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High Strain Rate Tension Tests

The Dynamic Mechanics of Materials Laboratory employs a direct tension split Hopkinson bar (SHB) for high strain rate tension tests.  The apparatus is made of two 7075-T6 aluminum bars with diameter of 0.5 in. The incident and transmitter bars are 12 ft. and 5.5 ft. long, respectively. The loading wave is generated by the release of a static tensile force that is initially clamped in the incident bar, see Figure 1. The waves during a test are measured at two locations on the incident bar (Gages A and B) and in one location on the transmitter bar (Gage C), see Figure1. At each location four 1000 Ohm strain gages are connected in a full Wheatstone bridge circuit configured to measure axial load.  The tension SHB apparatus at the Dynamic Mechanics of Materials Laboratory is shown on the left side of Figure 2.

A wide array of specimen geometries can be tested with this apparatus.  Typical specimen geometries include flat or axisymmetric specimens with a short, constant cross-section gage.  Notched flat and axisymmetric samples can also be tested to introduce multi-axial stress states.  The specimens are typically fastened to the bars using a high-strength, two-part epoxy.  Custom-designed mechanical fixtures can also be used for efficiency. Typical strain rates of 400 to 2000 s-1 are achievable with this apparatus. 

Click here for sample data from the tension SHB apparatus.

Figure 1. Sketch of a direct-tension split Hopkinson bar apparatus.

Figure 2. Direct-tension and Torsion split Hopkinson bars in the Dynamic Mechanics of Materials Laboratory.

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

Staab, G. H., Gilat, A., “A Direct-Tension Split Hopkinson Bar for High Strain-Rate Testing”, Experimental Mechanics, Vol. 31, 1991, pp 232-235.