Application of ultra high speed optical measurements to the study of metals viscoplasticity under very high dynamic mechanical loading

摘要

When the loading is highly dynamic, the prediction of dynamic structural response by means of numerical calculations needs optimized constitutive relations to describe plastic behavior of metallic materials. In order to extend the domain of strain rate and strain explored in classical characterization experimental programs , free expanding structure tests are particularly well suited to models evaluation by virtue of their inherently homogeneous stress state. The expanding shell test allows to load a material in the domain of high strain levels while strain rate is about 10~4 s~(-1). The experiment (experimental apparatus, measurements ...) is described with the difficulties encountered for the interpretation of the experimental data. The radial velocity is measured with the Doppler Laser Interferometry technique (DLI) for different locations (at 0°, 30° 45° from the revolution axis) in order to record border effects. The accuracy of this technique allows the evaluation for the shock wave propagation stage in the shell (a short characteristic duration of a few μs) and the plastic deformation (duration of tens of μs). Furthermore, expansion of the material sample is observed by means of ultra high speed cameras. The film is a good tool for examination of damage appearances, detonation products problems, fracture of elements of the experimental set up. Under some assumptions, the numerical transformation of radial velocities gives indications about the evolution of the strain, stress, strain rate and temperature rise (this last parameter is related to plastic work). It is also shown the correlation between experimental, analytical and numerical approaches. This review is presented for three materials:copper, tantalum and TA6V4. It is demonstrated that sensitivity to plastic contribution to the motion of matter under high dynamic deformation states is related to strength, density and melting temperature. The contribution of this test to modeling of constitutive equationsis also discussed and further works are finally proposed.