Glued laminated timber (glulam) is manufactured by gluing and stacking timber lamellas, which are sawn and finger-jointed parallel to the wood grain direction. This results in a sustainable and competitive construction material in terms of dimensional versatility and load-carrying capacity. With the proliferation of glued timber constructions, there is an increasing concern about safety problems related to adhesive bonding. Delaminations are caused by manufacturing errors and in service climate variations simultaneously combined with long-sustained loads (snow, wind and gravel filling on flat roofs). Several recent building collapses were related to bonding failure, which should be prevented in the future with a timely defect detection.
The goal of the thesis was the development of novel non-destructive testing methodologies capable of imaging the position and geometry of delaminations within the bonding planes of glulam. An ACU system prototype capable of detecting an ultrasound beam transmitted through up to 500mm thick glulam was developed, consisting of off-the-shelf ACU transducers, high-power pulsed excitation electronics and a low-noise amplification chain. A five-axes computerized scanning system and a low-cost micro-electromechanic sensors (MEMS) linear array design allowed ultrasound imaging with fix or independent transmitter and receiver transducer units. The bonding assessment was fundamentally based on the evaluation of the attenuation of the ultrasound beam, which significantly increases when transmitted through a material discontinuity (delamination) with respect to a defect-free glue line.