Strain concentration effects of wood knots under longitudinal tension obtained through digital image correlation

Knots are natural, inevitable structures in wood left by branches. Most of the negative mechanical effects of knots are caused by strain concentration effects and fiber variability. In this study, digital image correlation (DIC) was used to observe the global strain distribution around intergrown knots, encased knots, knotholes, and artificial holes in thin Japanese cedar (Cryptomeria japonica) tangential plates of different sizes subjected to tensile load. The results indicated that the strain around knots is more homogeneous than that around holes, and that the influence of wood anatomy is greater than that of the geometric factors of a hole. Knots have naturally optimized structures for branch junctions, altering wood properties, such as grain angle, microfibril angle, and branch collar, to disperse strain and prevent fractures. The fractures caused by four types of knots or holes differed. The specimens with artificial holes were the most consistent with strain concentration theory, but none of the specimens within the other three natural knots or holes exhibited fractures around knots. Although the strain peak of natural knots occurred at knot edges, this was unrelated to the fractures. Finally, the strain characteristics of the knot tissue revealed that the internal structure of intergrown knots was completely connected with surrounding tissues, meaning that they can deform synchronously with the specimen. By contrast, encased knots tissue exhibited an extremely low modulus in the transverse direction of the specimens (tangent axis of wood) because the strain was higher in the knot tissue than in the surrounding wood.