Determination of the Thuja Burr Material Symmetries by Direct Contact Ultrasonic Nethod on Spherical Specimens Mohammed El Mouridi 1,2* , Thierry Laurent 1 , Tancrède Almeras 1 , Olivier Arnould 1 , Abdelillah Hakam 2 , Joseph Gril 1 1 Laboratoire de Mécanique et de Génie Civil, Université de Montpellier 2, CNRS UMR 5508, CC 048 Place Eugène Bataillon, 34095 Montpellier, France {mohamed.el-mouridi, thierry.laurent, tancrede.almeras, olivier.arnould, joseph.gril}@univ-montp2.fr 2 Faculté des Sciences de Rabat, Équipe des Sciences et Technologie du Bois : 4, Avenue Ibn Battouta B.P. 1014 Agdal-Rabat, Maroc ahakam@fsr.ac.ma Key words: Material symmetry, anisotropy, ultrasonic method, direct contact, sphere, burr, wood, thuja. Abstract Thuja (Tetraclinis articulata (Vahl.) Master) is a species endemic to the South-western coast of the Mediterranean Sea and especially Maghreb. The thuja burr is an outgrowth found at the collar of thuja trees. The burr wood of thuja is composed of dark growths of high density, mixed in a "matrix" of woody tissue of density close to that of thuja. When properly worked, the wood of thuja burr reveals a remarkable speckled pattern, leading to a real aesthetic value (Figure 1). In order to document the physical and mechanical properties behaviour of burr wood of thuja, we developed a method for determining the material symmetries for any material with unknown anisotropy, by combining ultrasonic experimental methods and numerical computation. Figure 1a : Thuja burr Figure 1b : A machined plate from the wood burr of thuja An ultrasonic experimental device was developed to measure the elastic stiffness of a test specimen in a particular direction. Usually, the sample geometry used for direct contact ultrasonic methods is cubic or polyhedral allowing to obtain, on a single specimen of anisotropic material, 3 (cube) to 13 (polyhedron) measurements in different directions [1,2]. This number of measurements is not enough to determine the material symmetries on a single specimen. To overcome this limitation, an ultrasonic experimental device in direct contact on spherical specimens has been developed and improved (Figure 2). Using this geometry, the stiffness of the material can be measured in any number of directions on the same specimen. The result of this procedure is a “stiffness map”, giving stiffness as a function of specimen orientation. 51