Influence of structural parameters on the nondestructive evaluation of ash timber (Fraxinus excelsior L.)

The aim of this project is to analyse the influence of wood structure of ash timber products — as a result from different growth rates — on their performance, if nondestructive evaluation is applied. To do this, 261 beams have been tested with the method of natural frequency on flexural vibration. The following anatomical and structural parameters have been documented as independent variables on all tested beams: annual ring width, proportion of latewood, ring position, grain orientation, presence or absence of heartwood and other discolourations as well as the positions and dimensions of knots. The values obtained from the nondestructive evaluation have been put into relation with the MOR obtained through final destructive 4-point-bending test of all beams. The influence of the wood structure on this relationship has been analysed either with univariate regressions between single structural parameters and MOR, or including more than one of these structural parameters in multivariate regression models. The regressions between nondestructive parameters and MOR present lower levels of determination than the usual determination levels reported for softwood timber. For the ringporous species ash the influence of the wood structure could be more significant than for the more homogeneous wood structure of coniferous species and, consequently, should be analysed in detail. The main results of this analysis show only minimal influence of the ring structure and of the grain angle on the relationship between nondestructive values and MOR. The presence of heartwood does not influence this relationship. By far the most important parameter is the presence or absence of knots. Different knot parameters were measured and included into the models: KAR and vectorial models of position and dimension of the knots. This detailed measurement of the knots and its inclusion into multiple regression models allow a significant increase of the total level of determination and consequently an improved nondestructive quality assessment of ash timber products. Consequences for the practical application of the results in industrial sawmilling are considered.