On the development of new characteristic values for the evaluation of sheet metal formability

Abstract

Until now the r− and n-value have been used for the characterisation of the deep-drawing and stretch-forming ability of sheet metals. In this contex high r− and n-values indicated good deformation characteristics. New developments within the steel industry lead to new isotropic steel qualities with increased strength and very good deformation ability. Here, the r-value approximately is about or equal to one, which, with regard to a good deep-drawability, contradicts the assessment made up to now. The parameters already known for the description of the material behaviour don't consider the states of stress leading to failure, or they can only be applied to a spectrum of parts which has a geometry similar to that of the samples used in the test. This paper presents new ways of developing more precise characteristic values for the evaluation of deep-drawing and stretch-forming abilities of sheet metals, especially for the new isotropic steel qualities. For this, new testing methods considering the states of stress occurring in the individual zones of the drawing part were developed at the Institute for Metal Forming and Metal Forming Machine Tools (IFUM). These new testing methods are the cross tensile test used for the simulation of the ‘plain-strain’ state of deformation as well as the flange insertion test used for the combined tensile and compressive state of stress.