Stress gage system for measuring very soft materials under high rates of deformation

Abstract

Soft materials have seen continued growth in industrial importance, but are difficult to test at relevant, particularly at high, rates of deformation and relevant temperatures. This is mainly due to the low stresses supported by these materials, which mean that very sensitive force measurements are required. In this paper, a split-Hopkinson pressure bar method for testing very soft materials and elastomers at high rates of deformation is presented and applied. Experiments are conducted in compression on hydroxyl terminated polybutadiene, a very soft rubber, at strain rates of about 2000 s−1. Titanium alloy bars are used, and in addition to the usual strain gauges on the bars, forces at both ends of the specimen are measured using a piezoelectric material, lead zirconium titanate (PZT), which is much more sensitive than the quartz crystal gauges typically used in previous literature. The piezoelectric constant of PZT ranges between 290–630 × 10−12 C N−1, making it 100 times more sensitive than quartz crystal (2.3 × 10−12 C N−1). Results obtained from the experiments show that the gauges are able to measure the forces on both ends of the specimen with excellent signal to noise ratios.

High Strain Rate Mechanics of Polymers: A Review

“…2a, respectively. Typically, all three waves are measured using strain gauges mounted on the incident and transmitted bars; although quartz gauges [62][63][64] and interferometric techniques [59,65] have been used. Representative voltage-time curves are shown in Fig.…”

“…Increased sensitivity semiconductor strain gauges, which have a gauge factor *140 can be used to enable accurate measurement of small transmitted signals [74]. Additionally, piezoelectric gauges, such as x-cut quartz or lead zirconium titanate (PZT), can be placed in the experiment to provide enhanced sensitivity or to directly measure input and output forces [62][63][64]75]. Early experiments with quartz gauges placed them directly in contact with the specimen [63].…”

“…They found that the stress measurements using this modified system were three times more sensitive than a traditional steel bar. Kendall et al [64] utilized PZT crystals mounted on either side of the specimen near the ends of the incident and transmitted bars and were able to measure loads of less than 10 N on a soft, rubbery polymer.…”

“…However, rubbers were some of the first materials to be characterized in split Hopkinson bar experiments [1,2], and more recently a number of authors have proposed techniques to address these difficulties through modifications to the Hopkinson bar or other similar systems [57,66]. These include pulse shaping [62], low impedance Hopkinson bar materials to increase the transmitted force [76,77,81,82,148] or use of more sensitive force gauges to directly measure the force at the specimen bar interface [62][63][64]. Longer Hopkinson bars may be used to increase the duration of the experiment [53], as can direct impact systems [149], in addition, if stress gauges are combined with optical measurements of specimen deformation, the experiment duration is no longer limited by wave overlapping in the bars and longer durations can be achieved [150].…”

Experimentally Simulating High Rate Composite Deformation in Tension and Compression: Polymer Bonded Explosive Simulant

“…However, the very low strength of the HTPB prevented data from being obtained using this system, and instead a bar system was developed which used lead zirconium titanate (PZT) 1 stress gauges to directly measured the forces at the bar-specimen interfaces. This method for testing soft materials at high rates of deformation is an improvement to the Quartz crystal methodology of Chen et al [30], and is discussed further in in Kendall et al [31].…”