Peter Wyss

Tomography-Based Heat and Mass Transfer Characterization of Reticulate Porous Ceramics for High-Temperature Processing

Reticulate porous ceramics employed in high-temperature processes are characterized for heat and mass transfer. The exact 3D digital geometry of their complex porous structure is obtained by computer tomography and used in direct pore-level simulations to numerically calculate their effective transport properties. Two-point correlation functions and mathematical morphology operations are applied for the geometrical characterization that includes the determination of porosity, specific surface area, representative elementary volume edge size, and mean pore size. Finite volume techniques are applied for conductive/convective heat transfer and flow characterization, which includes the determination of the thermal conductivity, interfacial heat transfer coefficient, permeability, Dupuit–Forchheimer coefficient, residence time, tortuosity, and diffusion tensor. Collision-based Monte Carlo method is applied for the radiative heat transfer characterization, which includes the determination of the extinction coefficient and scattering phase function.

3D micro-scale deformations of wood in bending: Synchrotron radiation μCT data analyzed with digital volume correlation

A micro-scale three-point-bending experiment with a wood specimen was carried out and monitored by synchrotron radiation micro-computed tomography. The full three-dimensional wood structure of the 1.57x3.42x0.75mm(3) specimen was reconstructed at cellular level in different loading states. Furthermore, the full three-dimensional deformation field of the loaded wood specimen was determined by digital volume correlation, applied to the reconstructed data at successive loading states. Results from two selected regions within the wood specimen are presented as continuous displacement and strain fields in both 2D and 3D. The applied combination of synchrotron radiation micro-computed tomography and digital volume correlation for the deformation analysis of wood under bending stress is a novel application in wood material science. The method offers the potential for the simultaneous observation of structural changes and quantified deformations during in situ micro-mechanical experiments. Moreover, the high spatial resolution allows studying the influence of anatomical features on the fracture behaviour of wood. Possible applications of this method range from bio-mechanical observations in fresh plant tissue to fracture mechanics aspects in structural timber.

Tomographic Characterization of a Semitransparent-Particle Packed Bed and Determination of its Thermal Radiative Properties

Keywords: computerised tomography ; extinction coefficients ; Monte Carlo methods ; particle size ; porosity ; two-phase flow Reference EPFL-ARTICLE-184803doi:10.1115/1.3109261 Record created on 2013-03-04, modified on 2017-07-28

Time-lapsed assessment of microcrack initiation and propagation in murine cortical bone at submicrometer resolution

The strength of bone tissue is not only determined by its mass, but also by other properties usually referred to as bone quality, such as microarchitecture, distribution of bone cells, or microcracks and damage. It has been hypothesized that the bone ultrastructure affects microcrack initiation and propagation. Due to its high resolution, bone assessment by means of synchrotron radiation (SR)-based computed tomography (CT) allows unprecedented three-dimensional (3D) and non-invasive insights into ultrastructural bone phenotypes, such as the canal network and the osteocyte lacunar system. The aims of this study were to describe the initiation and propagation of microcracks and their relation with these ultrastructural phenotypes. To this end, femora from the two genetically distinct inbred mouse strains C3H/He (C3H) and C57BL/6 (B6) were loaded axially under compression, from 0% strain to failure, with 1% strain steps. Between each step, a high-resolution 3D image (700 nm nominal resolution) was acquired at the mid-diaphysis using SR CT for characterization and quantitative analysis of the intracortical porosity, namely the bone canal network, the osteocyte lacunar system and the emerging microcracks. For C3H mice, the canal, lacunar, and microcrack volume densities accounted typically for 1.91%, 2.11%, and 0.27% of the cortical total volume at 2% apparent strain, respectively. Due to its 3D nature, SR CT allowed to visualize and quantify also the volumetric extent of microcracks. At 2% apparent strain, the average microcrack thickness for both mouse strains was 2.0 microm for example. Microcracks initiated at canal and at bone surfaces, whereas osteocyte lacunae provided guidance to the microcracks. Moreover, we observed that microcracks could appear as linear cracks in one plane, but as diffuse cracks in a perpendicular plane. Finally, SR CT images permitted visualization of uncracked ligament bridging, which is thought to be of importance in bone toughening mechanisms. In conclusion, this study showed the power of SR CT for 3D visualization and quantification of the different ultrastructural phases of the intracortical bone porosity. We particularly postulate the necessity of 3D imaging techniques to unravel microcrack initiation and propagation and their effects on bone mechanics. We believe that this new investigation tool will be very useful to further enhance our understanding of bone failure mechanisms.

Tomography-based analysis of radiative transfer in reacting packed beds undergoing a solid-gas thermochemical transformation

A reacting packed-bed undergoing a high-temperature thermochemical solid-gas trans-formation is considered. The steam- and dry-gasification of carbonaceous materials tosyngas is selected as the model reaction. The exact 3D digital geometrical representationof the packed-bed is obtained by computer tomography and used in direct pore-levelsimulations to characterize its morphological and radiative transport properties as afunction of the reaction extent. Two-point correlation functions and mathematical mor-phology operations are applied to calculate porosities, specific surfaces, particle-sizedistributions, and representative elementary volumes. The collision-based Monte Carlomethod is applied to determine the probability distribution of attenuation path length anddirection of incidence at the solid-fluid boundary, which are linked to the extinctioncoefficient, scattering phase function, and scattering albedo. These effective propertiescan be then incorporated in continuum models of the reacting packed-bed.

