Effective atomic number and density determination of rocks by X-ray microtomography

Microtomography, as a non-destructive technique, has become an important tool in studies of internal properties of materials. Recently, interest using this methodology in characterizing the samples with respect to their compositions, especially rocks, has grown. Two physical properties, density and effective atomic number, are important in determining the composition of rocks. In this work, six samples of materials with densities that varied from 2.42 to 6.84 g/cm³ and effective atomic numbers from 15.0 to 77.3 were studied. The measurements were made using a SkyScan-Bruker 1172 microtomography apparatus operating in voltages at 50, 60, 70, 80, 90 and 100 kV with a resolution of 13.1 μm. Through micro-CT images, an average gray scale was calculated for the samples and correlation studies of this value with the density and the effective atomic number of samples were made. Linear fits were obtained for each energy value. The obtained functions were tested with samples of Amazonite, Gabbro, Sandstone and Sodalite.     

Structural and analytical studies of silica accumulations in <Emphasis Type="Italic">Equisetum hyemale</Emphasis>

Horsetail (Equisetum spp.) is known as one of the strongest accumulators of silicon among higher terrestrial plants. We use the combination of position-resolved analytical techniques, namely microtomography, energy-dispersive X-Ray elemental mapping, Raman microscopy, as well as small-angle and wide-angle scattering of X-rays, to study the type, distribution and nanostructure of silica in the internodes of Equisetum hyemale. The predominant silicification pattern is a thin continuous layer on the entire outer epidermis with the highest density in particular knob regions of the long epidermal cells. The knob tips contain up to 33 wt% silicon in the form of pure hydrated amorphous silica, while the silica content is lower in the inner part of the knobs and on the continuous layer. In contrast to the knob tips, the silica in these regions lacks silanol groups and is proposed to be in close association with polysaccharides. No mentionable amount of crystalline silica is detected by wide-angle X-ray scattering. The small-angle X-ray scattering data are consistent with the presence of colloidal, sheet-like silica agglomerates with a thickness of about 2 nm. From these results we conclude that there are at least two distinct forms of silica in E. hyemale which may have different functions. The close association of silica with cell wall polymers suggests that they may act as a polymeric template that controls the shape and size of the colloidal silica particles similar to many other biominerals and mineralised tissues. We propose that owing to its specific distribution in E. hyemale, a protective role and possibly also an important biomechanical role are among the most likely functions of silica in these plants. Figure 3D rendering of X-ray microtomography data from a dry Equisetum hyemale stalk. The red colour indicates high X-ray absorption values due to local silica accumulations     

Unveiling privacy: Advances in microtomography of coralline algae

Marine calcareous algae are widespread in oceans of the world and known for their calcified cell walls and the generation of rhodolith beds that turn sandy bottoms into a complex structured ecosystem with high biodiversity. Rhodoliths are unattached, branching, crustose benthic marine red algae; they provide habitat for a rich variety of marine invertebrates. The resultant excavation is relevant to sediment production, while is common that the fragments or the whole specimens result in vast fossil deposits formed by rich material that can be “mined” for biological and geological data. Accordingly, microtomography (μCT) may enable a detailed investigation of biological and geological signatures preserved within the rhodolith structure in a non-destructive approach that is especially relevant when analyzing herbaria collections or rare samples. Therefore, we prepared coralline algae samples and submitted them to a range of capabilities provided by the SkyScan1176 micro-CT scanner, including reconstruction, virtual slicing, and pinpointing biological and geological signatures. To this end, polychaetes and mollusk shells, or their excavations, coral nucleation, sediment deposits and conceptacles were all observed. Although a similar technique has been applied previously to samples of living rhodoliths in Brazil, we show, for the first time, its successful application to fossil rhodoliths. We also provide a detailed working protocol and discuss the advantages and limitations of the microtomography within the rhodoliths.     

