Verification of sonic tomography outcome through local testing of mechanical properties in historic timber beam

Since many years a number of NDT techniques have been developed and subsequentially refined for the aims of non-invasive diagnose on site of existing and historic timber members in constructions. Such methods, including drilling penetration resistance, surface stiffness, sonic transmission and tomography, ultrasounds, acoustic emission, GPR radar, IR thermography, exploit different principles and types of waves: mechanical, acoustic, electromagnetic signals. Therefore, in single or combined use, they are able to indirectly provide information about material geometry, density, mechanical properties, decay extent, presence of heterogeneities and knots, moisture. One of their main common advantages is the capacity to produce distribution plots from the measured parameter values, thus easing data interpretation and supplying info about investigated areas rather than punctual information. Those are among the benefits of image diagnostics. Even though positive examples of NDT applications are reported for timber structural elements, often the question arises if these results are only of qualitative nature or if estimation of mechanical properties can be considered of quantitative or semi-quantitative value. Starting from the signal velocity map obtained from sonic tomography investigation across a decayed transversal section from a historic timber beam, this contribution presents the outcome of autopsy of the beam followed by mechanical characterization of local properties of the material. The cutting of a beam slice in correspondence of the tomography tested section enabled visual correspondence of the degraded areas. Further cutting of the slice permitted to undergo small specimens to compression test in grain direction and to establish a relation between the outcome of NDT and destructive tests, point by point, with interesting consequences for the utility of use of NDT and for the preservation of historic timber.