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research-article  
Mahdi Saadati, Kenneth Weddfelt and Per-Lennart Larsson
J. Eng. Mater. Technol   doi: 10.1115/1.4039290
The focus in this work is towards an investigation of the fracture response of brittle materials with different specimen size loaded in diametral compression using different boundary conditions. The compacted zone underneath the loading points is assumed to be limited and only responsible for the load transition to the rest of the material. Therefore, the theory of elasticity is used to define the stress state within a circular specimen. A tensile failure criterion is used and the final load capacity is related to the formation of a sub-surface crack initiated in a probabilistic manner in a region in the vicinity of the loaded diameter of the specimen. This process is described by Weibull theory and it is assumed here that the growth of the sub-surface crack occurs in an unstable manner. Therefore the assumption in Weibull theory, that the final failure occurs as soon as a macroscopic fracture initiates from a microcrack is fulfilled. The concept of disc effective volume used in Weibull size effect is presented in a convenient way that facilitates the application of the model to transfer the tensile strength obtained from different methods such as three point bending and Brazilian test. The experimental results for Brazilian test on a selected hard rock is taken from the literature and a fairly close agreement is obtained with the model predictions.
TOPICS: Brittleness, Fracture (Materials), Boundary-value problems, Compression, Stress, Failure, Microcracks, Rocks, Size effect, Tensile strength, Disks, Elasticity
research-article  
Andrea J Felling and Darrel A. Doman
J. Eng. Mater. Technol   doi: 10.1115/1.4039291
Characterization of materials undergoing severe plastic deformation requires the careful measurement of instantaneous sample dimensions throughout testing. For compressive testing, it is insufficient to simply estimate sample diameter from an easily measured height and volume. Not all materials exhibit incompressibility, and friction during testing can lead to a barreled sample with diameter that varies with height. Video extensometry has the potential to greatly improve testing by capturing the full profile of a sample, allowing researchers to account for such effects. Common 2D video extensometry algorithms require thin, planar samples, as they are unable to account for out-of-plane deformation. They are therefore inappropriate for standard compressive tests which use cylindrical samples that exhibit large degrees of out-of-plane deformation. In this paper, a new approach to 2D video extensometry is proposed. By using background subtraction, the profile of a cylindrical sample can be isolated and measured. Calibration experiments show the proposed system has a 3.1% error on calculating true yield stress - similar to ASTM standard methods for compressive testing. The system is tested against Aluminum 2024-T351 in a series of cold upsetting tests. The results of these tests match very closely with similar tests from the literature. A preliminary finite element model constructed using data from these tests successfully reproduced experimental results. Diameter data from the finite element model undershot, but otherwise closely matched experimental data.
TOPICS: Deformation, Testing equipment, Testing, Finite element model, Yield stress, ASTM International, Materials characterization, Calibration, Errors, Algorithms, Friction, Aluminum, Dimensions
research-article  
Tarek Belgasam and Hussein Zbib
J. Eng. Mater. Technol   doi: 10.1115/1.4039292
Recent studies on developing dual phase (DP) steels showed that the combination of strength/ductility could be significantly improved when changing the volume fraction and grain size of phases in the microstructure depending on microstructure properties. Consequently, DP steel manufacturers are interested in predicting microstructure properties as well as optimizing microstructure design at different strain rate conditions. In this work, a microstructure-based approach using a multiscale material and structure model was developed. The approach examined the mechanical behavior of DP steels using virtual tensile tests with a full micro-macro multiscale material model to identify specific mechanical properties. Microstructures with varied ferrite grain sizes, martensite volume fractions, and carbon content in DP steels were also studied. The influence of these microscopic parameters at different strain rates on the mechanical properties of DP steels was examined numerically using a full micro-macro multiscale finite element method. An elasto-viscoplastic constitutive model and a response surface methodology (RSM) was used to determine the optimum microstructure parameters for a required combination of strength/ductility at different strain rates. The results from the numerical simulations are compared with experimental results found in the literature. The developed methodology proved to be a powerful tool for studying the effect and interaction of key strain rate sensitivity and microstructure parameters on mechanical behavior and thus can be used to identify optimum microstructural conditions at different strain rates.
TOPICS: Steel, Simulation, Mechanical behavior, Modeling, Optimization, Grain size, Ductility, Mechanical properties, Carbon, Constitutive equations, Design, Ferrites (Magnetic materials), Computer simulation, Finite element methods, Response surface methodology
research-article  
Yangchuan Li and Eric Fahrenthold
J. Eng. Mater. Technol   doi: 10.1115/1.4039293
Carbon nanotube based conductors are the focus of considerable ongoing experimental research, which has demonstrated their potential to offer increased current carrying capacity or higher specific conductance, as compared to conventional copper cabling. Complementary analytical research has been hindered by the high computational cost of large scale quantum models. The introduction of certain simplifying assumptions, supported by critical comparisons to exact solutions and the published literature, allows for quantum modeling work to assist experiment in composite conductor development. Ballistic conductance calculations may be used to identify structure-property relationships and suggest the most productive avenues for future nanocomposite conductor research.
TOPICS: Copper, Composite materials, Electrical conductance, Carbon, Design, Nanotubes, Electrical conductivity, Modeling, Carbon nanotubes, Nanocomposites
research-article  
Ahmad A Mousa, Gert Heinrich, Udo Wagenknecht and Omar Arrabeyat
J. Eng. Mater. Technol   doi: 10.1115/1.4037169
Exfoliated graphite (EXG) was prepared from commercially available natural graphite flakes (NGF), through strong acid treatment followed by thermal shock at 950 oC. The EXG sheets were characterized with respect to their thermal stability via thermo-gravimetric analysis (TGA) and Raman spectra. Their morphology and particle size were evaluated using scanning electron microscope (SEM) and particle size analyzer. The potential of EXG as reinforcement on the mechanical and thermal properties of the dynamically vulcanized polystyrene/styrene butadiene rubber (PS/SBR) composites were evaluated. The influence of EXG on the electrical properties of the composites was measured as well.
TOPICS: Composite materials, Thermal properties, Graphite, Particle size, Raman spectra, Thermal shock, Thermal stability, Electrical properties, Scanning electron microscopes, Styrene-butadiene rubber

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