Experimental Study on Flexural Behaviour of Diagonally Notched Wooden Beams Strengthened with Prestressed Superelastic SMA Wires

Timber structures constitute the primary form of ancient architecture in China and possess immense historical, artistic and scientific value. Having withstood the ravages of time over hundreds or even thousands of years, timber beams—one of the main load-bearing components in such structures—have largely sustained varying degrees of damage and require repair and reinforcement. Superelastic shape memory alloy (SMA) wires offer numerous advantages, including high strength, corrosion resistance, and stable recovery stress via stress-induced martensitic transformation, and are currently widely used in the field of concrete; however, research into their application for reinforcing timber beams remains limited. This study experimentally investigated the flexural performance of diagonally cracked Korean pine beams reinforced with prestressed Ni–Ti SMA wires. The results demonstrate that SMA reinforcement significantly enhances ultimate load-bearing capacity, with prestressing proving substantially more effective than simply increasing the number of wires. The optimal configuration—two SMA wires at 3% prestress—yielded the greatest improvement of 38.02% in ultimate capacity. Additionally, SMA reinforcement improved flexural stiffness, reduced compressive strain under equivalent loads, and lowered the neutral axis position, thereby enlarging the compression zone. These findings confirm that prestressed superelastic SMA wires offer a highly effective solution for strengthening diagonally cracked timber beams in existing structures.