An in-depth insight of the mechanical response of cellulose fibres by means of optical profilometry techniques

Degradation of historical paper is governed by a complex sequence of multi-physics phenomena occurring at multiple length scales, which typically result in the loss of structural integrity of the material at the macroscopic, sheet level. In order for conservators to take the most appropriate conservation decisions, it is key to gain an in-depth understanding of the intrinsic mechanisms that are responsible for paper degradation. This can be achieved only by the accurate characterization of the material behaviour at different length scales. This work proposes a novel experimental methodology to systematically characterize the mechanical response of individual cellulose fibres. An advanced micro-mechanical set-up is used that allows to perform uniaxial tensile experiments at the sub millimetre scale, in combination with in-situ high-resolution optical profilometry (white light interferometry) and digital image correlation techniques to achieve detailed strain measurements. The proposed methodology has never been applied for the analysis of single cellulose fibres, and thus requires dedicated tailoring of the testing methods, as well as the development of appropriate sample preparation procedures, which is particularly challenging for the case of ultra-delicate cellulose fibres. The present methodology has been finally applied to test two different cellulose fibre samples, providing an accurate estimate of the cross-sectional areas, the stiffness and the strength of the fibres. The application of the method to aged cellulose fibres may thus provide an improved understanding of the effect of cellulose degradation on the mechanical properties of paper.