivalence, numerical simulation methods were used to study the damage evolution process in sandstone. The research results indicate that the formation and distribution of fractures in the sandstone specimens are significantly affected by geometrical size and discharge voltage. By analyzing the interaction between stress waves and fracture propagation, combined with indoor experimental results, the fracture mechanism was revealed. The high temperature and shock wave generated by the plasma channel leads to the crushing zone near the electrode, while the circumferential tensile component of the stress wave can result in radial fractures, and the reflected tensile wave leads to circumferential and radial fractures near the boundary.

Investigation on mixed-mode II-III fracture of the sandstone by using eccentric cracked disk

Most of the fractures in deep rock mass are in compression condition. The deep rock failure is complex and different modes of failure play important roles. In this study, the central cracked circular disk specimen was improved by deviating from the distance between the crack on the circular surface and the center of the circle, and a new mixed-mode II-III fracture specimen called the eccentric crack disk (ECD) specimen was proposed. The ECD specimen was a simple structure and easy to machining with low cost. In addition, it could be tested in conventional loading machines without extra fixtures. The finite element was used to calculate mode I, mode II and mode III stress intensity factors for different loading angles and crack deviation distances. The critical loading angles of mixed-mode II-III fracture under different crack deviation distances were obtained. And the mixed-mode II-III fracture experiments were carried out with sandstone. The experimental results clearly revealed that the crack initiation started from the middle of the crack. The experimental data also conformed to the effective stress intensity factor criterion of mixed-mode II-III mode fracture from the literature.

Rate-dependent combined necking and fracture model for predicting ductile fracture with shell elements at high strain rates

This paper introduces a rate-dependent extension of the Hosford-Coulomb ductile fracture initiation model combined with a non-local localized necking criterion (rate-dependent DSSE-HC model) for predicting ductile fracture initiation in metals with shell finite elements at high strain rates. A Johnson-Cook-type hardening model, which consists of strain hardening, rate-sensitivity, and thermal softening terms, was adopted together with an associated flow rule. The temperature was treated as an internal state variable that was calculated from the plastic strain energy using a strain-rate-dependent weighting function between the isothermal and adiabatic conditions. The parameter associated with the localized necking locus for a specific strain rate was obtained using the instantaneous strain hardening rate. The proposed extension of the DSSE-HC model was implemented in a user-defined material subroutine and validated for virtual DP590 notched tension and punch-loaded circular disc specimens by comparing with fine mesh solid element solutions.

Diametral compression test method to analyse relative surface stresses in thermally sprayed coated and uncoated circular disc specimens

In firsts of its investigation, a diametral compression destructive testing method (also known as Brazilian test) was performed on thermally sprayed coated and uncoated circular disc specimens to compare relative surface stresses. The coating investigated had about 250 μm thickness deposited on 4.76 mm thick Hastelloy®X substrate discs of 20 mm diameter. In the instrumented experiment (diametral compression test) strain gauge rosettes were used to measure strains on two circular surfaces of disc specimen (coated and uncoated sides) and converted to stress values for analysis. Where comparisons were made, the experimental and finite element simulation results were in some agreement with overall understanding of the diametral compression testing behaviour. For coated specimen, test results convey that higher stresses exist within the uncoated side of the specimen rather than the coated side. Although the methods proposed would be deemed most comparable to real life scenarios (e.g. to quantify coating delamination strength and failure mechanics), this type of experimental investigation has certain advantages and limitations.

