Acoustic emission assisted fracture zone analysis of cellulose fibre materials

Abstract

An acoustic emission technique is used to quantify and position microfracture events ahead of a growing opening mode crack in paper materials containing different amounts of added starch. A mechanical model based on gradient-enhanced elasticity, containing an intrinsic length parameter reflecting the fibre-based materials microstructure, is applied to analyse the results. It is found in experiments that the addition of starch increases the tensile strength of paper significantly while the level of onset of microfracture nucleation at the crack-tip is only slightly increased. It is also found that the height of the process zone (zone in which microfractures ahead of the crack predominantly take place), measured from the crack plane, decreases with increasing amount of starch. The experimental and analytical results suggest that adding cationic starch to paper reduces the material’s sensitivity to gradients in the stress and strain fields and making the fibre network material more ‘continuum-like’. The experimental observations are shown to be qualitatively in agreement with the numerical results and lend confidence to the applied model.