The device installed is a direct tension/compression version of the so-called “Hopkinson bar”. It is a total of 15 meters long and consists of a 100 kN electromechanical preload jack, a 3.0 m long pre-tensioned bar, a 7.5 m long input bar, and a 4.0 m long output bar; at the end of the output bar there is a terminal block to stop the motion of the bars for safety reasons. Movement and acquisition is managed by a PC and by a National Instruments 16-bit card with simultaneous acquisition on 4 channels (M series). By exploiting the propagation of elastic waves in the bars, the device allows to perform compression and tension tests on specimens having dimensions (Dxh) from 5×5 to 20×20 mm at a strain rate typically varying from 102 to 104 s-1. The generation of the load wave is produced by the breaking of a sacrificial element, and can take place directly in the form of tensile or compressive wave. Using the same supports and the same jack, 4 different variants of Hopkinson bar can be used: depending on the material you want to test, you can use steel bars (17-4 PH), titanium (Ti-6Al4V), aluminum (7075T6) and plastic (PET). In this way, testing campaigns were conducted on many materials, from high-strength steel to polymers and elastomers, foams and low-density natural materials. The laboratory has a high-speed camera (Photron SA4), capable of acquiring up to 100,000 images per second; it is used to observe the evolution of failure and deformation in the sample during the test; the DIC technique can also be used for this purpose.
The Hopkinson bar allows to perform analyses and research concerning the dynamic behavior of materials and their sensitivity to strain rate. Effects typically found, but which must be studied on a case-by-case basis, can be an increase in resistance to increasing strain rate and reduction of deformation at break. For polymers, a ductile / fragile transition is often observed with increasing velocity.