Analyse du Processus de Fissuration dans le Bois sous Sollicitations Mécanique et Climatique : Apports de l'Emission Acoustique

摘要

The aim of this thesis is to consider what Acoustic Emission (AE) can provide as a tool for diagnosis and assessment of wood structures and works. We applied this tool and developed methods for tracking crack propagation. If our intention is using AE for the in-situ monitoring, we have to do experiments in laboratory to validate the methods developped. Loading tests under imposed displacement were conducted on Douglas samples (DCB constant inertia) subjected to a loading in mode I, for two levels of moisture content. In tandem of the acoustic emission emission system, an image acquisition system was used to record crack propagation on the main faces of the samples.First, we showed that the cracking kinetics correlates with the acoustic energy of the recorded signals. A global analysis, with unfiltered data, provides a good indication of the evolution of the damage within a structure. Then by performing a first data processing, by taking into account the acoustic events and by using correction curves of localization and amplitude, we were able to determine the position of one (or more) peak(s) of acoustic activity. The evolution curve of the position of this (these) peak(s) of activity gives a result similar of the crack tip evolution obtained with the digital image acquisition system. By this way we were able to estimate an average restoration level of critical energy Gc.In the purpose of making in situ applications, we have implemented this method during a creep test on a specimen with variable inertia. A previously loaded sample was placed to a relative humidity variation - from a humid atmosphere to a dry atmosphere. Only spot measurements of the crack tip position has been made. AE was able to take over when visual statements were absent. AE was also able to provide additional information on the initiation and propagation of cracks over a long term monitoring.The method for detecting peaks of activities by AE shows that acoustic events with high amplitude are located on both side of the crack tip obtained by imaging.Secondly, by comparing with the methods developed in composite materials, wood could be considered as such, we were able, by joint use of statistical tools, electronic imaging (SEM) and analysis of waveforms, to identify failure mechanisms which were present during testing.After processing data obtained on failure specific mode test (tensile, bending and shear), four clusters of events have been highlighted. By studying their respective waveforms and signal caracteristics, these families have been associated with the rupture of fiber, matrix cracking, debonding and delamination such as those found in composites. The study of SEM images made from owr samples has confirmed the presence of a multitude of mechanisms on the cracking path of the wood samples. This confirms the reasonableness of results obtained.From a quantitative perspective, two major failure mechanisms stand out: the breaking of fibers and matrix cracking. The evolution of these indicators may be a precursor to the ruin of a structure.The work undertaken in this thesis is a step in the use of AE in monitoring structures and wooden structures. It suggests other purposes for the use of AE in wood. We could imaging using AE in order to study the process zones by using mTDCB samples.