Understanding the distributions of strain within solid bodies undergoing plastic deformations has been of interest for many years in a wide range of disciplines, ranging from basic materials science to biology. However, the desire to investigate these strain fields has been frustrated by the inaccessibility of the interior of most samples to detailed investigation without destroying the sample in the process. To some extent, this has been remedied by the development of advanced surface measurement techniques as well as computer models based on Finite Element methods. Over the last decade, this situation has changed by the introduction of a range of tomographic methods based both on advances in computer technology and in instrumentation, advances which have opened up the interior of optically opaque samples for detailed investigations. We present a general method for assessing the strain in the interior of marker-containing specimens undergoing various types of deformation. The results are compared with Finite Element modelling.
A method for non-destructive characterization of plastic deformation in bulk materials is presented. The method is based on X-ray absorption microtomography investigations using X-rays from a synchrotron source. The method can be applied to materials that contain marker particles, which have an atomic number significantly different from that of the matrix material. Data were acquired at the dedicated microtomography instrument at beamline BW2 at HASYLAB/DESY, for a cylindrical aluminium sample containing W particles with an average particle diameter of 7 μm. The minimum detectable size of the maker particles is 1-2 μm with the present spatial resolution at HASYLAB. The position (x,y,z) of all the detected marker particles within 1 mm3 was determined as function of strain. The sample was deformed in stepwise compression along the axis of the cylinder. A tomographic scan was performed after each deformation step. After a series of image analysis steps to identify the centre of mass of individual particles and alignment of the successive tomographic reconstructions, the displacements of individual particles could be tracked as a function of external strain. The particle displacements are then used to identify local displacement gradient components, from which the local 3D plastic strain tensor can be determined. This allows us to map the strain components as a function of location inside a deforming metallic solid.