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Novel air-coupled ultrasound method for moisture-induced delamination imaging in individual bonding planes of historic glued timber

S. J. Sanabria and P. Schuetz and J. Neuenschwander and R. Widmann and P. Niemz
Proc. 18th International Symposium on Non-destructive Testing and Evaluation of Wood
Madison, WI, USA 2013

Abstract

The proliferation of glued timber construction raises increasing concern about safety problems related to in-service delamination of glue lines. In this work, a portable installation is presented based on a recently patented air-coupled ultrasound slanted lateral transmission (ACU, SLT) method. In comparison with the previous normal transmission (ACU NT) technology, the novel ACU SLT method is capable of separately associating lamination defects to individual bonding planes of arbitrarily high glued timber stacks. Moreover, the mechanically scanned transducers require only access to the lateral faces of the laminated timber beams. The impact of the natural variability of wood in the ultrasound signal is discussed and effectively minimized by combining ultrasound images acquires with specific transducer orientations. The installation was applied to assess a recently dismantled 90 year old roofing glulam member with significant climatic-induced delamination. The results are compared with feeler gauge visual inspection, laboratory X-ray computed tomography (CT) assessment and standard shear strength testing of stitch probes. The ACU SLT method was capable of assessing the geometry of delaminations not detected with the feeler gauge but consistently shown in the X-ray tomograms. Low-strength stitch probes were consistent with low signal amplitude regions in the ACU SLT images, the latter allowing full-scale non-destructive assessment of the structural member.


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@InProceedings{eth_biwi_01112,
  author = {S. J. Sanabria and P. Schuetz and J. Neuenschwander and R. Widmann and P. Niemz},
  title = {Novel air-coupled ultrasound method for moisture-induced delamination imaging in individual bonding planes of historic glued timber},
  booktitle = {Proc. 18th International Symposium on Non-destructive Testing and Evaluation of Wood},
  year = {2013},
  pages = {549-601},
  publisher = {USDA Forest Products Laboratory},
  keywords = {}
}