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Crack nucleation and propagation in microcrystalline-cellulose based granules subject to uniaxial and triaxial load
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.ORCID iD: 0000-0003-2709-9541
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Cracking patterns of four kinds of granules, based on the common pharmaceutical excipient microcrystalline cellulose (MCC) and subject to compressive load, were examined. The initial pore structure and the location of initial failure under uniaxial compression were assessed using X-ray micro-computed tomography, whereas contact force development and onset of cracking under more complex compressive load were examined using a triaxial testing apparatus. Smoothed particle hydrodynamics (SPH) simulations were employed for numerical analysis of the stress distributions prior to cracking. For granules subject to uniaxial compression, initial cracking always occurred along the meridian and the precise location of the crack depended on the pore structure. Likewise, for granules subject to triaxial compression, the fracture plane of the primary crack was generally parallel to the dominant loading direction. The occurrence of cracking was highly dependent on the triaxiality ratio, i.e. the ratio between the punch displacements in the secondary and dominant loading directions. Compressive stresses in the lateral directions, induced by triaxial compression, prevented crack opening and fragmentation of the granule, something that could be verified in the SPH simulations. These results provide corroboration as well as further insights into previously observed differences between confined and unconfined compression of granular media.

National Category
Pharmaceutical Sciences
Research subject
Pharmaceutics
Identifiers
URN: urn:nbn:se:uu:diva-347457OAI: oai:DiVA.org:uu-347457DiVA, id: diva2:1194512
Available from: 2018-04-03 Created: 2018-04-03 Last updated: 2018-04-12
In thesis
1. Confined Compression of Single Particles: Development of a Novel Triaxial Testing Instrument and Particle-Scale Modelling
Open this publication in new window or tab >>Confined Compression of Single Particles: Development of a Novel Triaxial Testing Instrument and Particle-Scale Modelling
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When predicting the performance of a powder compaction process, assessing the behaviour of the particles comprising the powder bed is of central relevance. Currently, however, no experimental methods are available for mimicking the multiaxial loading conditions imposed on the individual particles in a powder bed during compaction, and such analyses are therefore usually performed in silico. Thus, the purpose of this thesis is to introduce a novel experimental method that enables experimental evaluation of confined triaxial loading conditions on individual particles in the mm-scale.

The work underlying the thesis consists of three major parts. Firstly, the triaxial instrument was designed and developed, after which its performance was evaluated using nominally ideal elastic-plastic spheres as model materials. These initial experiments showed that the instrument was able to successfully impose confined triaxial conditions on the particles, something that was verified by finite element method (FEM) simulations.

Secondly, the triaxial instrument was used to investigate differences in deformation characteristics under uniaxial and triaxial loading conditions for four different microcrystalline cellulose (MCC)-based granules. It was shown that fragmentation, associated with unconfined uniaxial compression, was impeded under confined triaxial conditions, despite the emergence of cracks. In addition, it was observed that the primary crack always occurs in a plane parallel to the most deformed direction, and that the location of the largest pore has a pronounced influence on the path of the crack.

Thirdly, the influence of different triaxial loading ratios were evaluated on polymer spheres, after which a unified description of contact pressure development was devised. Data from these experiments were then successfully used to calibrate a contact model for simulating bulk powder compression with the discrete element method (DEM).

All in all, a novel experimental method has been established, which has proven useful as an alternative and complement to numerical studies when studying single particle deformation under confined triaxial conditions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 58
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Pharmacy, ISSN 1651-6192 ; 255
Keywords
Powder compaction, Compression, Single particle, Particle mechanics, Powder mechanics, Confined conditions, Triaxial, Apparatus design, Modelling
National Category
Pharmaceutical Sciences
Research subject
Pharmaceutics
Identifiers
urn:nbn:se:uu:diva-348091 (URN)978-91-513-0329-1 (ISBN)
Public defence
2018-06-05, B21, BMC, Husargatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-05-09 Created: 2018-04-12 Last updated: 2018-05-09

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Jonsson, HenrikÖhman-Mägi, CarolineAlderborn, GöranIsaksson, PerFrenning, Göran

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