In-situ stress analysis with X-ray diffraction for yield locus determination

One of the main problems in the field of sheet metal characterization is the inhomogeneous deformation fields caused by edge effects in the specimens. Except the uniaxial tension test, other tests like the plane strain tension test or the Miyauchi test suffer from this fact. For this reason, the desired ideal deformation state can only be achieved in the middle of the gauge area of the specimens. The inhomogeneous strain field in the gauge zone can be captured by optical strain measurement methods nowadays. However for the calculation of the yield stresses globally acting tool forces and a single averaged value for the instantaneous gauge area are used. Hence the inhomogeneity is an error source for the analytical determination of yield stresses. X-ray diffraction method is introduced to solve the problem caused by inhomogeneous deformation fields in the sheet specimens. In the proposed strategy, the strains on the specimen surface are captured by an optical measurement system during testing. At the same time the loading stresses in the gauge zone are measured by means of an X-ray diffractometer installed on a standard universal testing machine. With this method the stress-strain history of a material point on the specimen can be obtained throughout the test and this information is used to construct the yield locus of materials.