Analysis of Steam Formation and Migration in Firefighters’ Protective Clothing Using X-Ray Radiography

X-ray radiography was used to quantify evaporation and moisture transfer in a multilayer firefighter protective clothing system with defined wetted layers exposed to low thermal radiation. Evaporation was faster and took place at higher temperatures if the moisture was located in the outer layers of the clothing system. Moisture that evaporated in the outer layers of the clothing system was found to move inwards and condense in the inner layers and on the cap of the measurement cell. Results found in this study correlated well with the findings of our former study based on simple temperature distribution measurements to determine moisture transfer in protective clothing layers at low level thermal radiation.

Tomography-Based Determination of Effective Transport Properties for Reacting Porous Media

Reference EPFL-ARTICLE-174709doi:10.1115/1.4004842View record in Web of Science Record created on 2012-02-02, modified on 2017-05-10

Novel techniques for high-resolution functional imaging of trabecular bone

In current biological and biomedical research, quantitative endpoints have become an important factor of success. Classically, such endpoints were investigated with 2D imaging, which is usually destructive and the 3D character of tissue gets lost. 3D imaging has gained in importance as a tool for both, qualitative and quantitative assessment of biological systems. In this context synchrotron radiation based tomography has become a very effective tool for opaque 3D tissue systems. Results from a new device are presented enabling the 3D investigation of trabecular bone under mechanical load in a time-lapsed fashion. Using the highly brilliant X-rays from a synchrotron radiation source, bone microcracks and an indication for un-cracked ligament bridging are uncovered. 3D microcrack analysis proves that the classification of microcracks from 2D images is ambiguous. Fatigued bone was found to fail in burst-like fashion, whereas non-fatigued bone exhibited a distinct failure band. Additionally, a higher increase in microcrack volume was detected in fatigued in comparison to non-fatigued bone. Below the spatial resolution accessible with synchrotron radiation tomography we investigated native and fractured bone surfaces on the molecular scale with atomic force microscopy. The mineralized fibrils detected on fracture surfaces give rise to the assumption that the mineral-mineral interface is the weakest link in bone. The presented results show the power of functional micro-imaging, as well as the possibilities for AFM imaging (functional nano-imaging) in this context.

Processing, microstructure and electromechanical properties of Pb(Zr, Ti)O 3 fibers obtained by thermoplastic co-extrusion

Using a torque-rheometer, ceramic-polymer extrudable mixtures were prepared to produce lead zirconate titanate (PZT) fibers through a co-extrusion process. A 7.7 mm diameter PZT-polymer rod was coupled with a 24 mm diameter carbon black-polymer shaped cylinder. This so called preform composite was extruded through a 1 mm die. By this method, 300 µm PZT fiber structures were successfully achieved. The microstructure and the electromechanical properties of these co-extruded sintered fibers were investigated and compared to 300 µm diameter fibers produced by simple thermoplastic extrusion. Although the co-extruded fibers showed similar microstructure properties and only a slightly higher porosity than the extruded ones, the electromechanical performance was noticeably inferior.

Functional microimaging : an integrated approach for advanced bone biomechanics and failure analysis

Biomechanical testing is the gold standard to determine bone competence, and has been used extensively. Direct mechanical testing provides detailed information on overall bone mechanical and material properties, but fails in revealing local properties such as local deformations and strains or quantification of fracture progression. Therefore, we incorporated several imaging methods in our mechanical setups in order to get a better insight into bone deformation and failure characteristics. Our aim was to develop an integrative approach for hierarchical investigation of bone, working at different scales of resolution ranging from the whole bone to its ultrastructure. At a macroscopic level, we used high-resolution and high-speed cameras which drastically increased the amount of information obtained from a biomechanical bone test. The new image data proved especially important when dealing with very small bones such as the murine femur. Here the feedback of the camera in the process of aligning and positioning the samples is indispensable for reproducibility. In addition, global failure behavior and fracture initiation can now be visualized with high temporal resolution. At a microscopic level, bone microstructure, i.e. trabecular architecture and cortical porosity, are known to influence bone strength and failure mechanisms significantly. For this reason, we developed an image-guided failure assessment technique, also referred to as functional microimaging, allowing direct time-lapsed 3D visualization and computation of local displacements and strains for better quantification of fracture initiation and progression at the microscopic level. While the resolution of typical desktop micro-computed tomography is around a few micrometers, highly brilliant X-rays from synchrotron radiation permit to explore the nanometer world. This allowed, for the first time, to uncover fully nondestructively the 3D ultrastructure of bone including vascular and cellular structures and to investigate their role in development of bone microcracks in an unprecedented resolution. We conclude that functional microimaging, i.e. the combination of biomechanical testing with non-destructive 3D imaging and visualization are extremely valuable in studying bone failure mechanisms. Functional investigation of microcrack initiation and propagation will lead to a better understanding of the relative contribution of bone mass and bone quality to bone competence.