Structural Health Monitoring of Glued Laminated Timber with a Novel Air-coupled Ultrasound Method

Recent years have witnessed an increased use of glued timber products in construction, due to their highlights in versatility and sustainability. It is therefore a demanding necessity to develop non-destructive integrity assessment methods for structural health monitoring of timber constructions, both during production and utilization. Particularly, the integrity and load bearing capacity of the glue line need to be tested. Air coupled ultrasound (ACU) is a novel non-destructive method which is well suited for this purpose. The advantages with respect to traditional ultrasonic contact techniques are phenomenal reproducibility and unlimited scanning capabilities, together with full preservation of the properties of the inspected object. As part of an on-going project in the Swiss Federal Laboratories for Materials Testing and Research (Empa) together with the Laboratory for Wood Physics and Non-Destructive Testing Methods of ETH Zuerich, experiments were performed in samples consisting of two layers of glued laminated timber with artificially introduced delaminations. First, the main ultrasound propagation phenomena were modelled with dedicated analytical calculations and numerical simulations. Next the samples were scanned with a precise mechanical system and a Normal Transmission Mode setup and the ACU waveforms were digitized for each scanned position. A specific amplitude tracking algorithm together with a self-calibration procedure applied to the ultrasonic images were used to compensate for ultrasonic amplitude variations induced by wood heterogeneity. The mean amplitude variations in the orthotropic R-T plane and in the L axis were of 10 and 5 dB respectively. After an appropriate wood heterogeneity correction the uncertainty in the R-T plane was reduced down to 4 dB. Adhesion defects were reliably assessed for the investigated samples, as well in situations where adhesive was present in the glue line but no bonding existed between timber lamellas (dry glue), or where the adhesive spread during the pressing in an uncontrolled way to the desired non-glued regions. The ACU results were validated with an also novel Micro Focus Computed Tomography glue line assessment method. Specific wave propagation phenomena depending on the year ring orientation with respect to the insonification direction were also discussed.