Bending strength dependence of load-to-annual ring direction

We were studying the bending strength for load- to-grain direction at angle of 90° and load-to- annual tree ring direction at angle of 45° of almost 700 years old subfossil elm wood from Sava riverbed. Elm wood is widely used because it is solid and hard regarding its weight. However, due to the Elm Dutch disease and mass drying of elm trees in 1950s, nowadays it is almost impossible to find in natural forest stands of south-eastern Europe a tree of reproductive age. The value and uniqueness of the studied material lies also in the fact that the subfossil elm wood is even rarer than the recent one, and is highly appreciated for its aesthetic and mechanical qualities. Bending strength has been determined by a three-point bending test. The mean bending strength value is within the limits for recent elm wood. This investigation has shown that such load-to- annual ring direction has almost 20 % higher value for bending strength than for loads applied to the longitudinal-tangential surface (angle of 0°) and to the longitudinal-radial surface (angle of 90°). This proves that the ring orientation plays an important role in the measurement of mechanical properties of elm wood and that the measurement of the angle of 45° should always be considered for investigation of bending strength. The coefficient of determination of Weibull functions has been close to value of 1, so it has been concluded that the measured bending strength follows Weibull distribution.