Figure 1: Diagnostic Schematic of the Experimental Set-Up

http://www.ccm.udel.edu/reports-pubs/Spring02-Reviews/April24th/Gama/Gama.ppt

Figure 2: Striker Bar Assembly

Figure 3: Furnace Assembly

Figure 4: Physical set up and wave signals plotted as a function of time

http://www.tut.fi/units/ms/elm/laitteet/hopkinson(eng).htm

The Split Hopkinson Pressure Bar experiment is used to determine material properties under extreme loading conditions, particularly for materials under high pressure and high strain rate using induced wave propagation in along metallic elastic bar to measure the pressure produced during dynamic testing.  Mechanical engineers, and more specifically materials scientists, needed a testing method to gather material property information under extreme loading conditions such as fracture toughness, compressive and tensile strengths.  Bertram Hopkinson began development of the Split Hopkinson Pressure Bar experiment around 1914.  He studied the shape and evolution of stress pulses as they propagated down long rods.  This method for gathering property information has been used and widely accepted since the 1950’s.

        The Split Hopkinson Pressure Bar experiment is mainly composed of two separate Hopkinson pressure bars placed in series, with the sample to be tested sandwiched in between the two bars.    This physical set-up was however pioneered by Davies and Kolsky.  A striker bar, shown in figure 2,  is powered to strike the incident bar, which in turn sends an incident wave to the sample. The striker bar is accelerated by the means of a driving force supplied from a torsion bar spring mechanism.  The striker is first positioned in a yoke assembly and drawn back by the means of a hydraulic piston connected to the yoke by a shear pin. When the pin shears the torsion spring propels the striker bar to impact axially the incident bar.  Upon contact with the sample, part of this wave will continue through the transmitted bar and the other part will reflect back through the incident bar.    It is also required that the bar and the specimen be composed of the same materials and equivalent diameter.  Therefore upon impact a pressure pulse of approximately constant amplitude and of finite duration is obtained.  In order to the pressure bar data strain gauges are mounted  50 inches from each other to measure the strain-time histories of the incident , transmitted and reflected pulse.  Where the incident and transmitted wave signals represent the compressive loading and the reflected represents the tensile loading.  This data using the wave signals as a function of time is then sent to the signal conditioner and amplifier, then to the oscilloscope, and finally read into a specifically developed software interface for data analysis to determine the forces and velocities at the two interfaces between the pressure bars and the specimen can be determined.  A representation of this can be seen in figure 4.  The bar itself is held in place by the use of bushings that does not impede bar motion.  A split tube furnace, shown in figure 3, is also utilized for material testing on the Split Hopkinson pressure bar at elevated temperatures.  The furnace  is used to heat the specimen as well as a portion of the bars in axial contact with the specimen.  It is hinged to allow for easy access to the specimen and has an operating range from 200 to 1200 degrees Celsius.  The Split Hopkinson Pressure Bar experiment can measure mechanical behavior in compression, tension, and torsion.  The general set-up for this experiment is shown below in Figure 1.