Computing overall elastic constants of polydisperse particulate composites from microtomographic data

In this paper, we use the well-known Hashin-Shtrikman-Willis variational principle to obtain the overall mechanical properties of heterogeneous polydisperse particulate composites. The emphasis is placed on the efficient numerical integration of complex three-dimensional integrals and on aspects of the anisotropic material response of real tomographically characterized packs. For this purpose, we numerically calculate the complete statistics of real packs, which are numerically or tomographically generated. We use the parallel adaptive sparse Smolyak integration method with hierarchical basis to integrate complex singular integrals containing the product of probability functions and the second derivative of Green's function. Selected examples illustrate both the numerical and physical facets of our work. First, we show the reduction of integral points for integration in spherical coordinates. Then, we comment on the parallel scalability of our method and on the numerical accuracy associated with the integration of a singular function. Next, we validate the solver against the experimental data and verify the results by comparing it to a closed-form expression. To investigate the ability of our scheme to capture the anisotropic nature of packs, we study a lattice type system. Finally, we report on the elastic constants computed for the modeled anisotropic particulate system that is tomographically characterized.     

Thermal degradation of polyetherimide joined by friction riveting (FricRiveting). Part I: Influence of rotation speed

The thermal degradation of polyetherimides joined by friction riveting (FricRiveting) (Amancio Filho ST, Beyer M, Dos Santos JF. Verfahren zum Verbinden eines metallischen Bolzens mit einem Kunststoff-Werkstück - DE 10 2005 056 606 A1. Germany: Deutsches Patent- und Markenamt; 2007) has been investigated for varying rotation speeds. The rotation speed is an important variable to be understood in order to predict thermal degradation during this process. Investigated rotation speeds in the range of 1570-2199 rad/s resulted in high process temperatures (350-475 °C) and heating rates (up to 2 °C/100 rad/s), but only small heating times (<3 s). Thermal degradation was evaluated by gel permeation chromatography, Fourier transform infrared spectroscopy and X-ray computer microtomography. The results indicated that thermal degradation in the PEI polymer was mainly due to chain scission. Moreover, the small level of thermally degraded material (average drops of 10% in molecular weight) showed only a minor dependence on rotation speed. Although high peak temperatures and heating rates were present, the restricted variation and average values of heating time were insufficient to cause strong thermal changes in the joints of the studied rotation speed range.     

Microstructure characterization of granular materials

This paper presents techniques and algorithms to compute microstructure properties of irregular-shaped granulate assemblies utilizing 3D images. The techniques are capable of extracting microstructure properties of particles such as centeroid, particle size distribution, shape indices (i.e., sphericiy and angularity), number of contacts (i.e., distribution of coordination numbers), contact network, packing efficiency, distribution of local void ratio and radial distribution function. Such properties are critical parameters for micromechanical-based numerical models to capture micro- and macromechanical behavior of geomaterials. X-ray microtomography was used to reconstruct high-resolution 3D image of a natural sand system to represent granular materials. Microstructure properties of the sand system were computed and compared with properties of a computer-simulated image of periodic random spheres. Findings indicate that the use of simplified systems of idealized spheres to model micro- and macromechanical behavior of granular systems can lead to inaccurate results due to the differences in the microstructure between both systems. Methods presented in this paper enabled capturing a more realistic microstructure that can be incorporated in micromechanical models to better simulate, understand, or explain macroscale behavior of granular materials based on their actual microstructure.     

Simulation of the densification of real open-celled foam microstructures

Ubiquitous in nature and finding applications in engineering systems, cellular solids are an increasingly important class of materials. Foams are an important subclass of cellular solids with applications as packing materials and energy absorbers due to their unique properties. A better understanding of foam mechanical properties and their dependence on microstructural details would facilitate manufacture of tailored materials and development of constitutive models for their bulk response. Numerical simulation of these materials, while offering great promise toward furthering understanding, has also served to convincingly demonstrate the inherent complexity and associated modeling challenges.The large range of deformations which foams are subjected to in routine engineering applications is a fundamental source of complication in modeling the details of foam deformation on the scale of foam struts. It requires accurate handling of large material deformations and complex contact mechanics, both well established numerical challenges. A further complication is the replication of complex foam microstructure geometry in numerical simulations. Here various advantages of certain particle methods, in particular their compatibility with the determination of three-dimensional geometry via X-ray microtomography, are exploited to simulate the compression of “real” foam microstructures into densification. With attention paid to representative volume element size, predictions are made regarding bulk response, dynamic effects, and deformed microstructural character, for real polymeric, open-cell foams. These predictions include a negative Poisson's ratio in the stress plateau, and increased difficulty in removing residual porosity during densification.