Study by X-Ray microtomography of the horizontal vibration effects on sand densification

International audienceX ray microtomography experiments were performed in order to evaluate the densification of silica sand submitted to horizontal sinusoidal vibrations carried out at constant frequency (50 Hz) with controlled acceleration and deceleration Γ. Packing homogeneity was characterized using relative density distribution through 3D images of the relaxed samples. Information obtained from the images allowed us to evaluate data at grain scale: porosity and pore size distribution, number of contacts per particle, particle shape and size distribution were evaluated and linked to the densification process. Based on the internal analysis of samples, the results confirm and extend the conclusions of previous works regarding the 3-layer densification under vibration and the proposed optimized vibration cycle to get dense and homogeneous samples. They extend them to different initial packings. Additionally, significant correlations are found between density and local particle packing characteristics such as pores size distribution, or the number of contacts per particle

Using small-angle X-ray scattering to investigate the compaction behaviour of a granulated clay

The compaction behaviour of a commercial granulated clay (magnesium aluminium smectite, gMgSm) was investigated using macroscopic pressure-density measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray microtomography (XμT) and small-angle X-ray scattering (SAXS). This material was studied as a potential compaction excipient for pharmaceutical tabletting, but also as a model system demonstrating the capabilities of SAXS for investigating compaction in other situations. Bulk compaction measurements showed that the gMgSm was more difficult to compact than polymeric pharmaceutical excipients such as spheronised microcrystalline cellulose (sMCC), corresponding to harder granules. Moreover, in spite of using lubrication (magnesium stearate) on the tooling surfaces, rather high ejection forces were observed, which may cause problems during commercial tabletting, requiring further amelioration. Although the compacted gMgSm specimens were more porous, however, they still exhibited acceptable cohesive strengths, comparable to sMCC. Hence, there may be scope for using granular clay as one component of a tabletting formulation. Following principles established in previous work, SAXS revealed information concerning the intragranular structure of the gMgSm and its response to compaction. The results showed that little compression of the intragranular morphology occurred below a relative density of 0 · 6, suggesting that granule rearrangements or fragmentation were the dominant mechanisms during this stage. By contrast, granule deformation became considerably more important at higher relative density, which also coincided with a significant increase in the cohesive strength of compacted specimens. Spatially-resolved SAXS data was also used to investigate local variations in compaction behaviour within specimens of different shape. The results revealed the expected patterns of density variations within flat-faced cylindrical specimens. Significant variations in density, the magnitude of compressive strain and principal strain direction were also revealed in the vicinity of a debossed feature (a diametral notch) and within bi-convex specimens. The variations in compaction around the debossed notch, with a small region of high density below and low density along the flanks, appeared to be responsible for extensive cracking, which could also cause problems in commercial tabletting

Measurement of spatial variation of density of compacted powder using synchrotron microtomography

This research considers density change in powder metallurgy when compacting a powder blend in a die. In order to provide insights into particle behavior during compaction, atomized aluminum powders with an average particle size of 100 µm were used in two different compaction configurations. The compaction dies are cylindrical in shape: compaction configuration I displays an upper punch, a die, a lower punch, and a spacer. Compaction configuration II displays an upper punch, two dies, a lower punch, and a spacer with different geometry. The Synchrotron Microtomography scans were conducted on both compaction configurations. For compaction configuration I, the scan were acquired at axial strains of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% and 50%. The scans for compaction configuration II were acquired at axial strains of 0%, 3%, 6%, 9% and 12%. After scanning these compacted powders, the images were analyzed by means of computer codes that were written using Interactive Data Language (IDL) program. Programs of density map and image segmentation were used to analyze the image. Then, the spatial density maps and histograms of density variation were developed. Based on this data, the study generated curves of row statistical density, column statistical density, and compaction response in order to understand density change in compacted powder. This study revealed that Synchrotron Microtomography can be successfully applied to examine density variation and track microscopic deformations of aluminum powders under compaction. The density distribution within the compaction die was controlled by several factors, such as constitutive behavior of powder, friction interaction between powder and die wall, geometry of compaction die, and sample filling condition. Springback represents an important factor for influencing density distribution, especially during the scanning process. Moreover, the effect of Springback cannot be eliminated and can only be decreased in powder compaction. The aluminum compacts exhibit considerable side-to-side asymmetry which agrees with previous research findings. Finally, the generation of void gap between layers indicates the inefficiency in transmitting forces during compaction

Characterizing Three Dimensional Microstructures Formed by Particles: An Example of Electrophotographic Printing Toner

