Popis deformačního odporu za tepla v širokém intervalu termomechanických podmínek

Abstract

From the viewpoint of proper control of bulk-forming processes, so-called natural flow stress (i.e. response of formed material against an external force load) is also, inter alia, a very important knowledge. The flow stress is influenced by a value of strain which was put into the formed material and of other thermomechanical conditions accompanying the deformation (i.e. deformation temperature and strain rate). These dependencies are compiled for each material experimentally in a desired range of thermomechanical conditions. By this way, always a unique flow-curve-set is produced. For purposes of mathematical modeling of intensive hot forming processes, the ability of a united description of the flow-curve-set acquired in a very wide range of strains, of strain rates and of deformation temperatures is required. This type of the description is, however, complicated by a shape-variety of the individual flow curves, which is caused by the influence of physical actions which are closely intertwined with hot forming processes. In recent years, many different models were developed. These models allow a description of flow-curve-sets in some extent. Nevertheless, these models are more appropriate for the description of curves recorded at narrower ranges of the forming conditions, i.e. when the shapes of the curves are less different. Model, so universal that would allow an accurate united description of flow-curve-set consisting of the higher shaped-curve-diversity was not yet developed. Several of the so far derived models offer some variability of the description, but their accuracy is less than in the case of the description of the shape-similar flow curves. It was, however, possible based on the analysis of these so far derived models to reveal their advantages and disadvantages and by the subsequent modification of one of them to propose own unique model. Possibilities of the description of the new model were then tested on the experimental flow-curve-sets of three different materials. The new model was compared with so far most accurate or most frequently used models. Graphical and statistical comparison showed that the newly derived model enables to meet demanding requirements arising from the need of the united description of an experimental flow-curve-set acquired in a very wide range of thermomechanical conditions.