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Eingeladener Vortrag
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To better understand the mechanism of hydrogen assisted cracking (HAC), it is important to investigate the 3D structure of the cracks non-destructively. Since, cracks introduced by HAC are usually very small, conventional x-ray imaging methods often lack the required spatial resolution.
However, the detection of those cracks can be enhanced by taking advantage of refraction at interfaces within the sample.
To image this refractive deflection we employ analyser based imaging (ABI). In this work we aim at proving the enhanced crack detection of ABI by investigating an alluminum alloy weld.
Hydrogen in metals can cause a degradation of the mechanical properties with possible subsequent hydrogen assisted cracking (HAC). Though, the mechanism of HAC is not completely understood yet and thus suitable methods for in situ investigations to characterise the crack formation are needed. X-ray computed tomography (CT) is a well-known tool for analysing these properties. However, the effective resolution of the detector system limits the detection of small defects by CT. Analyser based imaging (ABI) takes advantage of x-ray refraction at interfaces between volumes of different density, i.e. of cracks, pores, inclusions, etc., within the sample to detect defects smaller than the resolution of the detector system. In this study, measurements on an aluminium alloy weld showed that ABI allows us to resolve the 3D structure of cracks undetected by absorption based CT. Prospective investigations will analyse HAC in steels.
We present an experimental approach to study the three-dimensional microstructure of gas diffusion layer (GDL) materials under realistic compression conditions. A dedicated compression device was designed that allows for synchrotron-tomographic investigation of circular samples under well-defined compression conditions. The tomographic data provide the experimental basis for stochastic modeling of nonwoven GDL materials. A plain compression tool is used to study the fiber courses in the material at different compression stages. Transport relevant geometrical parameters, such as porosity, pore size, and tortuosity distributions, are exemplarily evaluated for a GDL sample in the uncompressed state and for a compression of 30 vol.%. To mimic the geometry of the flow-field, we employed a compression punch with an integrated channel-rib-profile. It turned out that the GDL material is homogeneously compressed under the ribs, however, much less compressed underneath the channel. GDL fibers extend far into the channel volume where they might interfere with the convective gas transport and the removal of liquid water from the cell.
Röntgenstrahlung eignet sich besonders gut zur zerstörungsfreien Charakterisierung von Werkstoffen. Mit ihrer Hilfe lassen sich z.B. Risse und Poren in Werkstoffen erkennen. Die Risserkennbarkeit ist jedoch von der Rissgröße relativ zum Auflösungsvermögen des Detektors abhängig. Risse, Poren oder Einschlüsse können in Radiogrammen nur dann erkannt werden, wenn sie größer sind als das Detektorelement, das die Röntgenstrahlen registriert. Das hier vorgestellte neuartige Synchrotron-Refraktions-Computer-Tomographie-Verfahren ist jedoch in der Lage, auch Risse zu erkennen, deren Größe weit unterhalb des Auflösungsvermögens des Detektors liegt. Das Verfahren basiert auf dem bisher wenig genutzten Phänomen der Brechung von Röntgenstrahlen an den inneren Grenzflächen mikroskopisch strukturierter Objekte. Durch den Vergleich zwischen absorptionsbasierter und refraktionsbasierter Computer Tomographie Untersuchungen mit Synchrotronstrahlung an einer geschädigten Faserverbundprobe wird das Potenzial der Synchrotron-Refraktions-Computer-Tomographie demonstriert.
X-ray refraction is based on optical deflection of X-rays, similar to the well-known small angle X-ray scattering, but hundreds of times more intense, thus enabling shorter measurement time. We show that X-ray refraction techniques are suitable for the detection of pores, cracks, and in general defects. Indeed, the deflected X-ray intensity is directly proportional to the internal specific surface (i.e., surface per unit volume) of the objects. Although single defects cannot be imaged, the presence of populations of those defects can be detected even if the defects have sizes in the nanometer range.We present several applications of X-ray refraction techniques to composite materials:- To visualize macro and microcracks in Ti-SiC metal matrix composites (MMC);- To correlate fatigue damage (fibre de-bonding) of carbon fibre reinforced plastics (CFRP) to X-ray refraction intensity;- To quantify the impact damage by spatially resolved single fibre de-bonding fraction as a function of impact energy in CFRP laminates.An example of classic high-resolution computer tomography of an impact-damaged CFRP will also be presented, as a benchmark to the present state-of-the-art imaging capabilities. It will be shown that while (absorption) tomography can well visualize and quantify delamination, X-ray refraction techniques directly yield (spatially resolved) quantitative information about fibre de-bonding, inaccessible to absorption tomography.