Additive manufacturing of objects using powders can result in microscopic structures based on the properties of particles used. In additive manufacturing, piles of powder are consolidated by one or other mechanisms in order to form a part. The resulting microstructure within the part can impact its engineering performance. Predicting and controlling the engineering performance requires the characterization of the part’s material interior microstructures. H owever, obtaining the microstructure is difficult since the interior of a powder is a challenge to visualize. In this paper, unconsolidated powders are characterized. We have used Confocal Laser Scanning Microscopy to image three-dimensionally sediments of micron-sized poly-dispersed electro-photographic printing particles. Using image analysis tools, we have extracted each particle’s position and radius in selected sampling volumes. Through this methodology, we demonstrate the possibility to recreate the three-dimensional particle structures and extract values for morphological parameters for powder systems. The feasibility of direct and quantitative particle structural characterization can lead to much needed methods and tools to relate particle structures to the process of fabrication, the design of materials, and product performance in additive manufacturing.Mechanical Engineerin

Analysis of bulk behaviour of particles based on their individual properties

An in-depth understanding of bulk behaviour of particles based on their individual properties is a vital step for the powder handling industries, a good example is the selection of appropriate powder material and their flow consistency in additive manufacturing process which would have significant effects on the quality of the final products. Identification of the most reliable method to characterise powder flow behaviour in correlation to the conditions of powder spreading is still challenging. For instance, the low consolidation state of the powders within the process requires a characterisation technique which is capable of measurement for such conditions

3D Quantification of Particle Interaction of Compacted Powders Using Synchrotron Micro Tomography (SMT)

Synchrotron Micro Tomography (SMT) is a powerful, non-destructive scanning technique for studying the internal structure of materials. SMT was utilized for two applications in this thesis. The first application involves tracking particle rotation of aluminum powder under different compaction strains. The experiments were conducted on two geometrical configurations by applying axial load to compact the powder in the die and acquiring SMT scans at different strain levels. The SMT scans were processed using AVIZO visualization software for further analyses. The analyses included tracking the same particle at different compaction strain levels, analyzing their volume compressibility, and then quantifying their rotational behavior with respect to the z-axis and xy plane. Particles were first tracked, colored, and then 3D volume was generated. The main findings of this analysis include: 1) the volumetric strain of the particles decreased at high compaction strain due to breakage of the particles into small fragments and elastic volumetric strain of aluminum powder; 2) initially, particles showed no rotation, followed by significant rotation, due to an increase of compaction strains; 3) the majority of the particles exhibited significant rotations near the loading plate and the curved boundary; 4) the 3D shape of the tracked particles under different compaction strains provided a significant contribution to the research area of powders by demonstrating that particles change their shape during the application of compaction. SMT was utilized to quantify sand particles position during a Cone Penetration Test (CPT) as a second application. CPT is a fast and reliable in situ method for characterizing soil properties. A CPT was conducted on a sand specimen and the scans were acquired at different penetration depths using SMT. AVIZO was used to analyze the SMT scans with an objective of identifying how the particles change their position under different penetration depths. Individual particles were tracked and colored to perform this analysis. The results of the analyses include: 1) most particles near the top of the specimen moved upward during initial penetration, due to a small overburden pressure; 2) particles belonging to the middle and bottom of the specimen showed a downward movement with CPT advancement; 3) the tracked particles provided an insight into particle interaction with advancing cone penetration

In situ synchrotron tomography of granular deformation in semi-solid Al-Cu alloys

Optimising casting routes involving semi-solid deformation such as semi-solid processing and high-pressure die casting requires a fundamental understanding of the globule-scale mechanisms behind the macroscopic rheological behaviour. This thesis uses time-resolved 3D imaging to directly observe and measure semi-solid alloy deformation from a microstructural perspective. Under isothermal conditions and constant strain rates, deformation mechanisms both at the crystal scale and at the specimen scale were identified during deformation in globular Al-Cu alloys at 64-93% solid. Imaging and quantifying these mechanisms has led to the emergence of a refined understanding of semi-solid deformation based on granular material concepts. It is shown that globularised crystals (above fs> 60%) exclusively rearrange as individual grains during parallel plate compression and backward extrusion at low strain rates. Crystal-crystal interactions are identified for the two loading modes and are not accompanied by plastic deformation of the individual crystals during acquisition. The ubiquitous grain rearrangement is coupled with shear-induced dilation of the solid assembly, whereby the crystals push each other apart in order to accommodate the increasing strain. It is also shown that, for specimens lacking a liquid reservoir, shear-induced dilation causes menisci to be pulled into the specimen from the surface and additionally, at high solid fractions, internal pores to grow. The origins of cracking during semi-solid processing are explored in a granular framework and linked to the shear-induced dilation associated with the solid assembly which increases the initial width of the liquid channels between the grains. Finally, the discrete grain analysis is coupled with the bulk mechanical results to explore the shape of the stress-strain curve and relate it to the imaged and quantified behaviours. All solid fractions tend to the same final stress, hinting at the possibility of a critical state analogous to that in soil mechanics, although fully testing this hypothesis requires varying the confining pressure.Open Acces

Relating Representative Elementary Volume of Tortuosity to That of Porosity as Revealed from Computed Tomography Images

