Keywords: Wood, Acoustic Emission, Damage evolution, Failure mechanisms

Abstract: Due to the increase of ecological awareness, the use of wood in civil engineering structures like timber bridges has been recently raised. Nevertheless, wood is an anisotropic material which is organized in a complex hierarchical structure distributed acro ss multiple spatial scales. Hence, a thorough comprehension of the failure evolution and the damage mechanisms is required to avoid critical situations and to maintain safety standards. In this context, the acoustic emission (AE) method is appropriate for characterizing wood fracture process because it is sensitive to damage mechanisms on several length scales. It’s well known that tension loading of wood generates a high degree of AE (Niemz and Lûhmann, 1992). Hence, the tensile tests in the axial directi on were performed in this study. In order to establish the differences of AE characterestics, t hree wood species were investigated including Douglas fir, pectinea fir and poplar. The choose of theses species is essentially based on their structures: the l evel of the difference between early wood and late wood is high for the d ouglas fir , middle for the pectinea fir and low for the popular. It is believed that the homogeneity of woods influences significantly their damage modes and their fracture process. Moreover, the douglas fir and the white fir have the different anatomical characteristics even though both of them belongs to softwood. Their effects on AE signals can be so investigated. It is interesting to note that the d ouglas fir is widely used for co nstruction purposes . Actually, t he AE signals saved during the tests were analysed by means of unsupervised pattern recognition approach to identify the classes of AE signals. According to results, the following conclusions can be stated: The AE data of t ensile tests on three diffrent wood species indicate clearly different scenarios from damage initiation and accumulation up to final macroscopic failure. The forms and the magnitudes of AE activities are directly related to the annual rings structure . It is shown that the wood with abrupt transition from earlywood to latewood are more emissive than wood with gradual transition. Also, acoustic emission monitoring allows the establishment of criteria for predicting the critical levels of damage visible or no n visible.

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