Understanding and Predicting the Failure of Cellulosic Materials

• Applicant: Professor Dr.-Ing. Jaan-Willem Simon
• Subject Area: Mechanics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550562870

Paper and paperboard are gaining importance in numerous technically relevant applications, because they are extremely versatile, renewable materials and easily recyclable. Nevertheless, its material behaviour is still not sufficiently understood, and its modelling is underdeveloped. The challenge in the material modelling of paper is its multiscale nature. On the macroscale, paper shows a distinct elastoplastic behaviour coupled with progressively evolving damage. While these effects are well-understood for engineering materials such as metals, where mechanisms like plastic slip and growing voids or cracks on the microstructure are known to result in plasticity and damage on the macroscale, such effects are drastically different in paper. The reason that paper is so fundamentally different is the intrinsic microstructure, which consists of an unstructured fibre network. This fibre network structure dictates the macroscale material response. The elasto-plastic behaviour of single fibres, the breaking of individual fibres and fibre bonds, and friction between fibres result in what appears to be a combination of elasto-plasticity and progressive damage on the macroscale. While the presence of these mechanisms is widely acknowledged, the following two critical questions remain unanswered: (1) Which of these microscopic mechanisms is/are the dominant driver(s) of the macroscopic behaviour in various situations? (2) How do local plasticity and damage interact to result in the observed macroscale responses? Our ambition is to scrutinise the interaction of plasticity and damage resulting from the fibre network response. This requires, first of all, a sound experimental basis on the scales of single fibres, fibre bonds, fibre networks, and sheet. Further, these mechanisms will be built into a computational tool with a view to analysing the failure of cellulosic materials such as paper and paperboard. Addressing the critical missing links between microscopic effects and the resulting effective behaviour, particularly in the context of nonlinear deformation and progressive damage evolution, will constitute a breakthrough in relation to the existing research on these materials. To tackle this challenging multiscale problem, several innovations are necessary. First, the fibres and bonds have to be calibrated to experimental data that account for the influence of microfibrils. One very promising approach in that direction is adapting the wide-angle X-ray scattering technique to fibrous materials. Further, the fibre network microstructure must be resolved accurately, and statistical variations must be included. In addition, an innovative homogenisation scheme that enables consideration of damage needs to be developed. The final outcome of the project will be a fully validated numerical toolbox for failure analysis, so that paper-made structures can be designed for optimised structural performance.

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Co-Investigator: Dr.-Ing. Johannes Neumann
• Cooperation Partners: Dr. Caterina Czibula; Professor Dr. Ulrich Hirn