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The Joint Research Centre (JRC) is the European Commission's science and knowledge service which employs scientists to carry out research in order to provide independent scientific advice and support to EU policy.
The HOPLAB facility is used for the study of materials and of structural components to very fast dynamic loads, such as those due to blasts and impacts, where knowledge of the material behaviour under high strain-rates is necessary. HOPLAB is the World’s largest Hopkinson bar, extending for a length of more than 200 meters. A rectangular force pulse of up to 2 MN, of rise time 250 µs and of duration 40 ms can be generated. The machine has a maximum stroke of 700 mm. In the basic configuration the incident and transmitter bars have a diameter of 72 mm.
The incident strain pulse (εI) is generated by pre-stressing and abruptly releasing a long bar (100 m), which is the continuation of the incident bar of the machine. This pulse propagates along the incident bar with the velocity of the elastic wave (5500m/s for steel), with its shape remaining constant. When the strain pulse reaches the tested specimen, part of it (εR) is reflected back whereas another part enters the specimen and exits into the transmitter bar (εT), loading dynamically the specimen to failure. The relative amplitudes of the incident, reflected and transmitted pulses, depend on the mechanical properties of the specimen material. With a proper elaboration of these three pulse records the stress-strain relation of the material for a particular strain-rate can be determined, which is then used for numerical simulations (see EUROPLEXUS).
Thus, large material samples or structural components and sub-assemblies can be accurately tested under the required dynamic conditions. According to the needs of the experimentation, other Hopkinson bar devices are also available, which complement the laboratory and are suitable for performing dynamic testing using small material specimens.