Analysis of the diametral compression method for determining the tensile strength of transparent magnesium aluminate spinel

Attempts were made to determine the inherent tensile strength of a coarse-grained, hot-pressed magnesium aluminate spinel (MgAl₂O₄) using the diametral compression test. Thick (9.6 mm) disk specimens were machined from a large (356 mm square) plate of spinel. Two pairs of tungsten carbide (WC) platens, one with flat surfaces and the other with a 20° half-arc and radius matched to the disk diameter, were used to transfer the applied load. Specimens tested using the platens with the matched radius had strength values almost 50% higher than those tested using flat platens. Images of the fracture process captured using a high-speed camera showed that irrespective of the type of platens used, fracture consistently initiated at the loading interface, resulting in an invalid test. These results show that the diametral compression test method is not appropriate for determining the tensile strength of this spinel and it raises concerns about the applicability of the method for any advanced ceramic.     

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A multielectrode catheter for esophageal electrocardiography, using the cardia as point of fixation and reference, was tested in 10 normal adults. The ECG configurations derived from simultaneous multiple unipolar and bipolar leads in normal sinus rhythm are delineated. The bipolar recordings are particularly informative as they demonstrate the separate depolarization of each atrium, the distribution of atrial potentials in the sagittal plane with an obligatory shift in polarity at the midatrial level, and the pattern of left atrial depolarization. No previous report seems to have compiled these distinctive features.     

Enhanced Sintering Kinetics in Aluminum Alloy Powder Consolidated Using DC Electric Fields

Direct current (DC) electric currents were applied during sintering of aluminum alloy (AA5083) green powder compacts and it was found that the kinetics of sintering were greatly enhanced compared to samples processed without a field. In situ sintering kinetics during pressure-less sintering employing electric field strengths and amperages ranging from 0 to 56 V/cm and 0 to 3 A were quantified using digital image correlation. It was found that the application of a DC field during sintering results in a discontinuous change in volume at a critical temperature along with a transition in electrical properties of the compact from insulating to conductive. This effect is similar to the phenomena observed in the flash sintering process currently being actively researched for ceramic powder processing. The temperature at which the flash event occurs was found to be field strength dependent and doubling the field strength was found to decrease the flash temperature by 25 pct. Joule heating of the specimen was measured using thermal imaging and it was found to not contribute enough additional thermal energy to account for the substantially increased sintering rates observed in specimens processed using electric fields.

     

Equibiaxial Flexure Strength of Glass: Influence of Glass Plate Size and Equibiaxial Ring Ratio

Glass plays an important role in many engineering applications including transparent armor. Determining the actual strength of a glass is problematic because typical glass strength numbers are presented in the literature as ranges. Minor changes in surface finish, composition, test method employed, and processing all influence the mechanical strength of glass. A standardized method needs to be utilized to ensure that strength data can be compared universally. The use of a standard methodology is vital when comparing glass compositions or multiple sample lots of the same composition in order to attribute any real strength variations to the glass itself. While the procedures established in ASTM C1499 provide a good framework for glass strength measurement, flexibility in the experimental parameters of the standard allow for substantial strength variations when testing glass materials. The ratio between the load and support ring diameters, as well as the specimen dimensions, is shown to lead to variations in both the measured strength, as well as the number of valid/invalid tests based on where fracture initiates. The goal of this study is to quantify these effects in order to establish optimized testing conditions for determining the strength of float glass.