FAST DYNAMICS

At the material scale, the test devices allow to characterize the evolutions until failure in the range of strain rates commonly encountered in crash and impact situations [0.05;5000]s-1. Many complementary experimental devices are implemented to cover this range and characterize the behavior of materials over a very wide range of mechanical impedances: metals and alloys, composites and polymers, cellular materials, biological materials (related to the biomechanical activity of the impact).

The hydraulic machine sweeps a wide range of strain rates and meets high-speed testing requirements. They include tension/compression setups over a velocity range of 0.05 to 250s-1. It is accompanied by an electro-optical extensometer for strain measurements and a piezoelectric cell for stress measurement. A (stereo) image correlation device and ultra-fast cameras allow field measurement and shots ranging from 3000fps to 25000fps.

Hopkinson bar devices are experimental tools that allow stresses (tension, compression or torsion) to be performed at high strain rates (up to 5000 s-1). The principle of Hopkinson bars is based on the positioning of a sample to be characterized between an entry bar and an exit bar. A projectile is launched on the free end of the input bar, this shock generates a wave that propagates in the input bar. At the interface between the input bar and the sample, a part of this wave propagates in the sample and the other part is reflected. The transmitted wave crosses the sample and meets the interface between the sample and the output bar. Part of this wave is again reflected and again travels in the opposite direction through the sample.

This wave propagation thus generates a dynamic stress on the specimen. Measurement on the input and output bars of the wave passage allows us to return to the deformation of the specimen and the applied force. Depending on the nature of the specimen to be tested and the type of stress desired, various sets of bars can be used.