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Ultrasound computed tomography on standing trees: Accounting for wood anisotropy permits a more accurate detection of defects

Espinosa L., Brancheriau L., Cortes Y., Prieto F., Lasaygues P.. 2020. Annals of Forest Science, 77 (3) : 13 p..

DOI: 10.1007/s13595-020-00971-z

Context: Ultrasound computed tomography is a suitable tool for nondestructive evaluation of standing trees. Until now, to simplify the image reconstruction process, the transverse cross-section of trees has been considered as quasi-isotropic and therefore limiting the defect identification capability. Aims: An approach to solve the inverse problem for tree imaging is presented, using an ultrasound-based method (travel-time computed tomography) suited to the anisotropy of wood material and validated experimentally. Method : The proposed iterative method focused on finding a polynomial approximation of the slowness in each pixel of the image depending on the angle of propagation, modifying the curved trajectories by means of a raytracing method. This method allowed a mapping of specific elastic constants using nonlinear regression. Experimental validation was performed using sections of green wood from a pine tree (Pinus pinea L.), with configurations that include a healthy case, a centered, and an off-centered defect. Results: Images obtained using the proposed method led to a more accurate location of the defects compared to the filtered backprojection algorithm (isotropic hypothesis), considered as reference. Conclusion: The performed experiments demonstrated that considering the wood anisotropy in the imaging process led to a better defect detection compared to the use of a common imaging technique.

Mots-clés : tomographie; défaut du bois; ultrasonographie; ultrason; testage non destructif; arbre; anisotropie; orthotropie

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