Split Hopkinson Pressure Bars

We have recently completed the construction and commissioning of a new air-bearing based Split Hopkinson Pressure Bar (SHPB) as part of the HexMat EPSRC programme grant. Designed with 6 mm diameter maraging steel bars and having a 50 cm long striker it is capable of supplying a 140 μs loading pulse to millimetre sized metallic samples, see figure 38.

Designed specifically for precision measurements of low strain rate (102 to 104 s-1) deformation behaviour in titanium alloys (IMI834, Ti6242, Ti6246), the long loading pulse ensures a high global strain is reached. Due to their excellent strength and lightweight properties these alloys are common throughout the aerospace industry where they have uses in blades, rings, and discs as well as in airframes and structural components.

This particular set-up allows the direct investigation into the formation of adiabatic shear bands, a widely encountered phenomenon in metals subjected to high strain rate loading and often resulting in catastrophic failure from brittle-like fracture. Adiabatic shear bands often occur during foreign-object impact, where a hard object impacts the rotating fan blades at high speed. The velocity of impact prevents thermal conduction leading to an increase in temperature. At a macroscopic level shear bands arise from local softening caused by increased temperatures in the region of plastic flow.

With the ability to test millimetre sized samples with high strength we are able to study the shear behaviour within single grains of Ti-64. By creating top-hat samples where a single grain is localised in the region of high shear the effects of heterogeneity and differing crystallographic orientations can be tested. See figure 39.

]Design and construction of the new bar was completed between March and June in collaboration with ERASMUS students, Quentin de Menech and Alexandre Wirtzler from the National Engineering School of Metz (ENIM). The success of the project has led to future students from ENIM joining the ISP. The relatively compact size of the apparatus also allows transportation to large external facilities such as synchrotrons where X-ray imaging of the internal structure of the targets can be performed.

This new facility complements our previously established SHPB apparatus used by both the ISP and the CBIS. Projectile velocity is measured using a photodiode array whilst wave profiles within the (Titanium or Inconel) bars are monitored using type AFP 500-90 semiconductor gauges which are manufactured by Kulite Semiconductor Products INC. The complimentary rail and precision rail-slider design offers flexibility to the system and for bars of differing length and material composition to be interchanged with ease.