X‐ray computed tomography is an important tool for non‐destructively evaluating the 3‐D microstructure of modern materials. To resolve material structures in the micrometer range and below, high brilliance synchrotron radiation has to be used. The Federal Institute for Materials Research and Testing (BAM) has built up an imaging setup for micro‐tomography and ‐radiography (BAMline) at the Berliner storage ring for synchrotron radiation (BESSY). In computed tomography, the contrast at interfaces within heterogeneous materials can be strongly amplified by effects related to X‐ray refraction. Such effects are especially useful for materials of low absorption or mixed phases showing similar X‐ray absorption properties that produce low contrast. The technique is based on ultra‐small‐angle scattering by microstructural elements causing phase‐related effects, such as refraction and total reflection. The extraordinary contrast of inner surfaces is far beyond absorption effects. Crack orientation and fibre/matrix debonding in plastics, polymers, ceramics and metal‐matrix‐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. The technique is an alternative to other attempts on raising the spatial resolution of CT machines.
For the first time metal matrix composites have been investigated by 3D computed tomography combined with enhanced interface contrast due to X-ray refraction. The related techniques of refraction topography and refraction computed tomography have been developed and applied during the last decade to meet the actual demand for improved non-destructive characterization of high performance composites, ceramics and other low-density materials and components. X-ray refraction is an optical effect that can be observed at small scattering angles of a few minutes of arc as the refractive index n of X-rays is nearly unity (n = 1 − 10−6). Due to the short X-ray wavelength, the technique determines the amount of inner surfaces and interfaces of nanometer dimensions. The technique can solve many problems in understanding micro and sub microstructures in materials science. Applying 3D refraction computed tomography, some questions could be clarified for a better understanding of fatigue failure mechanisms under cyclic loading conditions.
Analyser-based imaging expands the performance of X-ray imaging by utilizing not only the absorption properties of X-rays but also the refraction and scatter rejection (extinction) properties. In this study, analyser-based computed tomography has been implemented on imaging an articular cartilage sample, depicting substructural variations, without overlay, at a pixel resolution of 3.6 microm.
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 (1⋅10 -5 ). 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.
X-ray computed tomography is an important tool for evaluating the three dimensional microstructure of modern materials non-destructively. To resolve material structures in the micrometre range and below high brilliance synchrotron radiation has to be taken. But materials of low absorption or mixed phases show a weak absorption contrast at there interfaces. A Contrast enhancement can be achieved by exploiting the refraction of X-rays at interfaces. This technique was developed and applied at the NDT department of the Federal Institute for Materials Research and Testing (BAM) during the last decade. It meets the actual demand for improved non-destructive characterisation of high performance composites, ceramics and other low density materials and components. The technique is based on Ultra Small Angle Scattering (USAXS) 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. The extraordinary refraction contrast of inner surfaces is far beyond absorption effects and hence especially useful for materials of low absorption or mixed phases, showing similar X-ray absorption properties. Crack orientation and fibre-matrix debonding in plastics, polymers, ceramics and metal-matrix-composites after cyclic loading and hydro thermal aging can be visualized. By combining the refraction technique with the computed tomography technique the three dimensional imaging of the micro structure of the materials is obtained. In most cases the investigated inner surface and interface structures correlate to mechanical properties. Recent results with a sub-micrometer resolution will be presented.
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