ORALS

Narrow fine grain layers of material are often generated in the vicinity of frictional interfaces in manufacturing processes as a result of severe shear deformation. These layers change some surface properties of machine parts. The latter affects the performance of structures and machine parts under service conditions. Therefore, it is of importance to develop a method to connect parameters of manufacturing processes and parameters that characterize properties of fine grain layers generated by these processes. The strain rate intensity factor is the coefficient of the leading singular term in a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. Such expansions are available for several material models that are often adopted to describe the response of material in metal forming processes. The objective of the present paper is to develop a general approach to use the strain rate intensity factor for predicting the evolution of material properties within the fine grain layers. The present paper includes a conceptual approach, experimental results on upsetting and drawing and a special numerical method for calculating the strain rate intensity factor. The latter is necessary since the strain rate intensity factor appears in singular solutions and conventional finite element methods are not capable of calculating this factor. The method proposed is based on the method of characteristics. Two criteria for the thickness of the fine grain layer are considered.