Chapter 21 Dynamic Compressive Tests of Alumina Dumbbells Using a Spherical Joint Steven Mates, Richard Rhorer, and George Quinn Abstract The dynamic compressive strength of ceramic armor materials is difficult to obtain experimentally due to the sensitivity of fracture strength to small misalignments, end effects, and surface effects. In this work we introduce a spherical joint into a compression Kolsky bar to investigate whether the joint can alleviate bending stresses due to minor misalignment in compression tests on alumina dumbbell specimens. Tests are conducted both with and without the spherical joint, and high-speed (75,000 frames/s) three-dimensional Digital Image Correlation (3D DIC) is used to measure both the strain field on the dumbbell specimen and motion, if any, in the spherical joint. Results indicate that the spherical joint is extremely sensitive to eccentric loading and in most cases increases rather than decreases the bending stresses in the test, leading to lower apparent fracture strengths. Keywords Kolsky bar · Dynamic fracture strength · Dumbbell specimens · 3D DIC · High-speed video Introduction The design of ceramic armor systems requires precise knowledge of the dynamic mechanical behavior of the ceramic armor materials, including dynamic compressive fracture strength, which is typically measured using a Split Hopkinson Pressure Bar, or Kolsky Bar [1]. The fracture strength of brittle materials is very difficult to measure accurately because of the extreme sensitivity of the test results to mechanical alignment, end effects, and surface effects. Researchers at the U.S. Army Research, Development and Engineering Command (RDECOM) have developed a dumbbell-shaped specimen to avoid premature fracture from end splitting due to tensile stresses developed during elastic punching, lateral strain mismatches, and/or friction that can occur when simple cylindrical specimens are used. As part of this research effort, the U.S. Army RDECOM organized a limited round robin test effort involving three laboratories. In this paper we report on the testing performed at the National Institute of Standards and Technology (NIST) as part of the limited round robin. Because dumbbell specimens often have much larger aspect ratios compared to usually short cylindrical or cubic specimens used for dynamic testing, they might be susceptible to premature failure from bending. Alignment of the Kolsky bar interface is therefore crucial if bending is to be minimized during dynamic compression testing of dumbbell specimens. Alignment quality is usually checked by doing Kolsky bar tests with no sample and with the bar ends pressed tightly together. Perfect alignment results in complete transmission of the elastic loading pulse and no reflected pulse. While this condition is achievable in practice, in the present study we investigated whether bending stresses caused by small misalignment of the interface could allow large axial loads to be transmitted while minimizing bending in a dynamic compression test of a ceramic dumbbell specimen. The present study compares fracture strength experiments performed with and without a spherical joint. The joint consists of a precisely-machined spherical interface placed on the transmission side of the specimen in Kolsky bar test. High-speed, three-dimensional digital image correlation (3D DIC) is used to measure the motion of the specimen and the platens during testing. From these measurements, specimen rotation is measured by comparing displacement vectors of the cylindrical ends of the dumbbell specimens with the original specimen axis. If the specimen rotates during the test, bending stresses are likely. The rotation observed in the DIC data are correlated to the apparent fracture strengths. Finally, the effect of the spherical S.Mates ( ) · G. Quinn National Institute of Standards and Technology, Gaithersburg, MD, USA e-mail: smates@nist.gov R. Rhorer Rhorer Precision Engineering, Gaithersburg, MD, USA © Society for Experimental Mechanics, Inc. 2020 L. E. Lamberson (ed.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-30021-0_21 119
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