In vitro bio-stability screening of novel implantable polyurethane elastomers

Abstract A series of novel biomedical TPCUs with different percentages of hard segment and a silicone component in the soft segment were synthesized in a multi-stage one-pot method. The kinetic profiles of the urethane formation in TPCU-based copolymer systems were monitored by rheological, in line FTIR spectroscopic (React IR) and real-time calorimetric (RC1) methods. This process-analytically monitored multi step synthesis was successfully used to optimize the production of medical-grade TPCU elastomers on preparative scale (in lots of several kg) with controlled molecular structure and mechanical properties. Various surface and bulk analytical methods as well as systematic studies of the mechanic response of the elastomer end-products towards compression and tensile loading were used to estimate the bio-stability of the prepared TPCUs in vitro after 3 months. The tests suggested that high bio-stability of all polyurethane formulations using accelerating in vitro test can be attributed to the synthetic design as well as to the specific techniques used for specimen preparation, namely: (i) the annealing for reducing residual polymer surface stress and preventing IES, (ii) stabilization of the morphology by long-time storage of the specimens after processing bevor being immersed in the test liquids, (iii) purification by extraction to remove the shot chain oligomers which are the most susceptible to degradation. All mechanical tests were performed on cylindrical and circular disc specimens for modelling the thickness of the meniscus implants under application-relevant stress conditions.

Fracture studies of Ultra-High Performance Concrete using dynamic Brazilian tests

Ultra-High Performance Concrete (UHPC) is a new generation of concrete which shows improved performance and higher strength than traditional concrete. Despite of the huge potential in application, technical information about this material is still limited. This contribution presents a study on the dynamic fracture of UHPC.Two different Brazilian discs, namely circular disc specimens and flat end specimens, are tested in a Split-Hopkinson-Pressure Bar (SHPB) setup. The Brazilian disc specimens are impacted by a pressure pulse which generates a strain rate of 30s-1. From the resulting forces the dynamic tensile strength is deduced and, in addition, the dynamic elastic modulus of UHPC material is determined.A data acquisition system records the strain history during the test by means of strain gauges mounted on the bars and on the specimens. During the tests a high-speed photograph system is focused on the specimens and a subsequent fractographic analysis identifies the effect of the loading rate on the surface roughness. Additionally, a numerical fracture analysis using the extended finite element method has been performed and shows a good agreement with the experiments.

Fracture prediction of rocks under mode I and mode II loading using the generalized maximum tangential strain criterion

The effects of specimen geometry on mode I and mode II fracture toughness (i.e., KIc and KIIc), and the ratio of KIIc to KIc for a given rock material were derived on the basis of a generalized maximum tangential strain (GMTSN) fracture criterion, in which the effects of T-stress and radial stress on rock fracturing can be taken into consideration. To verify these theoretical predictions, laboratory tests using semi-circular bend (SCB) and center-cracked circular disc (CCCD) specimens were conducted. Experimental results indicate that the GMTSN criterion is capable of estimating the geometry dependence of KIc and KIIc, the ratio of KIIc to KIc and the fracture initiation angle of rocks. Moreover, the GMTSN criterion can provide better estimates for the experimental results than some other frequently-used fracture criteria. In addition, our study reveals that the SCB fracture test using asymmetric bottom supports can be identified as a suitable testing method to estimate the upper bound of mode II fracture resistance.

Investigation Study on Determination of Fracture Strain and Fracture Forming Limit Curve Using Different Experimental and Numerical Methods

Forming Limit Curve (FLC) is a well-known tool for the evaluation of failure in sheet metal process. However, its experimental determination and evaluation are rather complex. From theoretical point of view, FLC describes initiation of the instability not fracture. During the last years Digital Image Correlation (DIC) techniques have been developed extensively. Throughout this paper, all the measurements were done using DIC and as it is reported in the literature, different approaches to capture necking and fracture phenomena using Cross Section Method (CSM), Time dependent Method (TDM) and Thinning Method (TM) were investigated. Each aforementioned method has some advantages and disadvantages. Moreover, a cruciform specimen was used in order to cover whole FLC in the range between uniaxial to equi-biaxial tension and as an alternative for Nakajima test. Based on above-mentioned uncertainty about the fracture strain, some advanced numerical failure models can describe necking and fracture phenomena accurately with consideration of anisotropic effects. It is noticeable that in this paper, dog-bone, notch and circular disk specimens are used to calibrate Johnson-Cook (J-C) fracture model. The results are discussed for mild steel DC01.

