tested to 0.5 strain if a metallic bar (c¼5000 m/s) was used. Because modified servo-hydraulics are capable of strain rates up to 100/s, this technique could provide enough overlap for complete testing throughout the intermediate strain rate regime if a laboratory had acquired both systems. 22.2.2 The Serpentine Bar Approach Figure 22.2 shows the setup of a serpentine, or folded, bar that can provide increased time duration to achieve large strains. A serpentine bar has the advantage over a conventional long bar in the since the stress wave, propagating from the sample, can be transferred into a series of tubes. These tubes are impedance matched to the original solid bar to eliminate the reflection due to the added tubes, and the joint is made small and stiff to reduce the reflections at the joint. Tubes have been successfully used previously to trap the stress wave energy in recovery Hopkinson bar setups [16]. The recovery Hopkinson bar uses a tube that is located near a flange on the free end of the bar to allow a certain amount of bar movement before the stress wave enters the tube and is trapped from returning to the specimen. Furthermore, this setup has been shown to transmit the stress wave very well when careful consideration is taken in designing the transfer flange. Here, we adapt this concept for increasing the stress wave possible in a given bar length, rather than trapping a shorter stress wave inside a detachable tube. The main difference in our setup is that in the serpentine bar, a series of tubes are rigidly connected at alternating ends of the bar. The setup shown in Fig. 22.2 shows a serpentine bar with two attached tubes that multiply the effective length of the bar by a factor of three. If manufacturing techniques permit, any number of tubes can be attached to increase the effective length of the bar. With attachment of this bar to a direct Hopkinson bar loading system as the transmitted bar, or to the fixed end of a servo-hydraulic load frame, elimination of high frequency ringing with long time duration can be achieved efficiently in a bar of substantial reduced length. 22.3 Prototype Fabrication A serpentine transmitted bar with a single attached tube, designed for compression testing, was selected to provide a conceptual demonstration. This setup was also compared directly to a longer bar of equal effective length, detailed in the Experiments section. The serpentine bar was built using a 15 mm diameter rod of 1.5 m length with a 1 m long tube (giving an equivalent length of 2.5 m), both being the same grade 350 maraging steel. The diametrical gap between the tube and the rod was 1.5 mm and the tube was machined to match the effective cross sectional area of the solid rod. Joining the bars was performed using a gas-tungsten arc welding (GTAW) process without filler. Threaded connections between the bars were also attempted but the load did not transmit smoothly through the joint. Bronze rings were placed in the gaps in between the bar and tube at 300 mm intervals to ensure rigidity of the bar inside the tube. Two gage stations were placed near the specimen approximately 300 mm apart from each other with the closest strain gage 100 mm from the sample to provide load sampling and monitoring of any reflections that may occur. The entire bar was then mounted on a SHPB frame in a similar fashion to conventional Hopkinson bar mounting. Images of the fabricated serpentine bar are shown in Fig. 22.3. The physical limitations of making a serpentine bar became clear during the fabrication of the prototype. If each tube would have the same characteristic impedance, i.e. cross sectional area, then the tube’s thickness becomes very small as the number of tubes increases. A practical limitation of two tubes for a 12–15 mm diameter bar was observed during the project. However, and initial larger diameter rod would allow more attached tubes. A 25 mm diameter rod may be able to achieve four tubes. Fig. 22.2 Setup of our proposed serpentine transmitted bar loaded in compression, with two strain gage stations (blue), the specimen location (white), and bushings (grey) 22 Robust Intermediate Strain Rate Experimentation Using the Serpentine Transmitted Bar 169
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