Prediction of timber bending strength on basis of bending stiffness and material homogeneity assessed from dynamic excitation

The potential of utilizing resonance frequencies corresponding to edgewise bending modes for predicting the bending strength of timber is investigated. The research includes measurements of axial and transversal resonance frequencies, laboratory assessment of density, static bending stiffness and bending strength of 105 boards of Norway spruce of dimensions 45 × 145 × 3,600 mm³. It is shown that Eb,1, (MOE based on the resonance frequency of the first bending mode) gives a higher coefficient of determination to the bending strength than what Ea,1 (MOE based on the first axial resonance frequency) does. It is also shown that resonance frequencies corresponding to higher bending modes can be used in the definition of a new indicating property, the measure of inhomogeneity (MOI). This is a scalar value representing the lack of fit between the true, measured resonance frequencies and the expected (assuming homogeneity) resonance frequencies of a board. The results show that using the MOI as a third indicating property, in addition to Eb,1 and density, increases the coefficient of determination with respect to bending strength from R2 = 0.69 to R2 = 0.75.