Tortuosity and porosity are significant micro-scale parameters that have a major impact on many environmental processes and engineering applications that are usually implemented at a macroscopic scale. Thus, it is essential to quantify the corresponding representative elementary volume of these micro-scale properties to study and improve the understanding of different environmental applications, such as flow and transport of contaminates in porous media systems. 3D x-ray microtomography images of sand systems that have different sizes and geometry were used to compute two important microscopic properties such as porosity and tortuosity. Corresponding representative elementary volumes (REV) of these systems were computed by developing an efficient algorithm using Matlab. Representative elementary volume (REV) of tortuosity was related to that of porosity for each system, to determine whether an REV for porosity is sufficient to define REV for tortuosity. Findings revealed that for regular particles geometry REVmin of porosity was less than REVmin of tortuosity. However, for irregular particles geometry, the REVmin of porosity and REVmin of tortuosity were similar. This indicates that REVmin value of porosity depends on the geometry and the structure of the porous media. Whereas, REVmin value of tortuosity is not affected by the geometry. In addition, the number of particles required to reach REV region was found to be an easier method to be reflected upon.يعد التعرج والمسامية من الخصائص ذات تأثير كبير على العديد من العمليات البيئية والتطبيقات الهندسية. ليتم تطبيق هذه الخصاص على نطاق التطبيقات والعمليات الهندسية، لابد من ايجاد الحجم التمثيلي لهذه الخواص التي تعتبر صغيرة الحجم، وذالك لدراسة وتحسين فهم التطبيقات البيئية المختلفة، مثل نقل تلوث المياه الجوفية في الوسائط المسامية ونشر الغازات في بنية الوسائط المسامية المعقدة. تتمثل الأهداف الرئيسية لهذه الأطروحة في استخدام الصور المجهرية ثلاثية الأبعاد لخمسة عشر نظاما كل نظام يتسم باختلاف شكل الجزيئات واختلاف اقطارها وذالك لحساب خواص مجهرية مهمة مثل المسامية والتعرج على نطاق واسع. من خلال تطوير خوارزمية فعالة باستخدام برنامج وذالك للعثور على الحجم التمثيلي للمسامية والتعرج MatLab تحديد ما إذا كان الحجم التمثيلي للمسامية كافية لتعريف ، وإيجاد علاقة بينهم. بالإضافة إلى ذلك الحجم التمثيلي للتعرج .كشفت النتائج أنه بالنسبة للجزيئات ذات الشكل المنتظمة فأن الحجم التمثيلي للمسامية اقل من الحجم التمثيلي لتعرج. في المقابل بالنسبة للجزيئات غير منتظمة الشكل فأن الحجم التمثيلي للمسامية كان مساويا للحجم التمثيلي لتعرج. بمعنى اخر، تتأثر قيمة الحجم التمثيلي للمسامية بالهندسة وبنية وسائط المسام. بينما، لا تتأثر قيمة الحجم التمثيلي للتعرج تم العثور على عدد الجسيمات المطلوبة للوصول ، بالهندسة وبنية وسائط المسام. بالإضافة إلى ذلك إلى وحدة الحجم التمثيل

Measuring the Particle Packing of l-Glutamic Acid Crystals through X-ray Computed Tomography for Understanding Powder Flow and Consolidation Behavior

The morphology of free-flowed and gravity consolidated crystal powder beds of the alpha and beta polymorphic forms of l-glutamic acid, together with a detailed analysis of particle density and microstructure within alpha form tablets using state -of-the-art X-ray computed tomography (XCT), is presented. The Carr’s index is measured to be 19.7 and 35.2 for the bulk powders of the prismatic alpha form and needle-like beta form, respectively, revealing the alpha forms increased powder flowability versus the beta form. XCT reveals the alpha form consolidates under gravity more efficiently than beta, where the final measured bed density of the alpha form is 0.724 g/cm3 compared to 0.248 g/cm3 for the beta form, which is found to be caused by the inability of the beta particles to pack efficiently along their needle axis. Tabletting studies reveal that the alpha form consolidates into compacts of intermediate tensile strength, whereas the beta form cannot be compacted under these conditions. XCT analysis of tablets formed from α-form crystals reveals two discrete density regimes, one low-density region of fine powder which accounts for 53.8% of the compact, and high-density regions of largely intact single crystals which account for 44.2% of the compact. Further analysis of the tablet microstructure reveals that the crystal particles are generally orientated with their basal {0 0 1} plane, normal to the compaction force and that small microcracks which appear within the particles generally occur perpendicular to the surface and are orientated through possible {1 1 0} and {1 0 1} fracture planes. XCT also reveals evidence for incipient transformation between the meta-stable alpha to stable beta phase at concentrations below that detected using laboratory X-ray diffraction. The results show that XCT can accurately measure the extent of tapping induced densification and reveals the powder bed mesostructure characteristics and tablet microstructure for the two polymorphic forms of alpha and beta l-glutamic acid