  In fibrous paper materials, an exposure to a variation in moisture content
causes changes in the geometrical and mechanical properties. Such changes are
strongly affected by the inter-fibre bonds, which are responsible for the
transfer of the hygro-mechanical response from one fibre to its neighbours in
the network, resulting in sheet-scale deformation. Most models developed in
literature assume perfect bonding between fibres. In the 3D reality, there is
some flexibility in the bond region, even for the perfectly bonded fibres,
because of the possibility of deformation gradients through the fibre
thickness. In earlier 2D idealizations, perfectly bonded fibres were assumed,
implying full kinematic constraint through the entire thickness of the sheet.
The purpose of the present study is to assess the effect of this assumption.
Using a homogenization approach, a random network of fibres is generated with
different coverages and modelled using finite elements. In order to understand
the role of bonding between fibres on the hygro-expansive behaviour of a
network, a bond model is developed. In this model, the fibres are modelled
using 2D regular bulk finite elements and the bonds are represented by
interfacial elements of finite stiffness, which are introduced between each
pair of fibres bonded in the network. These embedded interfacial elements form
a connection between two respective fibres, allowing relative displacements
between their mid-planes. The hygro-elastic response of networks obtained with
this bond model is investigated by varying the bond stiffness and the network
coverage under the application of mechanical loading and changes in moisture
content. Furthermore, the bond model is used to analyse the influence of
inter-fibre bonds on the anisotropic response of the paper fibre network.
