HOPKINSON’S BAR FOR SIMULTANEOUS TRACTION AND TORSION TESTS
Thanks to the invention it is possible to carry out dynamic tension and torsion tests with a Hopkinson bar type apparatus. A loading system at one end of the apparatus simultaneously generates a torsion wave and a compression wave, which travel more rapidly along the bars and reach the opposite free end to rise as a tensile wave. In this way, the torsion wave and the tensile wave deform the sample simultaneously. The device allows to carry out tests on different materials with high deformation in states of mixed stress-torsion stress, or of pure torsion or pure traction, with elongation and rotation values comparable to those of standard testing machines. Plasticity and damage behavior can be studied in unprecedented strain rates and stress states compared to standard Hopkinson bar systems.
The system consists of sixteen titanium bars aligned and joined for a total length of about 90 m (the prestressed, inlet and outlet bars are respectively 9, 57 and 20 m long), two electromechanical loading systems for twisting and pulling simultaneously the prestressed bar, two rigid clamps and two end stops for traction and torsion, devices specially designed for locking the specimen between the inlet and outlet bars. The device, currently at TRL 8, allows to:
- Overcome the limit relating to the simultaneous generation of compression and torsion waves and the ability to guarantee the synchronous arrival of the two waves to the specimens;
- Perform more types of tests, depending on how the preload is applied and eventually released (dynamic tension, dynamic torsion, dynamic torsion – static tension / compression and dynamic tension – dynamic torsion);
- Record the entire history of stress and tension values;
- A torsion angle of approximately 360 ° and a longitudinal displacement of approximately 50 mm.
- Material characterization for energy absorption and protection systems;
- High-speed deformation processes;
- Viscoplastic constitutive modelling.
- Permits combined tension-torsion tests with Hopkinson bar configuration;
- Permits very large deformations in elongation and rotation;
- Records the entire story of stress and strain up to failure;
- Study the plasticity and damage behaviour in unprecedented strain rates and stress states.