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High Strain Rate Torsion Tests

The Dynamic Mechanics of Materials Laboratory uses a torsion split-Hopkinson bar apparatus that is used to investigate the high strain rate behavior of materials subjected to shear stress-states.  The apparatus is comprised of two, 0.875 in diameter 7075-T6 aluminum bars. The incident and transmitter bars are 12 ft. and 7 ft. long, respectively. The torsion loading wave is generated by the release of a torque that is initially clamped in the incident bar, see Figure 1. The wave test data are measured at two locations on the incident bar (Gages A and B), and at one location on the transmitter bar (Gage C), see Figure 1.  At each location four 1000 Ohm strain gages are configured in a full Wheatstone bridge circuit for torque measurment. The torsion SHB apparatus at the Dynamic Mechanics of Materials Laboratory is shown on the right side of Figure 2.

The specimens are flanged thin-walled tubes (shaped like spools) which are fastened to the bars using a high-strength, two-part epoxy. Typical shear strain rates of 400 to 5000 s-1 are achievable using this apparatus.  The torsion SHB has two advantages over compression and tension SHBs.  First, shear waves are not susceptible to dispersion (the wave changes shape as it travels down the bar).  Second, radial inertia effects in the specimen, for example necking in a tension specimen and "barreling" deformation in a compression specimen, do not effect the experimental data.

Click here to view sample experimental data from the torsion SHB apparatus shown in Figure 2.

Figure 1. Sketch of a torsion split Hopkinson bar apparatus.

 

Figure 2. Torsion and tension split Hopkinson bars at the Dynamic Mechanics of Materials Laboratory.

 

More detail concerning this experimental technique can be found in the following reference:

Gilat, A. “Torsional Kolsky Bar Testing”, Mechanical Testing, Vol. 8, ASM Handbook, ASM International, 2000.