Impact Toughness Properties of Polymer Latex Modified Concrete Composites

The paper focuses on the impact strength and toughness behaviour of SBR based polymer concrete reinforced with crimped polypropylene fibres. A total of 48 slab specimens were casted and tested on a manual type fabricated impact testing apparatus. Circular disc specimens of 400 mm diameter concrete slabs were used to test the impact strength. In the polymer modified concrete the dosage of fibre used were 0.1% and 0.3% to the volume fraction of the concrete. Toughness parameters such as first crack, ultimate, fibre concrete and post crack toughness were evolved from the experimental study. It is observed that ultimate toughness of polymer fibre concrete (372.78N-m) is higher compared to plain concrete (139.79Nm) due to absorption of energy by SBR latex polymer and bridging of polypropylene fibres compared to plain concrete. The effect of fibres in absorbing the impact load and cracks was shown in post crack toughness of polymer modified concrete with increase in the dosage of fibres from 0.1% to 0.3%.

The Effect of Outdoor Weathering on Color Stability of Silicone and Acrylic Resin, Pigments-A Comparative Evaluation: An in vitro Study

ABSTRACT Maxillofacial prosthetic materials either resin or elastomer used for the fabrication of facial prostheses have their own physical and mechanical properties. Maxillofacial prosthesis frequent replacement because the elastomer or acrylic resin and its pigments undergo color changes. Purpose Evaluate the effect of outdoor weathering on color stability of silicone and heat-cure acrylic with two different pigments. Materials and methods Total of 80 circular disk specimens, 40 silicone elastomer and 40 heat-cure clear acrylic resin were fabricated in the prepared mold and two dry earth pigments burnt sienna and cobalt blue was added to the specimens and it was subjected to simulated outdoor weathering. Before and after weathering, CIE values of both control and weathering groups were recorded by using spectrophotometer and tabulated. The obtained data were subjected to statistical analysis. Results Silicone elastomer with burnt sienna showed more color change when exposed to weathering followed by silicone with cobalt blue. Heat-cure with cobalt blue had shown some color change, heat-cure and burnt sienna were the most color stable. Conclusion From the study, it can be concluded that heat-cure acrylic resin with burnt sienna most color stable than silicone elastomer. How to cite this article Lekha K, Kumar HD, Meshramkar R, Nadiger RK. The Effect of Outdoor Weathering on Color Stability of Silicone and Acrylic Resin, Pigments-A Comparative Evaluation: An in vitro Study. Int J Prosthodont Restor Dent 2015;5(3):81-85.

Experimental analysis of mixed mode crack propagation in brittle rocks: The effect of non-singular terms

The fracture behavior of Neyriz marble is investigated both experimentally and theoretically by using two different test configurations: (1) the center cracked circular disk specimen subjected to mode I loading, and (2) the edge cracked triangular samples under mixed mode loading. The experimental results show that there is a noticeable difference between the values of mode I fracture resistance obtained from these two test configurations. The aim of this paper is to justify this difference by using a modified maximum tangential stress criterion which takes into account the effects of non-singular stress terms (T-stress, A3 and B3) in addition to the singular terms. The same criterion is then extended to estimate the mixed mode fracture resistance of triangular specimen from the mode I fracture resistance obtained from the circular specimen. It is also shown that the criterion provides better estimates for the experimental data when uses A3 in the calculation of the fracture process zone length.

Impedance Spectroscopy Characterization of a Piezoresistive Structural Polymer Composite Bulk Sensor

Abstract In this study, self-monitoring characteristics of a new piezoresistive structural polyester polymer composite bulk sensor were investigated under various loading conditions using impedance spectroscopy (IS). IS was used to estimate the bulk resistance of the piezoresistive polymer composite and the contact resistance of the two-probe measuring system used, along with the stress and shape dependence of these resistances. The piezoresistive behavior of cylindrical specimens in compression and circular disk specimens in bending and splitting tension showed repeatable characteristics. The contact resistances were about 1 % and 4.4 % of the bulk resistances for the cylindrical and circular disk specimens, respectively. The contact resistances were relatively small compared to the bulk resistances, and the change in contact resistance with applied stress was quantified. The average piezoresistivity coefficient for cylindrical specimens was 0.015 MPa−1 in compression, and for the circular disk specimens the coefficients were 3.1 × 10−4 MPa−1 and 4.62 × 10−4 MPa−1 under bending and splitting tensile loading, respectively. An incremental stress-resistivity model was used to predict the observed behavior of the piezoresistive polymer composite.

