Hopkinson Spring for dynamic characterization of materials at intermediate 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.
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)
- 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