Politecnico di Torino - Corso Duca degli Abruzzi, 24 - 10129 Torino, ITALY

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Hopkinson Spring for dynamic characterization of materials at intermediate strain rate

Dynamic testingHopkinson springIntermediate strain rate


A novel system for performing dynamic testing of materials at intermediate strain rates. It is designed to generate and transmit tension or compression waves having a low propagation speed. The mechanical wave reaches the test specimen after travelling through a series of properly dimensioned springs, according to the Split Hopkinson Pressure Bar scheme or through a single long spring. The mechanical properties of the material are measured by means of a load cell located between the specimen and the end spring and a non-contact strain measuring system. The apparatus provides mechanical waves of high duration time, a long test stroke, and has a moderate overall encumbrance relative to the state of art.

Technical features

The apparatus structure, rearranging the Split Hopkinson Pressure Bar, allows the propagation of mechanical waves at low speed (50 ÷ 70 m/s versus 5000 m/s of the Hopkinson bars) and deforms the specimens at an intermediate strain rate regime (10 ÷ 200 1/s).

In this apparatus the specimen is subjected to a mechanical perturbation originating from properly sized springs (prestressed, input, output) to transfer a slow wave that will deform it in tension or compression. The proposed solution, currently at TRL 2 stage, aims to overcome some technical issues present in the state of the art and regarding the measurement of material properties at intermediate strain rates, with the following benefits:

  • Unperturbed measurement of specimen resistance (free from inertial effects)
  • Horizontal machine extension and overall compact size (3m)
  • High maximum transmissible dynamic forces (20 kN) over a considerable duration (10-12 ms)
  • High machine stroke useful to deform the specimens (up to 40 mm)

Possible Applications

  • Material characterization for energy absorption systems
  • Technological processes at intermediate strain rates (forging, sheet metal forming, …)
  • Characterization of soft materials with different mechanical behavior between quasi-static and dynamic regime


  • Permits testing at intermediate strain rates on materials
  • Compact machine size
  • Undisturbed force signals
  • High strain rate and test duration