The Split-Hopkinson Pressure Bar allows for
dynamic deformation at high strain rates (103-104 /s). The SHPB
in our lab is a compression testing rig, though
plans are being made for a tensile testing apparatus. The schematic of the basic Hopkinson bar setup shows two long
cylindrical
bars of the same diameter, called the incident and transmitted bars. In a
compression test, the sample sits sandwiched between the incident and
transmitted bars. A striker bar of the same
diameter is propelled using a gas gun so that it strikes the incident bar
squarely on the end. This generates a compression wave which travels down the
incident bar. Part of the wave is reflected and part is transmitted through the
interface with the specimen. The stress pulse continues through the specimen and
into the transmitted bar. The incident stress pulse and transmitted stress pulse
are measured in real-time using strain gages on the incident and transmitted
bars. If the two bars remain elastic and wave
dispersion ignored, then the measured stress pulses can be assumed to be the
same ones acting on the sample. One could procure the stress strain curves from
this. The high frequency transducers and the digital storage oscilloscope help
in acquire and storing data at high rates. We will use the technique to
deform materials at high strain rates and at different temperatures and up to
different strains. The deformed samples are then characterized will be
characterized using the TEM to facilitate the understanding of deformation
mechanisms.
The last stages of the fabrication are underway - the
maraging steel bars are being aged and machined. The gas gun is firing as
designed.
Some images of the SHPB apparatus are shown below.
