2017 Volume 2
Oxfordpp 489-594Three-dimensional Deformation and Damage Mechanisms in Forming of Advanced Structures in PaperAbstractPDF
There is a large potential for wood-fiber based materials such as paper and board to contribute to lightweight structures in several applications, particularly packaging. Fiber-based packaging materials have important advantages in comparison to fossil-based plastics regarding biodegradability, recyclability and renewability. Individualisation has become a crucial criterion for the use of packaging solutions and forming of advanced paperboard structure is a key technology for manufacturing of such packaging shapes. New sustainable packaging concepts are creating a need for paper materials with considerably enhanced properties.
Paper and board are in manufacturing of geometrically advances structures in general subjected to complex and often little known multi-axial states of loading and deformation that are necessarily quantified by conventional measures for paper performance. Today, commercial paperboard is optimised for folding and printing, and not for applications involving forming of advanced structures. It is like-wise important to design the manufacturing process to meet the particular properties of paperboard. Manufacturing methods that are suitable for metals and plastics are inevitably not suitable for paper and board since the deformation and damage mechanisms of fibre network materials are different from metals and plastics.
In this paper recent findings in the literature on 3D forming of paper and paperboard structures are reviewed. In particular, deformation and damage mechanisms involved in pertinent forming operations and how they are related to paper and board properties in order to enhance the development of new advanced paper materials and structure are analysed.
In the last decade, there have been major advancements in the development of geometrically advanced 3D paperboard structures including technological advances of various forming process, enriched understanding of the importance and influence of process parameters, and new paperboard materials with significant improved forming properties. However, there is still a lack of knowledge regarding the deformation mechanisms of these complex systems and particularly regarding the influence of friction. One remedy would be the enhancement of numerical simulation tools. Optimisation of existing forming processes and development of new ones as well as tailored paper and board materials with properties customised to the demands of exiting and new 3D forming processes will also play important roles. This development is only in its beginning and major progress is expected in the near future.
Oxfordpp 595-609Focused Ion Beam Tomography as A Means for Characterization of CNF in A Paper Matrix.AbstractPDF
A method to study Cellulose Nanofibril (CNF) distribution in three dimensions within a paper matrix-in-situ-was developed. Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) tomography was used to investigate the distribution of cellulose nanofibres in thee dimensions within a paper structure. Sufficient resolution and material contrast was contained using both secondary and back-scattered electrons in volumes as large as 103 um3. Challenges and approaches to achieve this are discussed, both with respect to the microscopy technique and with respect to image processing and volume reconstruction. A range of recorded images and reconstructed 3D volumes show the technique capable of resolving CNF in a paper matrix. Results presented show CNF within the paper matrix forming capsules enclosing filler particles. These capsules are seen to only infrequently be in physical contact with the enclosed particles. Similar separation between CNF and enclosed filler particles was tested and confirmed in CNF films with 10 wt% added ground calcium carbonate.
Oxfordpp 611-625Synchrotron Tomographic Imaging of Softwood Paper: A 4D Investigation of Deformation and Failure MechanismAbstractPDF
Fibre pull-out and fibre fracture are the two dominant failure mechanisms of softwood paper. For the first time, 4D synchrotron X-ray tomographic imaging (three spatial directions plus time) was performed to observe these mechanism in-situ during tensile deformation of softwood paper handsheets. The experiments were conducted on three handsheets, produced from pulp that was low consistency refined at 0 k Wh/t and 100 kWh/t and wither air-dried in restraint or freeze-dried. The fibre deformation was found to be highly complex; initially being accommodated via straightening of the fibres resulting in fibre separation and then complete fibre pull-out or fibre fracture. The 3D strain fields, computed by Digital Volume Correlation, revealed increasing out-of =plane deformation in samples with decreasing inter-fibre bonding. Further, the relation between the L2 norms of the out-of-lane strain fields with displacement was computed and found to follow a second order polynomial, with an increasing slope in samples with reduced inter-fibre bonding. It was then shown that the accumulated out-of plane deformations could be used as a metric to quantify the relative contribution of inter-fibre bond breakage, and subsequently, fibre pull-out during tensile deformation of handsheets. The results demonstrate that 4D imaging provides new insights into paper deformation mechanisms.
Oxfordpp 627-649A Micro-mechanical Modelling Study of Drying Restraint Effects on the Hygro-mechanics of Paper SheetsAbstractPDF
In this contribution we show how fibre activation and micro-buckling of fibre walls may explain, quantitatively, differences in the hydro-mechanical response of paper sheets due to the presence or absence of mechanical restraint during their fabrication. To this end, both effects are incorporated in an idealised micro-mechanical model of the fibre network. The model is used to predict the response of the network to wetting-drying cycles, as a function of the degree of restraint during production. Restrained-dried networks are predicted to exhibit an irreversible hydroscopic strain upon first wetting and a different reversible hygro-expansivity coefficient, compared with free-dried networks, which match well with experimental values reposted in the literature.
Soft fibre networks, typically seen in bathroom tissues, kitchen towels, and personal-care products, have properties that are intricately affected by the details of fibre geometry, 3D-network structures, and processing conditions. Designing such materials and products for better performance, while controlling cost, is especially a challenge in today’s fast paced product development. This paper concerns the development of a new, robust computational design platform for the design of soft fibre networks.
We have used particle-based methods, particularly, Discrete Element Method (DEM), to model fibres, fibre networks, their properties and performance, and also unit processes for manufacturing. Unlike other computational methods, this method has advantages to model discrete and non-homogeneous materials, complex geometries, and highly non-linear dynamic problems, such as large deformation (flow), contact/non/contact, fracture, and fragmentation.
With this approach, fibres are represented by a series of connected spherical particles in different lengths and geometries (curl, kinks, twists). Fiber networks are created by the deposition of those fibres under gravity, followed by the subsequent consolidation under pressure. here processes have shown an interesting transition phenomenon from a highly fluidic granular system to a fragile soft solid. The network is then subjected to a creping process, a critical process of tissue-making. The model was able, not only to reproduce unique crepe frequencies, but also unprecedented details of the destruction of fibre network structures and fibre failure (dusting) during creping. Typical tensile tests, thickness-direction compression tests, and softness tests have been also performed to demonstrate unique deformation characteristics of low-density, low-basis weight fibre networks.
This computational design system based on DEM provides a promising platform for exploring large parameter space of new material/product design.