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Nondestructive testing of built in timber - new applications

Frühwald, K. / Hasenstab, A.

Undetected decay can result in sudden failure of the construction. In the usual structural safety analysis of timber structures, defects, damages and material heterogeneities and their extent in the interior of wooden construction components are estimated by visual inspection of drilling cores and drilling resistance. Both reveal information only about this single measuring point. Therefore repeated sampling is necessary, which still may be insufficient to assess larger buildings. The destruction from drilling cores must be considered (not suitable for statically highly stressed parts, limited in heritage-protected buildings). With the visual inspection of the drilling cores without further magnifying device, decay can only be detected in the advanced stage. Incipient fungal attack, which already has a significant influence on the mechanical properties, cannot be detected. The drilling resistance method and drilling cores are combined with the ultrasonic echo technique and are suitable for structural health monitoring of timber constructions. With the laminar ultrasonic echo method (macroscopic) characteristics of wood affecting the strength and stiffness (e. g. knots, cracks fungal decay) can be detected. Undamaged areas can be defined very quickly, the extension of damaged areas can be localized. The results of point wise drilling resistance method correlates with the hardness of the wood and - like for drilling cores - its density and, subsequently, the fungal infectron (at microscopic level). A detection of the structural characteristics affecting the strength (especially knots, cracks and slope of grain) is not guaranteed.