Multi-scale mechanical and viscoelastic behaviour of major and lesser-used Congo Basin hardwoods: effects of radial position and moisture content

Abstract

This study investigates the multi-scale mechanical and viscoelastic behaviour of nineteen Congo Basin hardwood species, including thirteen major commercial timbers and six lesser-used substitutes. A comprehensive approach was applied, integrating apparent density, intra-tree radial variability, and moisture content (12–28%) to evaluate static properties (MOE, MOR, compression and tension), fracture toughness (G_IC), and dynamic properties (storage modulus E′ and loss factor tan δ). Peripheral wood consistently exhibited the highest static and dynamic performance, confirming its relevance for structural applications. Dense species such as Azobé, Okan and Wengé reached MOE values above 22 GPa and displayed low viscoelastic damping, whereas light species such as Ayous and Okoumé showed lower stiffness and higher energy dissipation. Increasing moisture content reduced elastic moduli by up to 35% and increased damping, with more pronounced effects in low-density species. Lesser-used substitute species exhibited mechanical and viscoelastic properties consistent with those of their respective mechanical groups, supporting their potential use as alternatives in construction. Overall, the results highlight the dominant role of density and radial position in governing stiffness, strength and viscoelastic response, while quantifying moisture effects critical for tropical service conditions. This dataset provides a robust basis for mechanical grading, species substitution and sustainable utilisation of Congo Basin hardwoods.