Geometrically nonlinear behaviour of actively twisted and bent plywood

The current work aims to investigate the geometrically nonlinear behaviour of flexible plywood strips that undergo large displacements during the formation process of a structural element. Such deformations allow bending -active structures to reach curved geometries from initially planar elements. As our literature review points out, firstly, twist for plywood materials is not well studied, and secondly, the detailed determination of the stress state after the formation process of such structures, is usually oversimplified or even ignored. To this end, a finite element model using laminated shell theory is employed and calibrated against experimental data retrieved from physical prototypes, by paying particular attention in the accurate tuning of its boundary conditions. Furthermore, destructive tensile tests of twisted strips were implemented and their ultimate strength was compared with the numerical model. Our aim is to explore the material's stress state during the strip's 90(o) twist coupled with bending in two different directions, which is crucial for understanding the strip's further behaviour as a structural element. In addition, we conduct parametric studies to showcase the non -negligible impact of geometric aspects on stress distribution along the strip. The results show that the utilisation ratios, using the Tsai-Hill failure criterion in terms of strength, for the case of the constructed beam element, are quite uniform throughout the strip and below 50%, apart from local stress concentrations. Finally, the parametric studies highlight the need to comprehend the material response to inform geometry within a structurally sound environment in order to unlock new possibilities for using thin plywood strips in lightweight structural and architectural applications, as demonstrated by the built beam elements.