Mechanical behaviour and temperature measurement during dynamic deformation on split Hopkinson bar of 304L stainless steel and 5754 aluminium alloy

The forming process of massive products at ambient temperature and at high speed of loading has not led to many scientific investigations up to now. Its understanding involves mechanical and thermal aspects that are strongly linked together (thermo-mechanical coupling). The adiabatic process generated at high strain rates, due to the short duration of the test that does not allow for thermal equilibrium, can induce thermal softening in the billet and modifications of the metallurgical microstructures. The tests are done with 304L stainless steel and 5754 aluminium alloy. A split Hopkinson bar is used for strain rates up to 2000(s-1). During the test, the temperature at specimen surface was measured with an infrared multi-detector (with a resolution area 43 mu m x 43 mu m and an frequency acquisition equal to 1 MHz). The measurement system allows for a temperature measurement along a line of the specimen surface. The focusing system is designed to eliminate the geometric and chromatic aberrations induced by the lenses and it allows for measurements at high strain rates with short specimens. With this system, it is shown that the temperature field is homogeneous along the sample during the complete duration of loading. Consequently, the Taylor-Quinney coefficient can be deduced from temperature measurements.