Acoustic methods for timber structural health assessment

The aim of the thesis is the evaluation of the feasibility of a new Non-DestructiveTesting (NDT) technique for structural timber assessment based on the sound recordings analysis of impact hammer tests. The research workflow can be summarised as follows. First, an extensive bibliographic study was carried out both on the current state-of-theart of timber assessment methods and on the physical aspects specific to the examined technique (i.e. wave propagation, structural vibration, sound radiation, etc.). Second, a series of experimental tests were performed to examine the potentiality of the proposed diagnostic technique to identify decay and/or damage inside solid wood beam elements. The recorded audio data were then analysed using a specifically developed MATLAB® code, extracting several parameters to be used as reference to deduce information about the timber element condition. In the bibliographic survey, carried out at the beginning of the PhD work, a series of European and International documentation and guidelines about structural timber assessment were reviewed and analysed. A summary of the most common Non-Destructive Testing (NDT) and Semi-Destructive Testing (SDT) techniques was compiled, with a more detailed inquiry into the so-called stress-wave based techniques since they are closely related to the topic of the thesis. A general overview of the theory of wave propagation, both in solid media and fluid media, was then reported setting the stage for a clearer understanding of the sound radiation phenomenon involved in the proposed assessment method. The principles of audio analysis and digital audio recording were studied, and different types of data post-processing procedure were considered but a particular emphasis was reserved to frequency domain analysis and spectral audio features extraction. For the experimental part of the thesis a first step was taken by testing 10 salvaged timber beam specimens (i.e. timber elements recovered from a structure being dismantled), 5 of them in sound conditions and 5 of them in decayed conditions as confirmed by visual examination, moisture content measurement and static testing (i.e. evaluation of the element modulus of elasticity). After these preliminary investigation, additional testing was performed on 20 new-sawn timber beam specimens (spruce solid wood grade C24) evaluating the influence of different variables, such as type of boundary conditions, point of impact or type of microphone used for the recording (i.e. smartphone built-in microphone and calibrated electret microphone). Six of the total 20 specimens were chosen for further testing after two alternative damage scenarios were applied to the timber beams. A horizontal crack, one eighth of the total beam span long and located at the mid-height of the cross section, was applied either at the beam ends (simulating shear failure) or at the midspan section (simulating failure perpendicular to the grain for hanging loads). Results appear promising, however further testing is required to extend the method validity and statistical relevance.