Analysis of Bending-Induced Uncertainty in Dynamic Tensile Strength Measurements Using Theta Specimens

Abstract Measurement of the tensile strength of brittle materials faces experimental challenges related to specimen gripping and alignment. Because of these challenges, indirect methods such as the Brazilian disk specimen, which convert compression loads into tensile stress states via geometry, are commonly used in both quasistatic and dynamic loading regimes. The majority of these indirect methods subject the material to a non-uniform multiaxial state of stress, however, the thin central web of “theta" specimens results in a uniform state of uniaxial stress over the majority of the gage section of the specimen. Prior work, which used numerical simulations to evaluate the use of “theta" specimens under dynamic loading conditions, showed the dynamic loading could induce transverse oscillations in the central web of the specimen, disrupting the state of uniaxial stress. In the current work, an existing theta specimen geometry is adapted for use in Kolsky bar experiments to achieve a strain rate of $$\sim$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 400 $$\hbox {s}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>s</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> in PMMA specimens. High-speed images captured during experiments confirm the presence of transverse oscillations in the central web of the specimen, which perturb the assumed uniaxial state of stress and add uncertainty to the measured material strength. These oscillations are analyzed analytically, and the results are presented in a non-dimensional form allowing for the bending-induced measurement uncertainty to be quantified for a range of materials and experimental parameters. The experimental results and corresponding analysis indicate that the theta specimen combined with Kolsky bar loading provides a viable methodology for conducting high rate tensile tests on brittle materials.