Effect of Poisson's ratio on the mechanical properties and behaviour of auxetic bolts withdrawal from holes in beech wood

The work involves utilising new auxetic connectors for wooden structures, which will enhance their efficiency and competitiveness compared to conventional connectors. Due to the unique properties of materials and structures with a negative Poisson's ratio, called auxetics, it was decided to provide scientific evidence for the engineering application of auxetic bolts to timber connections that can improve reaction to axial loads that can cause the fastener to withdraw from the timber or reduce its stiffness. The study aimed to check the effect of Poisson's ratio on the withdrawal force of auxetic bolts from holes in wood (using beech wood as an example) and the distribution of radial pressures in the joint, depending on the angle of the wood fibres. It was also decided to determine the effect of Poisson's ratio on the change in absorption energy during bolt withdrawal. Experimental studies were conducted on steel auxetic bolt models produced using 3D printing. Numerical calculations included the determination of radial pressures during bolt withdrawal. Based on the results, it was demonstrated that negative Poisson's ratio values lead to an increase of up to 42.9 % in the bolt withdrawal force in the tensile stress state compared to conventional bolts. The amount of energy absorbed also increases. In the case of auxetic bolts, the voids deform, pressing with their sharp edges against the wood surfaces and generating more friction and higher radial pressures.