Micro-radiography and tomography for high resolution NDT of advanced materials and microstructural components

Quality assessment of advanced materials like high strength ceramics and fibre reinforced composites requires (e.g. because of the brittleness of ceramics) detection and characterization of early damage and defects as, for examples, cracks and pores even with linear dimensions of only 10 mym. Structures of the same size must be distinguishable in components of precision mechanics and electronics. This paper discusses microfocal X-ray radiography and high resolution computed tomography. The latter can be looked at as a nondestructive microscopic technique imaging cross sections of an object. Results of investigations using a tomographic unit with a resolution capability of the order of magnitude of 20 mym are presented. The development of high strength ceramics and the application of these materials in machine building demands the detection and characterization of very small defects. Since these materials are very brittle the crack is the most critical kind of defect. From a fracture me chanics point of view, the limit of detection lies significantly below 50 mym for crack-like defects (1). Fibre reinforced composites require a similar detection capability. Their strength and reliability depend on orientation and fibre-resin distribution as well as on the size and concentration of the included pores. Porosity can lead to a lack of bonding between fibre and matrix and consequently to delaminations. Furthermore, microstructural electronic components need a careful nondestructive inspection. Problems are delamination of glued parts, orientation of layers, and soldering joints. The detection and size determination of such small defects requires the application of advanced NDT-methods. Especially in complex shaped parts, the investigation can be carried out by microfocal X-ray radiography without the enormous scanning mechanism necessary for other high resolution methods like ultrasonics. High resolution radiography is achieved by microfocal X-ray tubes. In radiography, a l l the details of a three dimensional object are projected to a two-dimensional plane. Complete size determination and orientation of defects and microstructures can only be found by multiangle radiation, finally leading to X-ray computed tomography. This technique enables imaging of single planes within an object and therefore gives information about the 3 dimensional distribution and location of defects in the object. The micro tomography can be looked at as a nondestructive microscopic technique imaging cross section of an object without the necessity of cutting out slices. This paper describes the equipment for microradiography and micro computed tomography and dicusses their application for NDT of some advanced mat