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A microscale three‐point bend experiment on wood has been carried out. The full 3D strain field of the microscale wood structure has been determined by use of digital volume correlation, based on reconstructed 3D image data acquired with synchrotron radiation micro‐computed tomography. The wood specimen, which measures 1.57 × 3.42 × 0.75 mm3, was scanned in different load states along the three‐point bend load cycle, from unloaded state to failure. The correlation algorithm is based on a Chebyshev polynomial description of the displacements, which allows a continuous representation of the 3D deformation fields. The methodology of the correlation algorithm is described thoroughly and its performance is tested for a 3D structure that is exposed to a virtual pre‐defined deformation. The performance is tested both for noise free volume data as well as for structures with additive noise content. The performance test shows that the correlation algorithm resolves the applied deformation satisfyingly well. In the real experiment, on wood microstructure, the displacement fields show a structural behaviour that is consistent with what is expected for a specimen exposed to three‐point bend. However, there are also anomalous effects present in the displacement fields that can be coupled to characteristic features in the cellular structure of the wood. Furthermore, 3D strain calculations based on the obtained displacement data shows a concentration of tensile strain in the region where the specimen eventually collapses. The experimental results show that the use of X‐ray‐based tomography with high spatial resolution in combination with digital volume correlation can successfully be used to perform 3D strain measurements on wood, at the microscale.

This paper presents a new approach to estimate two-and three-dimensional affine transformations from tomographic projections. Instead of estimating the deformation from the reconstructed data, we introduce a method which works directly in the projection domain, using parallel and fan beam projection geometries. We show that any affine deformation can be analytically compensated, and we develop an efficient multiscale estimation framework based on the normalized cross correlation. The accuracy of the approach is verified using simulated and experimental data, and we demonstrate that the new method needs less projection angles and has a much lower computational complexity as compared to approaches based on the standard reconstruction techniques.

Abstract. Soot particles can significantly influence the Earth's climate by absorbing and scattering solar radiation as well as by acting as cloud condensation nuclei. However, despite their environmental (as well as economic and political) importance, the way these properties are affected by atmospheric processing of the combustion exhaust gases is still a subject of discussion. In this work, individual soot particles emitted from two different vehicles, a EURO 2 transporter, a EURO 3 passenger car, and a wood stove were investigated on a single-particle basis. The emitted exhaust, including the particulate and the gas phase, was processed in a smog chamber with artificial solar radiation. Single particle specimens of both unprocessed and aged soot were characterized using near edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning electron microscopy. Comparison of NEXAFS spectra from the unprocessed particles and those resulting from exhaust photooxidation in the chamber revealed changes in the carbon functional group content. For the wood stove emissions, these changes were minor, related to the relatively mild oxidation conditions. For the EURO 2 transporter emissions, the most apparent change was that of carboxylic carbon from oxidized organic compounds condensing on the primary soot particles. For the EURO 3 car emissions oxidation of primary soot particles upon photochemical aging has likely contributed as well. Overall, Correspondence to: M. Ammann (markus.ammann@psi.ch) the changes in the NEXAFS fingerprints were in qualitative agreement with data from an aerosol mass spectrometer. Furthermore, by taking full advantage of our in situ microreactor concept, we show that the soot particles from all three combustion sources changed their ability to take up water under humid conditions upon photochemical aging of the exhaust. Due to the selectivity and sensitivity of the NEXAFS technique for the water mass, also small amounts of water taken up into the internal voids of agglomerated particles could be detected. Because such small amounts of water uptake do not lead to measurable changes in particle diameter, it may remain beyond the limits of volume growth measurements, especially for larger agglomerated particles.

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