Abstract
Several laboratory studies and experiments have demonstrated the usability of polymer films filled with electrically conductive filler as piezoresistive material. Applied to adhesives, the glue lines of wood products can achieve multifunctional—thus bonding and piezoresistive/strain sensing—properties. Based on critical load areas in timber constructions, upscaled test setups for simplified load situations were designed, especially with regard to a stress-free electrical contact. In a second step, another upscaling was done to small glulam beams. Based on an experimental test sequence, the piezoresistive reactions as well as the behaviour until failure were analysed. The results show in all cases that a piezoresistive reaction of the multifunctionally bonded specimens was measurable, giving a difference in the extent of relative change. Additionally, measured phenomena like inverse piezoresistive reactions, electrical resistance drift and the absence of a piezoresistive reaction were discussed, based on additional strain analysis by digital image correlation. A model of macroscopic and microscopic strains influencing the piezoresistive reaction of the electrically conductive bond line in wood was used to explain all experimental results. Finally, a first scale-up of piezoresistive bond lines from laboratory samples to glulam beams was possible and successful.
Highlights
Timber structures are exposed to environmental influences and must be preserved by constructive wood protection and monitored by inspection intervals
While constructive wood protection elongates the usability of the structure, inspection intervals help to prove the integrity of infrastructure, public or re-utilised buildings
All results clearly show the piezoresistive reaction (PR) of the multifunctional adhesive bond line could be used to detect stresses
Summary
Timber structures are exposed to environmental influences and must be preserved by constructive wood protection and monitored by inspection intervals. While constructive wood protection elongates the usability of the structure, inspection intervals help to prove the integrity of infrastructure, public or re-utilised buildings. The additional costs for integrating these sensors into timber structures are high due to the necessary highly specialised staff and the time-consuming installation. Usage of electrically conductive adhesives as sensory layers can help at this point. The integration of these multifunctional adhesives during the manufacturing of glulam beams, cross-laminated timber or other engineered wood products can help to decrease the production costs and integrate sensors at points inside the engineered product
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