Formability and Flammability of Thin Walled Veneer Structures

Abstract

Wood has always been a valuable engineering material, and specific technological advancements have drastically increased its usefulness. Forming of veneers has always been an innovative aspect of value addition and end product preparation. However, the postforming distortions experienced at the bending planes have always been a critical aspect of consideration and have affected the effective use of plywood structures. The current thesis primarily focuses on the thermoforming of plywood structures to form specific geometries through controlling and minimising the post-forming shape distortions at the forming planes. There have been significant advancements in the empirical representation of the thermoforming process, although there is still considerable potential for improvements. In order to do so, the detailed knowledge of the structural aspects and the complicated woodforming procedure should be adequately understood. In view of that, a detailed parametric analysis was carried out for a single curvature vee-bending test to quantitatively evaluate the post-forming distortions (spring-back and spring-forward or negative spring-back). The variability and final contributions of each parameter were studied by a statistical data evaluation. The forming temperature and pre-forming moisture content were found to have the greatest influences, while the experimental results could be satisfactorily simulated by calculations based on the established equation. The ideal forming parameters and the desired thickness to bend radius ratio were acquired and applied to carry out a multiple bend analysis. Wood veneers are usually categorised as an anisotropic natural composite with interconnected fibres bound in a predominant lignin matrix. Thus, the thermoforming process can have significant post-forming distortions due to the existing differences in the in-plane and out-of-plane dimensional changes caused by the changing temperature and moisture. These variations on the bending plane can have a significant influence on each other in multiple bend situations. A comprehensive multiple bend analysis was carried out through in-situ and kinematic strain analysis to observe one bending plane's effects on the other. The forming techniques were also used to propose an empirical equation representing plywood's thermoforming in a broader range of the most influential parameters. Having understood the thermoforming process and with the ability to control the post-forming shape distortions, channel manufacturing through minimisation of the distortions was carried out. Top-hat sections were produced to perform another statistical analysis on the in-situ curing time and forming temperature to report the parameters which gave minimum shape distortion. A new modification was further introduced in the forming process in the form of an extended in-situ curing time. The modified method helped in successfully achieving negligible spring-back or spring-forward while preparing the top-hat sections. With this knowledge, developable curvatures were analytically achieved, and developable surfaces on plywood were produced for the first time without any wrinkling or other defects. The shape-distortion cancellation further helped in producing partially formed compartments or chambers (in a reduced scale) from thin-walled veneer structures, which were then subjected to the fire performance study. A detailed experimental analysis was carried out on the fire performance of standard and formed plywood structures, and the results were compared to the traditional straight-walled structures used. Eventually, a numerical model based on Fire Dynamics Simulator (FDS®) was prepared, which replicated both the standard samples and the partial compartments. The successful prediction of the numerical models, compared to the experimental results, also helped extend the work towards a non-dimensional evaluation of the effects of the forming radius to the half-span ratio on the fire-reaction properties of the compartments. In view of that, the fire-reaction properties of the corrugated and honeycomb cores and sandwich panels with those hollow cores were tested and compared to the counterparts made from natural fibre (flax) reinforced fire-retardant composites. The plywood structures out-performed the composites in many aspects, pointing towards the possibility of having a hybrid veneer composite with good mechanical and fire performances. Thus, the viability of developing biodegradable, renewable and sustainable fire-retardant hybrid plywood was established.