M. Mihaliková Research of Strain Distribution and Strain Rate Change in the Fracture Surroundings by the Videoextensometric Methode

Steels applied in the automotive industry can be classified under the AHSS category, which includes dual-phase (DP) steels, complex-phase (CP) steels and transformation-induced plasticity (TRIP) steels. The UHSS category mainly includes martensitic steels. The above-mentioned steel groups are well pressable, show an excellent combination of strength, service life and absorption of strain energy, strain hardening and good weldability 1 . Microalloyed (HSLA – High Strength Low Alloyed) steels show a fine ferrite-pearlite structure with a small addition (max. 0,15 %) of one element or the combination of elements of the Al, Ti, Nb, V group. The microalloying elements are bound to carbon and nitrogen, while the nature of microalloying effects is connected with the dissolubility of TiC, VC, NbC carbides, AlN, TiN nitrides and Ti(C, N) carbonitrides in austenite and ferrite and with the hardening mechanisms. Such designed steels enable car designers to implement the ideas of reduction of the weight of a structure and increase of safety of passengers in case of an accident 1 . The material characteristics of steel sheet in the forming process are also significantly influenced by external factors. The strain rate is a significant external factor and the intensity of its influence on the behaviour of material during the forming process, and hence on the material characteristics, is a function of its internal structure. With increasing the strain rate, the critical slip stress increases, the yield point intensively increases, the ultimate tensile strength increases and the material deformation characteristics change. Therefore it is necessary to know the behaviour of material in the forming process at increased rates, as well as its material characteristics. Measuring the material characteristics using the tensile test at high strain rates is very difficult and therefore possibilities to evaluate the material characteristics using modified tests are looked for 2, 3 . In the paper, the behaviour of three automotive steel sheets was observed. The steels were loaded with static uniaxial stress at the loading rate of 1,3 mm/min. The strain distribution at the Rm value and immediately before fracture was sensed using the videoextensometric technique. There are several non-contact strain-sensing methods. They are based on the illumination of the sensed surface with white and laser light. The signal obtained