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.
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.