Mechanical properties of secondary quality beech (Fagus sylvatica L.) and oak (Quercus petraea (Matt.) Liebl.) obtained from thinning, and their relationship to structural parameters

International audienceContext: Using secondary qualities and small logs of hardwoods such as beech and oak for engineered wood products is an increasingly important issue due to the technological challenges of processing smaller logs and denser woods. Secondary quality hardwoods are expected to have less variation in mechanical properties compared to softwoods with high juvenile wood content.Aims: The first objective of this study was to investigate the radial variation in wood properties of suppressed growth beech and oak trees obtained from thinning operations. The second objective was to develop a model to predict the mechanical properties of these hardwood species based on their structural parameters.• MethodsThe microfibril angle, ring wood density, and ring width from the pith to the bark were determined using an X-ray densitometer. The modulus of elasticity and modulus of rupture were evaluated on the small clear specimen using a three-point bending test. The wood density, grain angle, and microfibril angle of this small clear specimen were also measured.ResultsThe results show that the juvenile wood in oak has a wider ring and higher microfibril angle, whereas it has wider latewood and higher microfibril angle in beech. For both species, the juvenile wood occupies a very small area, less than 5 cm from the pith. The mechanical properties of oak and beech wood from suppressed growth trees are comparable to properties reported in the literature for dominant trees. The modulus of elasticity of oak was best predicted using wood density, grain angle, and microfibril angle. The modulus of rupture of oak is better predicted with wood density and grain angle, whereas it is best predicted with wood density alone for beech.ConclusionJuvenile wood found in the suppressed growth trees of both hardwoods can be used in place of mature wood. It is important to take structural parameters into account when predicting the mechanical properties of hardwood species