  The state-of-the-art in paper mechanics calls for novel experimental data
covering the full-field hygro-expansion of inter-fiber bonds in paper, i.e.,
the 3D morphological changes and inter-fiber interactions. Therefore, a
recently developed full-field single fiber hygro-expansion measurement
methodology based on global digital height correlation is extended to
orthogonally bonded inter-fiber bonds, to investigate their full-field quasi-3D
hygroscopic behavior. A sample holder has been developed which enables the
quasi-3D characterization of the initial geometry of individual inter-fiber
bonds, including the fiber thickness and width along the length of the fibers
as well as the degree of wrap around and contact area of the bond, which are
vital for understanding the inter-fiber bond hygro-mechanics. Full-field
hygroscopic testing revealed novel insights on the inter-fiber interactions:
(i) the transverse hygro-expansion of each fiber strongly reduces when
approaching the bonded area, due to the significantly lower longitudinal
hygro-expansion of the other bonded fiber. (ii) The relatively large transverse
strain of one fiber stretches the other crossing fiber in its longitudinal
direction, thereby significantly contributing to the sheet scale
hygro-expansion. (iii) Out-of-plane bending is observed in the bonded region
which is driven by the significant difference in transverse and longitudinal
hygro-expansion of, respectively, the top and bottom fiber constituting the
bond. A bi-layer laminate model is derived to rationalize the bending
deformation and an adequate match is found with the experimental data. Under
the assumption of zero bending, which represents constrained inter-fiber bonds
inside a paper sheet, the model is able to predict the contribution of the
transverse strain in the bonded regions to the sheet-scale hygro-expansion.
