Critical section effect during derivation of settings for grading machines based on dynamic modulus of elasticity

Testing of the critical section between the load points in four-point bending testing is required according to [DIN EN 384:2010–08, Structural Timber–Determination of Characteristic Values of Mechanical Properties and Density (Brussels European Committee for Standardization)]. However, strength grading machines based on measurements of the dynamic modulus of elasticity (MOEdyn) benefit as the strength of the weakest section is not always measured when deriving settings. According to the current procedure for derivation of settings, violation of the critical section requirement is accepted. The extent of such violation and its impact on grading settings of strength classes are quantified in two ways using about 3500 recently tested specimens of Norway spruce: empirically and modelled. The results show that for strength grading machines based on MOEdyn, the settings would be higher if the weakest section was always placed between the inner load points. The current procedure, however, leads to lower settings and hence better yields in practice. The settings for machines based on MOEdyn are up to more than 20% higher if the lowest bending strength of the entire board is applied. To treat different grading principles equally, a correction factor for settings of MOEdyn-based grading machines should be introduced–especially for lower strength classes. A simplified factor between 1.00 and 1.20 for standardization is recommended.