Cast plaster dries with different densities depending on the surrounding media. Liquid plaster filled into a lubricated casting mould will acquire a surface boundary of high density, once set. The second and third cast layers into the still moist form will dry to a lower density. Later additions of plaster, due to sculptural reworking and restorative measures, will also have discernible densities. With computerized tomography (CT) the density in each volume element can be measured. With 3D - μCT the total body of a sculpture can be scanned to a high spatial resolution. Cracks within and cuts through the original cast become visible along with internal structures and armouring. The results from two studies on plaster statues (by Christian Daniel Rauch and Honoré Daumier), done in support of the conservation process as well in the intent of revealing a relative chronology within a series, are presented and placed into the art-historical context.
X-Ray Refraction Topography techniques are based on Ultra Small Angle Scattering by micro structural elements causing phase related effects like refraction and total reflection at a few minutes of arc as the refractive index of X-rays is nearly unity (1x10<sup>-5</sup>). The extraordinary contrast of inner surfaces is far beyond absorption effects. Scanning of specimens results in 2D-imaging of closed and open pore surfaces and crack surface density of ceramics and foams. Crack orientation and fiber/matrix debonding in plastics, polymers and ceramic composites after cyclic loading and hydro thermal aging can be visualized. In most cases the investigated inner surface and interface structures correlate to mechanical properties. For the exploration of Metal Matrix Composites (MMC) and other micro structured materials the refraction technique has been improved to a 3D Synchrotron Refraction Computed Tomography (SR-CT) test station. The specimen is situated in an X-ray beam between two single crystals. Therefore all sample scattering is strongly suppressed and interpreted as additional attenuation. Asymmetric cut second crystals magnify the image up to 50 times revealing nanometer resolution. The refraction contrast is several times higher than "true absorption" and results in images of cracks, pores and fiber debonding separations below the spatial resolution of the detector. The technique is an alternative to other attempts on raising the spatial resolution of CT machines. The given results yield a much better understanding of fatigue failure mechanisms under cyclic loading conditions.
In 1997 we presented some correction techniques for image intensifier images. In the mean time flat panel detectors are often used instead of. The visible contrast of the 16bit flat panel is much higher then with the same digitisation from intensifier images. This misleads users of CT-systems with flat panel detectors to expect far better results. Nevertheless all of the previously described corrections have to be done here too, if an artefact free image is the aim. This gets most important, if an automated evaluation shall be used to extract features from CT images. The main advantage of the new proposed correction technique is that the detector intrinsic scattered radiation (stray light) is corrected with a fast two dimensional filter. Also the right interaction with other corrections like beam hardening and object-scattered radiation is of importance, examples will be shown. The corrected 2D detector images enhances the quality of cone beam CT results in respect to their geometrical distinctness so that geometrical measurements and reverse engineering results get comparable with 2D CT measurements.
Results are shown on the µ-CT scanner for bigger objects or for objects with higher X-ray absorption which was set up at BAM. The system is equipped with a bipolar 320kV micro focus tube and a flat panel detector of amorphous Si with 400mm side length, room and system temperatures are regulated.
Some very small parts from regular cellular (spherical) metals were investigated in a strength test, which was done inside the (mu) CT machine with resolutions down to 5 micrometers . The aim was to inspect the behavior of the walls of cellular metals under load, and to find out how the buckling or breaking starts. The results are needed for theoretical calculations, which will be used to predict the behavior of large structures of these materials. Also the pore size and pore size distribution for cellular metals is calculated and a method to separate walls and nodes is presented. CT insights of cellular metals as well as investigations of data by means of image processing is shown.
At BAM 3D-computerized tomography (3D-CT) using x-ray cone beam and area detectors is established as a standard method for materials testing and development. Up to now main applications concerned fiber reinformed plastics and ceramics, density distribution in ceramics, powder metallurgical parts and archaeological objects. Spatial and density resolution depends on the object and on the combination x-ray source - detector system. The maximum spatial resolution is 5 micrometers using a transmission target and 12 micrometers using a standard micro focus tube together with an image intensifier as detector. The main problem of image intensifiers applied to 3D-CT is the rather bad contrast ratio of about 20:1. An object dependent correction for the light scattering in the image intensifier in combination with bam hardening correction is performed at BAM. This contribution will point out the advantages and disadvantages of different detector systems and results will be shown on test samples and selected investigation from our ongoing work.