Bending properties of commercial wood-based panels by NDT methods

This thesis work focuses mainly on the application of non-destructive testing (NDT) methods on wood-based panels (WBP) in order to estimate the bending properties. To prove the accuracy and applicability of these methods on WBP, their results are correlated with results from a standardized static bending test. The behavior in different climate conditions and the application on panels of larger sizes is also questioned to provide an indication about strong points and boundaries of NDT methods applied on WBP.The bending properties are of major importance, especially for materials suited to bear loads. Bending stiffness, represented by the modulus of elasticity (MOE), is an expression of the deflection rate of a material under load. The bending strength, represented by the modulus of rupture (MOR), is an expression of the maximum load withstood by a material before rupture.Before testing, the material is acclimatized in three climate conditions: dry (20°C, 35% RH), standard (20°C, 65% RH) and wet (20°C, 85% RH), to understand the bending properties variation and how the NDT methods are affected by the variation in moisture content.The materials used are seven types of WBP, in particular four types of particleboards (PB), one type of high-density fiberboard (HDF), one type of dual density PB (with high and low density areas along the production direction) and one type of light-weight panel (Board-on-stiles, a composite panel of HDF, PB and paper honeycomb).To test the bending properties the following NDT methods are considered: transversal resonance vibration and longitudinal resonance vibration with the use of the BING system and the time-of-flight with the use of Fakopp Ultrasonic Timer and Silvatest Trio. The resonance vibration methods, transversal and longitudinal, are based on the relation between resonance vibration properties and bending properties of a material. The relation with bending properties also exists for the stress wave velocity (SWV) through a material, calculated with the time-of-flight method. The dynamic MOE resulting from these tests is then correlated with the static MOE and MOR from the static bending test.The NDT methods resulted to be reliable on WBP, with generally high levels of correlation between dynamic MOE and static MOE and MoR. The highest correlation value for MoE is with the transversal resonance vibration while the highest for MOR is with the longitudinal resonance vibration. The results of the dynamic MOE for all the NDT methods are higher than the static MOE, as confirmed also in the literature; the average ratio between the dynamic and the static MOE is, for example, up to 1,6 for WBP in standard climate condition, tested with Fakopp U.T.. These results are extremely higher than values suggested by previous studies. Moreover, the ratio increases with increasing relative humidity of the climate condition. The results from the tests on larger sizes suggest a possible application in this field. The time-of-flight method is suitable for in-plane uniform materials, like the PB and HDF, while the transversal resonance methods give also a good representation of the properties of the dual density PB and the light-weight panel.