The use of the direct impact Hopkinson pressure bar technique to describe thermally activated and viscous regimes of metallic materials
The influence of strain rate over domains involving the thermal activation and the viscous drag behaviour of the dislocations is discussed. While it is recognized that the Koslky–Hopkinson technique or split Hopkinson pressure bar technique can generate data up to the upper strain-rate limit of the thermal-activated regime, it is necessary to use a direct impact Hopkinson pressure bar technique to access the viscous regime. Data generated with this technique are presented for a series of metals, including steel, nickel, copper and tungsten alloys. The motivation to generate such data is provided through three industrial applications.
Application of photon Doppler velocimetry to direct impact Hopkinson pressure bars.
Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion, and faster achievement of force equilibrium. Currently, these advantages are gained at the expense of all information about the striker impacted specimen face, preventing the experimental determination of force equilibrium, and requiring approximations to be made on the sample deformation history. In this paper, we discuss an experimental method and complementary data analysis for using photon Doppler velocimetry to measure surface velocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system. We discuss extracting velocity and force measurements, and the precision of measurements. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains in fully dense metals, and improvement for all strains in slow and non-equilibrating materials.
Symmetric Taylor testing procedures for material strength ranging from 400 to 2000 MPa
Symmetric Taylor testing procedures to evaluate dynamic mechanical properties under uniaxial compression of a variety of materials including aluminum alloys, tungsten alloys and steels are described. For material strength greater than 800 MPa, symmetric Taylor testing involved Teflon sabots deforming along with the Taylor specimens. For low strength materials, 400-800 MPa, the test is conducted with Teflon ring supports that can be easily crushed. The testing procedures have been validated though numerical simulation of tests conducted with a tungsten alloy and an aluminium alloy.