In-situ assessment of density and material properties in timber structures by non-destructive and semi-destructive testing

Licentiate thesis, 2012

ABSTRACT In the assessment of timber structures, diagnostic investigations of the structural members and connections are generally necessary. The assessment often requires appropriate non-destructive testing (NDT) and semi-destructive testing (SDT) techniques. Improved, new methods based on scientific knowledge and guidelines are needed for their application, so the principal goal of the project is to develop guidelines and recommendations on how to perform assessment of existing timber structures with reference to their condition and structural capability, as well as assessment strategies. In the first part of the thesis, the research focused on the development of an X-ray image calibration procedure, which enables the determination of density properties for the in-situ assessment of timber structures. This non-destructive method is useful for evaluating the internal condition, since wood density has a strong relationship with several mechanical properties. An X-ray procedure to calibrate for the influence of significant density differences in wood specimens, thickness and moisture was applied, which the images were analysed and evaluated. Finally, a calibration wedge was set up to verify the procedure on timber beam specimens, resulting in good agreement and an average accuracy of ~97%. The values obtained from the image calibration resulted in very good linear correlation between the measured density and the greyscale from X-ray images. The main advantage compared with conventional techniques is the detection and quantification of the internal condition of timber that may reduce the mechanical properties of the structure. This study shows good potential when it comes to the development of a viable tool for in-situ assessments of timber structures and could be used indirectly in analyses of structural behaviour. In the second part, the research focused on a systematic in-situ assessment strategy of the Vasa warship, including the prediction of the density, the stiffness properties and the influence of PEG (Poly Ethylene Glycol). This was made possible by combining non-destructive in-situ testing techniques, particularly X-ray, with mechanical and chemical tests. A global non-destructive assessment of the mechanical properties is needed to predict both the strength and stiffness of the Vasa oak. The proposed procedure can be used in situ with satisfactory results for the evaluation of timber properties. A conversion of a three-dimensional ship model from the Vasa warship to model the real structural behaviour might create difficulties due to the complexity of the material properties that are needed for input. The PEG content clearly has a negative influence on the strength and stiffness properties. Satisfactory agreement between the density and stiffness properties was reached through cross-correlation to PEG content. Keywords: In-situ assessment, non-destructive testing (NDT), semi-destructive testing (SDT), density determination, X-ray, calibration procedure, Vasa warship.