Potential of microwave scanning for determining density and tension strength of four European hardwood species

As an effect of changing forest management—away from softwood monocultures to more robust mixed stands—the availability of hardwood on the European timber market increases. Thus, a more diversified spectrum of hardwood products is required between the established uses in furniture and energy production. Glued timber products are a promising option in this respect. One important prerequisite for efficiently producing glued hardwood products is to establish hardwood strength grading. To this end, the current paper explored the potential of microwave scanning, stand-alone or combined with the measurement of dynamic stiffness, to estimate the tension strength of ash, beech, sweet chestnut and oak lamellas. In this preliminary study, combining microwave and dynamic stiffness measurement showed much potential for hardwood strength grading for all four species; for beech and sweet chestnut, coefficients of determination (\({r}^{2}\)) beyond 60% could be achieved, which is on a level with established softwood grading principles. For ash and oak, \({r}^{2}\approx 45\%\) was observed, which is acceptable for machine strength grading. The paper also considered measuring density using microwaves. Such a density measurement was found to be as accurate for hardwoods as for softwoods.