D04 - Strain history

D04 - Characterization of "colored conditions" by measurement of strain history during forming

Tensile testing is one of the standard methods used to characterize materials which provide widely used and recognized properties such as mating limit, elongation at break and strain hardening. In this respect, a transfer of this type of characterization to descriptors for the analysis of micro samples seems to be useful. A direct conversion of the tensile test to the spherical specimens of the SFB is not possible in the high-throughput method due to the specimen geometry and size as well as the tensile forces to be applied.

In this project, however, high pressures initially cause compressive stresses in the specimens, which are altered by deflecting the material flow in multi-stage forming tools in such a way that transient areas arise in which approximately uniaxial and predominantly homogeneous tensile stresses prevail. Descriptor values are determined from the measured resulting strains and set in relation to macroscopically determined material properties.

The test rig for this purpose consists of an electrohydraulic forming device and an integrated speckle photography system, which is to record in-situ deformations of individual sample regions with high resolution. From these measured strains, strains simulated with reference material data and the actual force or energy in the experiment, meaningful descriptors are formed which are processed into predictor functions in the sense of the overall concept of the SFB.

As forming method for the pressing of the partly very hard micro samples into multi-stage bulk forming channels, a flexible punch on the basis of electrohydraulic forming is used, which has not been described so far in the literature. Accordingly, basic tests must be carried out to understand and master this technology. In particular, the identification of usable process parameter ranges and the assessment of the reproducibility of the forming process are essential prerequisites for a reliable descriptor determination. Particular attention is paid to the achievable forming step size, which must be set low (< 100 µm) so that the distortion of the specimen surface measured by measurement does not exceed the low percentage range, thus preventing a decorrelation of the speckle images and enabling strain measurement.

As forming method for the pressing of the partly very hard micro samples into multi-stage bulk forming channels, a flexible punch on the basis of electrohydraulic forming is used, which has not been described so far in the literature. Accordingly, basic tests must be carried out to understand and master this technology. In particular, the identification of usable process parameter ranges and the assessment of the reproducibility of the forming process are essential prerequisites for a reliable descriptor determination. Particular attention is paid to the achievable forming step size, which must be set low (< 100 µm) so that the distortion of the specimen surface measured by measurement does not exceed the low percentage range, thus preventing a decorrelation of the speckle images and enabling strain measurement.