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Ron H.J. Peerlings: Dimensional stability of paper sheets undergoing hygroscopic loading

19 Nov 2015   13:00-14:00

Significant dimensional variations may occur in paper materials when subjected to changes in moisture content. Gradients of the moisture content in the plane of the sheet, in particular, may result in instabilities and out-of plane deformation of the sheet, which are problematic in e.g. printing operations.

Moisture induced deformations are governed by the swelling of individual fibres, which is transferred through inter-fibre bonds within the fibrous network. Complex interactions between mechanical and hygro-expansive properties take place in the bonding areas, affecting the overall material response.

We study these mechanisms experimentally, by a combination of Scanning Electron Microscopy, Optical Microscopy and Digital Image Correlation. In particular, we aim to quantitatively characterise them at the scale of individual fibres and bonds, as well as at the sheet level.

At the same time, models are developed to bridge these two scales, i.e. to predict the sheet-level hygro-expansivity as a function of the properties of the fibre network ? and in particular of the bonds; in many existing network models, the role of inter-fibre bonds is not explicitly incorporated.

A first version of the model is based on a periodic meso-structural model of the discrete fibrous network, which considers the free fibre segments and inter-fibre bonds. Despite its simplicity, the reference unit cell enables to naturally take into account relevant microstructural features such as network structure, fibre and bond geometry and hygro-mechanical properties. The proposed model is solved analytically, allowing to recover the paper sheet's anisotropic hygro-mechanical response in terms of effective constitutive constants and effective hygro-expansive coefficients, based on the coupling at the microstructural level between hygroscopic and mechanical behaviour. A comparison with experimental results obtained from the literature shows that the presented approach is an accurate tool to predict the overall paper response and to study how it is influenced by the microscale parameters (e.g. fibre orientation, dimensions, mechanical strength).

In ongoing work the model is refined, in particular geometrically, by considering a random network of fibres. Periodic homogenisation is employed to extract the effective sheet-level expansivity and mechanical properties from unit cell computations. We in particular study the influence of anisotropy of the fibre orientation distribution on these effective properties and compare them with predictions of the earlier, highly idealised model.

This is a joint work with Emanuela Bosco, Mary Bastawrous, and Marc Geers, Eidhoven University of Technology, the Netherlands.

Fakulta stavební ČVUT/Faculty of Civil Engineering, Thákurova 7, B 366
Contact person
Jan Zeman, zemanj@cml.fsv.cvut.cz
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