For most materials, the symmetry group is known a priori and deduced from the realization process. This allows many simplifications for the measurements of the stiffness tensor. We deal here with the case where the symmetry is a priori unknown, as for biological or geological materials, or when the process makes the material symmetry axis uncertain (some composites, monocrystals). The measurements are then more complicated and the raw stiffness tensor obtained does not exhibit any symmetry in the Voigt's matricial form, as it is expressed in the arbitrarily chosen specimen's base. A complete ultrasonic measurement of the stiffness tensor from redundant measurements is proposed. In a second time, we show how to make a plane symmetry pole figure able to give visual information about the quasi-symmetries of a raw stiffness tensor determined by any measurement method. Finally we introduce the concept of distance from a raw stiffness tensor to one of the eight symmetry classes available for a stiffness tensor. The method provides the nearest tensor (to the raw stiffness tensor) possessing a chosen symmetry class, with its associated natural symmetry base.
We report on the statistical analysis of experimental measurements of the crack roughness for a fracture initiated from a straight notch in a granite sample. The crack surface is shown to develop a self-affine character over a correlation length &which depends on the distance to the notch y as a power law & cc y" characterized by a dynamic exponent 1/a estimated to be 1.2 -+ 0.15. The self-affine roughness exponent below the correlation length is found to be 0.80 2 2 0.05. The scaling is obtained from two maps of the surface, consisting each in more than lo6 data points.
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