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Split Hopkinson Pressure Bar

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(Right) Actual image of assembled SHPB.
(Left) Final SHPB SolidWorks model prior to assembly

What is a Split Hopkinson Pressure Bar?
A Split Hopkinson Pressure Bar (SHPB or Kolsky bar) is an apparatus used to characterize the unique stress-strain response of materials during dynamics loading. They make use of wave mechanics, measuring the part of a stress wave produced by a striker which is not absorbed by a sample to work backwards and determine the effects felt by

a sample material. The stress wave is measured by two high sensitivity strain gauges on identical bars sandwiching the sample and is shown schematically to the right. These devices are sold commercially for around $20,000 but with a research group at Northeastern University,

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Schematic overview of SHPB apparatus

I have been working to design and construct a miniature SHPB in order to characterize the stress-strain response of polymers and metals during loading achieving a strain rate in the order of 1000-4000 s^-1 for 20% of this price.

My research:

Each SHPB must be custom made for a desired family of materials (metals, polymers, ceramics...etc) and a target strain rate range. Typically, SHPB apparatuses achieve strain rates in the order of 100 – 1000 s^-1 and are around

20m in length but my research group is interested in characterizing strain rates several magnitudes greater than this.

My tasks as part of this group has been to learn the relevant theory behind the SHPB and develop a method to allow for our desired high strain rates. The method that I have worked for the past year to develop requires the device to be shrunk from 20m to < 5m in total length to impart a shorter incident pulse. In doing so I have developed what is known as a miniature SHPB because it is miniature. 

After months of research and parameter calculation I worked to develop an FEM simulation in Abaqus to model the device and determine the strain rates that will be achieved.

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Abaqus FEA models of SHPB system and anticipated stress response

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Machined triangular supports for bar axial alignment

Once all the ground work was done and all of our key parameters were set, I worked to produce the rest of the device: I developed a gas gun to accelerate a striker utilizing fluid mechanics knowledge, coded a velocity measurement system and launch system using Arduino, and made an in depth CAD model detailing every part of the apparatus in order to make sure that we were on track to start ordering and machining parts. Once our parts were machined (see left) we were able to start constructing our device.

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SolidWorks model of gas gun and velocity measurement systems

My group was able to get to Northeastern University’s Innovation Campus in Burlington, Massachusetts to start the assembly process of our device. The assembly took us a few days to complete before we moved into the more laborious task of calibrating the system. The SHPB functions based on a few key assumptions, one of which being the imparting of a one-dimensional wave pulse. To reach this idealized condition, I put a lot of effort into making sure of precise manufacturing tolerances for integral components, but the system still needs to be meticulously axially aligned to yield correct results. 

For a more detailed description of the work and calculation that went into the system design, feel free to look at this executive summary of  the SHPB apparatus that I wrote or a poster I presented at Northeastern SOURCE in 2021:

Me presenting SHPB poster at Northeastern SOURCE

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