Citation: Martins, C.; Moltini, G.; Dias, A.M.P.G.; Baño, V. Machine Grading of High-Density Hardwoods (Southern Blue Gum) from Tensile Testing. Forests 2023, 14, 1623. https://doi.org/10.3390/f14081623 Academic Editor: Milan Gaff Received: 22 July 2023 Revised: 3 August 2023 Accepted: 9 August 2023 Published: 11 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Machine Grading of High-Density Hardwoods (Southern Blue Gum) from Tensile Testing Carlos Martins 1,2, * , Gonzalo Moltini 3,4 , Alfredo M. P. G. Dias 1,2 and Vanesa Baño 3,5 1 SerQ–Innovation and Competence Forest Centre, Rua J, Nº 9, 6100-711 Sertã, Portugal 2 University of Coimbra, ISISE, ARISE, Department of Civil Engineering, 3030-788 Coimbra, Portugal 3 Cesefor, Polígono Industrial Las Casas, Calle C, Parcela 4, 42005 Soria, Spain 4 Advanced Forest Research Doctorate Program, MONTES (School of Forest Engineering and Natural Resources), Universidad Politécnica de Madrid, Calle de José Antonio Novais, 10, 28040 Madrid, Spain 5 InnovaWood, The European Forestry House, 66, Rue du Luxembourg, 1000 Brussels, Belgium * Correspondence: carlosmartins86@gmail.com Abstract: Hardwoods commonly have high mechanical properties, which makes them interesting for structural use, but softwoods dominate the structural timber market in Europe. Tensile strength classes are recommended for engineered wood products. However, current European standards do not provide tensile strength classes for hardwoods and the declaration of tensile properties from machine grading in the industry is not yet possible. The present paper aims to contribute to the revision of European standards through the technical group CEN/TC124/WG2/TG2: Tensile strength classes for hardwoods, of the European Standardisation Committee. An experimental campaign which involved machine grading and tensile testing of over 569 boards of Southern blue gum (Eucalyptus globulus Labill.) from Spain and Portugal was made. Six new tensile strength classes were defined, from ET24 (f t ,0,k = 24 N/mm 2 , E t ,0,m = 18 kN/mm 2 and ρ k = 590 kg/m 3 ) to ET42 (f t ,0,k = 42 N/mm 2 , E t ,0,m = 23 kN/mm 2 and ρ k = 640 kg/m 3 ). Machine grading made possible the definition of six strength class combinations. Four combinations resulted in 40% of the sample being assigned to the higher strength class, with low percentages of rejection (varying between 1% and 14%). This demonstrates the high mechanical properties of the species and the performance improvement of machine grading with respect to current visual grading. Keywords: dynamic modulus of elasticity; EN 14081-2; Eucalyptus globulus Labill.; indicating property; mechanical properties; non-destructive testing; strength class combinations; tensile strength 1. Introduction Portuguese and Spanish forests are mainly occupied by hardwoods, these having registered a significant increase of area in the last decades. According to the most recent National Inventories, in Spain, data from 2019 refers to 55.5% (10.184 million hectares) of forest land occupied by hardwoods (mixed stands not included) [1]. On the other hand, in Portugal, hardwoods represent 63.7% (2.055 million hectares), mixed stands not included [2]. In both countries, these percentages have been increasing over the last 50 years. Southern blue gum (Eucalyptus globulus Labill.) is considered a fast-growing species and, according to Cesarolli et al. [3], in Europe covers 1.3 million hectares of forest, mainly in the Iberian Peninsula (more than 80%), France and Italy. Currently, it is one of the most representative hardwood species in the Iberian Peninsula with 619.9 kha in Spain (data from 2019) [1] and 845 kha in Portugal (data from 2015) [2]. Eucalyptus is mainly used in the pulp and paper industry, energy production and as firewood. However, cascading use of wood leads to a need to adapt the sector to the circular economy and bioeconomy strategies. A piece of wood may experience several reuse, recovery and/or recycling loops, where high-quality wood is used for high-value products (solid wood or engineered wood products), with lower-value products employing wood in transformed forms over Forests 2023, 14, 1623. https://doi.org/10.3390/f14081623 https://www.mdpi.com/journal/forests