Influence of the Loading and Strain Rates on the Strength Properties and Formability of Higher-strength Sheet

Cold formability of material is influenced by all factors participating in the forming process. The crucial factors include the sheet material, whose formability is the most often evaluated according to its yield point, tensile strength and their ratio, elongation, uniform deformation, their combination, etc. [1]. In the forming process, these material characteristics are significantly influenced by the strain rate and the intensity of its influence on the material characteristics depends on the internal structure of the material [2, 3]. The study of the influence of high strain rates on the material characteristics using a standard tensile test is very difficult in terms of both the technical equipment and the interpretation of the measured results. Therefore possibilities to determine the material characteristics using modified tests are looked for [4 6]. The presence of a notch on the test bar also influences the material characteristics as a result of a stress change, and its effect is expressed using a coefficient, which must The paper analyses the influence of the loading rate in the interval from 1 to 1000 mm/min, which corresponds to the tensile machine working range, on the strength properties and the formability characteristics obtained on standard and notched test bars made of H340 LAD steel strips. The combination of the loading rate and the test bar type made it possible to obtain the relationships of monitored variables in the strain rate interval from 10–4 to 10 s–1. In this interval, the strength properties of the tested strips thick 1, 1,5 and 1,8 mm exponentially increase, but formability does not change up to the strain rate of 1 s–1.