Analysis of the compressive response of Nano Fibrillar Cellulose foams

• We model NFC foams through 2-D random structures incorporating internal contact.
• We validate a custom beam element by simulating compression of honeycomb structure.
• We study the size effect for random structures modelled with internal contact.
• Non-straightness of cell walls is shown to influence initial stress–strain response.
• Order of magnitude agreement achieved in simulated response compared to experiments.

Nano Fibrillar Cellulose (NFC) is fast emerging as a biomaterial with promising applications, one of which is cellular foam. The inner structure of the foam can take various shapes and hierarchical micro-structures depending on the manufacturing parameters. The compressive response of foams developed from these materials is currently a primary criterion for the material development. In this work, we focus on the connection between the non-linear part of the response and the inner structure of the material. We study the effect of internal contact and its contribution to gradual stiffening in the energy absorbing region and accelerated stiffening in the densification region of the large strain compressive response. We use the finite element method in this study and discuss the applicability and efficiency of different modelling techniques by considering well defined geometries and available experimental data. The relative contribution of internal contact is singled out and mapped onto the overall compressive response of the material. The effect of initial non-straightness of the cell walls is studied through superposing differing percentages of the buckling modes on the initial geometry. The initial non-straightness is seen to have a significant effect for only strains up to 1%. The secant modulus measured at slightly higher strains of 4%, demonstrates lesser effect from the non-straightness of cell walls. The simulations capture the compressive response well into the densification regime and there is an order of magnitude agreement in between simulations and experiments. We observed that internal contact is crucial for capturing the trend of compressive response.