Machine strength and stiffness prediction with focus on different acoustic measurement methods

Strength grading is an important step for the production of homogenous and high-quality solid wood material. In particular, for hardwoods, the use of non-visible characteristics is indispensable. Dynamic MOE (MOEdyn) is an important parameter widely used for grading of softwoods and applicable to hardwoods as well. There are two common ways to measure MOEdyn – ultrasound (US) wave propagation and longitudinal stress wave (LSW) propagation. Both methods are used in practice, however, due to the different measurement techniques behind them, the results differ. Current paper analyses the stiffness and strength prediction accuracy for several temperate European hardwood specimens and stress the differences between the two measurement systems. The performance was analysed with regard to grading techniques, testing modes for the mechanical properties (tension and bending) and wood qualities. For more than 2861 pieces of European ash (Fraxinus excelsior), European beech (Fagus sylvatica), European oak (Quercus spp.) and maple (Acer spp), the MOEdyn was measured using both techniques, and destructive tests (tension and edgewise bending) were applied. The results show that LSW has higher prediction accuracy compared to the US MOEdyn. The prediction accuracy for both methods and tensile application can be increased by calculating MOEdyn with average density. Furthermore, the results support the species independent strength grading of hardwoods. Further research on the effect of different wood qualities and sawing pattern is required.