A Constant Acoustic Impedance Mount for Sheet-Type Specimens in the Tensile Split-Hopkinson Bar

Abstract

This paper addresses a problem well-known amongst practitioners of the Split-Hopkinson Tension Bar method: attaching a flat test specimen made from sheet material to the cylindrical input-and output bars. To date, slotting the bar ends and gluing the specimens into these ends with high-strength adhesives is the gold standard. However, this approach is not universally applicable because some materials are difficult to bond, and the adhesion surface is limited by the bar diameter, meaning that only small width specimens can be tested. In contrast, the hitherto published mechanical clamping mechanisms typically introduce excessive additional mass into the Split-Hopkinson system which detrimentally affects wave propagation and thus causes errors in the stress-strain signals. We circumvent this problem by designing a mechanical clamping device which has the same acoustic impedance as the bar material and is suitable to securely attach specimens with a width larger than the bar diameter. The benefits of our new clamping device are demonstrated by reporting tensile stress-strain data for Polycarbonate at high strain rates. The data is free from unwanted oscillations and enables accurate determination of dynamic strength and stiffness.

Mechanical Behavior of Multi-Material Single-Lap Joints under High Rates of Loading Using a Split Hopkinson Tension Bar

“…Often, an external thread is used to keep the specimen in position during the experiment [29,30]. Different mounting methods have been used for an SHTB, but most investigations conducted experiments with symmetric flat specimens which consisted of one material only [30][31][32][33][34]. It is important that all mounting devices enable the transfer of high loads to the specimens and do not disturb the elastic waves.…”

“…A disturbance of such elastic waves happens when the impedance changes. The impedance of a structure depends on the cross-sectional area (A), the density (ρ), and the sound velocity (c) of the material, which leads to Equation (1) [34]:…”

“…Following the investigation of Ganzenmüller et al [34], a constant impedance mounting device was developed to minimize such impedance changes. This device was made of the same material and diameter as the bars (titanium), and it was designed with a constant cross-sectional area from the beginning of the interface between the bars and the device to the end of the mounting device.…”

A Simple Glassy Polymer Model

“…Elevated strain rate testing at [1120, 1760, 2300]/s was performed with a split-Hopkinson tension bar (SHTB) with 16 mm diameter aluminium bars, the setup of which is described in detail in [9]. The sheet-type specimens are mounted to the bars with a specialized clamping device which does not detrimentally affect wave propagation as shown in [10]. For the SHTB, strain measurement is performed locally on the specimen with an optical extensometer at an acquisition rate of 200 kHz.…”

“…Now, each histogram bin contains between 1 and N samples, which allows the definition of an average and an uncertainty estimate, for which we use the discrete standard deviation. The amount of scatter between the individual experiments is very low, see [10] for details. To reduce clutter, the individual experimenst are not shown here.…”