Mechanical Designs
Strain Rates and Hopkinson Pressure Bar
Although standard quasi-static testing techniques are adequate for defining material characteristics under normal loading conditions, they cannot replicate the deformation that occurs at higher strain rates. The need to characterize material properties under higher strain rates is especially prevalent in situations where materials will experience high impact loading conditions, such as an impact loading due to a projectile that impacts a manned fighting vehicle. If a strain rate dependent material property is needed, it is important to collect this information for the strain rates associated with the desired application. For instance, if a micrometeoroid impact on a satellite is studied (where impact velocities are in the range of 10 km/s), rate dependent properties collected from a hydraulic servomechanism testing machine are impractical and unrealistic.
Bertram Hopkinson (January 11th, 1874 – August 26, 1918) was a British patent lawyer and a Professor of Mechanism and Applied Mechanics at Cambridge University. In 1914, the Hopkinson pressure bar was first suggested by Hopkinson as a method to measure stress pulse propagation in a metal bar. Later, Herbert Kolsky (September 22, 1916 – March 9th, 1992) a British physics education, refined the Hopkinson’s technique by using two Hopkinson bars in series. This is now known as the split Hopkinson pressure bar.
A split Hopkinson pressure bar is one of the most versatile testing apparatuses for determining the dynamic mechanical properties at high strain rates from 102/s to 105/s. When appropriately configuring the setup of a split Hopkinson pressure bar, testing of tension, compression, and shear specimens to generate the respective stress-strain curves are possible. Furthermore, testing of three-point bending and compact tension fracture toughness specimens to obtain dynamic fracture toughness values are possible.
Split Tensile Hopkinson Pressure Bar
A modified innovative split tensile Hopkinson pressure bar (SHTB) was designed using AutoDesk Inventor Professional, machined entirely at the University of Massachusetts Machine machine shop, and validated with 6061 aluminum specimens. This design uses a U-shaped striker bar as a projectile and offers several advantages over classical STHB designs such as 1.) longer stress pulse duration with respect to U-shaped striker bar length, 2.) ability for a wide range of pulse shaping methods, 3.) easy access to loading bar and striker bar, 4.) ability to fire with low air pressures, and 5.) efficient manual reloading.
Zoom in of Pressure Chamber Piston Pulling Rod
A pre-chamber is filled with helium to a corresponding pressure that is proportional to the desired strain rate. A polyvinyl chloride (PVC) hose connects the pre-chamber to the pressure chamber, with a solenoid valve between the two. The displacement of the piston within the pressure chamber follows a polytrophic process. In order to dynamically launch the U-shaped striker bar into the Impact flange, there needs to be a piston inside the pressure chamber. This piston is attached to a pulling rod with a flange hook, when compressed gas infiltrates the pressure chamber then the piston displaces. This in return accelerates the U-shaped striker bar towards the impact flange. As the volume increases due to displacement of the piston, this causes the piston to deaccelerate, in which the flange hook loses contact with the U-shaped striker bar.
Pressure Chamber Piston Pulling Rod
In order to dynamically launch the U-shaped striker bar into the Impact flange, there needs to be a piston inside the pressure chamber. This piston is attached to a pulling rod with a flange hook, when compressed gas infiltrates the pressure chamber then the piston displaces. This in return accelerates the U-shaped striker bar towards the impact flange. As the volume increases due to displacement of the piston, this causes the piston to deaccelerate, in which the flange hook loses contact with the U-shaped striker bar.
The pressure chamber piston pulling rod designed is depicted in exploded assembly gif and via the balloon-annotated section view.
The assembly of the pressure chamber piston pulling rod was done in two phases. The first phase was to assemble the pulling rod assembly while the pulling rod is inserted into the front-end cap. The use of 0.005” thick brass shims aided the installation of two or three round 014 size standard AS568 silicone O-rings. With the pulling rod inserted into the front-end cap and O-rings in position of the internal groove, then piston was secured to one end of pulling rod while the flange hook to the other. Both were fixed with a 1/2-13 locknut. The a round 242 size AS568 silicone O-ring was slipped onto position of the piston groove of the piston. The second phase was to mount the mount the end caps to the cylinder with the round 243 size standard AS568 silicone O-ring in position on each end cap face groove. Petroleum jelly was used to keep the O-rings in place during the assembly process, and Way oil was used to lubricate the inside surface of the pressure chamber cylinder to allow unrestricted movement of the piston. Similar designs rate the maximum internal pressure to be 180psi.