New calibration method for high and low triaxiality and validation on SENT specimens of API X70

The determination of the exact mechanical properties of material is essential for an optimal and safe design of linepipes. It is especially important for the prevention of over-engineering and the reliable assessment of complex accidental loading, such as extreme bending due to loss of buoyancy of support, or abrupt ground movement. Currently, the focus of research in offshore deepwater installations and linepipes is towards pre-cracked structures with high triaxiality stress states and complex loading histories. At the same time, low triaxiality stress states must be correctly studied in order to represent shear dominated failure in pipes. A comprehensive experimental and numerical program was undertaken to determine the mechanical properties of the traditional API X70 grade of steel. The material was characterized for anisotropic plasticity, fracture initiation and uncracked ductility for various states of stress. The same material was also used for pre-cracked fracture toughness assessment. The experimental program included flat and round specimens. The first type of tests on flat butterfly-shaped, central hole, notched and circular disk specimens; were selected to address the low stress triaxiality range. Tests on round notched bar specimens and SENT fracture mechanics tests extended the characterization and verification process to higher stress triaxiality values. This program covered a wide range of stress conditions and demonstrated their effect on the material resistance to crack extension. Each test conducted was numerically simulated using solid finite element models, matching the exact geometric and loading history features. The numerical simulation provided information on the local stress and strain fields around the location of the potential or existing cracks. Based on the above hybrid experimental/numerical technique tailored for pipe applications, the MMC fracture model was calibrated. The model relates the material ductility not only to stress triaxiality but also to the Lode parameter. The predictive capabilities of the MMC were then evaluated in the case of SENT testing, used extensively in the pipeline industry. It was shown that the present fracture model calibration can describe fracture behavior of SENT experiments.

Size and Geometry Effects on Rock Fracture Toughness: Mode I Fracture

In this paper, the effects of specimen size and geometry on the apparent mode I fracture toughness (K c) of an Iranian white marble (Neyriz) are studied. A number of fracture tests were conducted on center-cracked circular disk (CCCD) specimens with different radii to investigate the size effects on K c. The experimental results demonstrate that the apparent fracture toughness increases in bigger specimens. In order to explain the experimental results, the modified maximum tangential stress (MMTS) criterion is used, where higher order terms of the Williams’ series expansion are included in the maximum tangential stress criterion. It is shown that the MMTS criterion provides good estimates for the apparent fracture toughness of Neyriz marble, obtained from fracture tests of edge-cracked triangular specimens. It is, therefore, concluded that the proposed criterion is able to account for the size and geometry effects on the fracture resistance of rocks simultaneously.

Evaluation of the Role of Pores during Strength Testing in Compacts Made from Different Particle Size Fractions of Sucrose

The purpose of this study was to investigate the role of pores in the fracture of circular compacts and to predict compact properties and critical crack lengths. Four different particle size fractions of sucrose, ranging from 20 to 500 microm, were compressed into circular discs (i.e. flat tablets) and rectangular beam specimens of porosity between 30 and 14%. Modelling of the relationship between the tensile strength of the circular discs and the compact porosity indicated extensive fragmentation during compaction for particles in the size range of 250-500 microm, accompanied by a change in densification mechanism for very coarse particles (355-500 microm). When determining the critical stress intensity factor from rectangular single edge notched beam specimens by 3-point bending, an apparent influence of particle size on the values could be seen, whereby here the results indicated that the critical particle size for fragmentation to occur is about 20-40 microm. It was possible to predict the critical stress intensity factor of the compacts from the median pore size and the tensile strength of the circular disc specimens by interpolation of the critical crack length for propagation to occur. The results indicated that for sucrose compacts regardless of their porosity, the pores themselves acted as stress concentrators, not as sharp cracks. For sucrose compacts, crack propagation hence proceeds most likely along grain boundaries.

