Anisotropic Shear Modulus due to Stress Anisotropy

By special measurement technique it is shown that the dynamic shear modulus in homogenous isotropic sand samples depends upon the direction of stress. Furthermore it can be demonstrated that the shear modulus is not related to the first invariant of stress tensor as assumed hitherto, but to the respective stress component itself. Measurements were performed making use of the cross correlation method and stochastic shear wave excitation by a special shear wave exciter that avoids the generation of p waves. Polarized shear waves were generated in a cubical sand sample and the shear wave velocity was measured independent of the value and the direction of the stress in the sample. Due to the sensivity of the correlation method small vibration amplitudes could be applied, such that a linear stress-strain relation could be assumed. It was shown that the placing technique of the sand had no influence upon the test results.