Measurement of in-plane elastic moduli of composites with a circular disk specimen and piezoelectric sensors

Many test specimen configurations and test methods have been used for measuring the in-plane elastic constants of orthotropic composites. While the measurement of the Young's modulus is straightforward, the shear modulus determination is more difficult. Most of the experimental methods require more than one specimen for the measurement of all the in-plane elastic constants. In the proposed method, a single test specimen in the form of a circular disk is sufficient. The Young's modulus and the shear modulus are measured with piezoelectric sensors that produce and detect dilatational and shear waves, respectively. The experimental techniques involved and the possible methods of interpreting the results are explained. The results obtained were compared with those determined for bar specimens of the same material, using piezoelectric sensors and strain gauges. The comparisons are encouraging. POLYM. COMPOS., 26:542–551, 2005. © 2005 Society of Plastics Engineers

Determination of Elastic Constants of Composites with a Circular Disk Specimen

The in-plane elastic constants of orthotropic composite materials need to be determined in order to perform stress analysis and design of composite structures. Several specimen configurations have been proposed and the majority suffer from one of two drawbacks: either multiple specimens are required or the experimental method involves a scientific rather than an engineering approach. In the present investigation a simple procedure involving a single calibration specimen is suggested. The method utilises a theoretical solution available in the literature for an orthotropic circular disk under diametral compression, measured strains at specific locations on a circular disk specimen, and a least squares procedure that gives average rather than localised elastic constants.

Measurement of thermal diffusivity of isotropic materials using a laserflash method and its validation by finite element analysis

Two experimental arrangements have been used to measure the thermaldiffusivity at room temperature using laser pulse heating. One uses circulardisc specimens and the other rectangular plate-like specimens to measure bothone-dimensional and three-dimensional heat transfer. In both experimentalconfigurations a pulsed laser beam which has a circular cross section isirradiated onto the front surface of the specimen. The time dependence of thetemperature response of a circular area on the back surface of the specimen ismeasured using a radiation detector. The experimental data is then used tocalculate thermal diffusivity using appropriate analytical expressions. Thetheory of these methods is outlined and results are discussed. In addition,finite element models have been generated for one-dimensional andtwo-dimensional transient thermal analysis of the specimens. These results areshown to be in close agreement with the results of experiments within thebounds of experimental error, hence, validating the experimentalmeasurements.

Antimicrobial abilities of various dentine bonding agents and restorative materials

Objectives: The purpose of this study was to observe and measure the in vitro effect of various composite restorative materials and dentine bonding agents on the growth and adherence of oral bacteria believed to be responsible for recurrent caries in humans and on micro-organisms commonly used to evaluate the effectiveness of disinfecting agents. Methods: Five sets of dentine bonding agents and composite resins and ten species of micro-organisms were used. Circular disc specimens of each composite set were placed onto inoculated plates. Zones of growth inhibition around specimens were measured after incubation. On other plates, specimens were placed alone for 48 h, removed, and then the micro-organisms added. Also, the composite sets were placed into sterilized bovine incisors and suspended into sucrose-containing broth inoculated with Streptococcus mutans for 3 days. Adhering materials were disclosed and scored. Results and conclusions: Four of the composites sets produced statistically similar ( P > 0.05) inhibitory zones. The Gram-negative rods and the Staphylococcus aureus were the most resistant micro-organisms. The five composites sets produced the same ( P > 0.05) reduction in bacterial accumulation (> 60%). Aging of the specimens in water for periods up to 4 weeks prior to exposure to S. mutons did not affect product activity.