ON DETERMINATION OF ORTHOTROPIC MATERIAL MODULI FROM ULTRASONIC VELOCITY DATA IN NONSYMMETRY PLANES

This paper reports analysis of the reconstruction of elastic constants from ultrasonic velocity measurements in nonsymmetry planes of unidir ectional composite materials. It is shown that the nonlinear least-squ are optimization method is stable to initial guess selection and data scatter and can be used routinely to measure the full set of nine elas tic constants for orthotropic materials. Simple analytical expressions are derived for phase velocities in arbitrary directions in an orthot ropic material with low transverse-to-fiber anisotropy. Using these, c oefficients of sensitivity of phase velocities to elastic constants ar e obtained in closed form and used to find the optimal wave propagatio n directions for elastic constant measurement. The changes in velocity due to elastic constant variation calculated by the exact theory agre e well with the predictions from the sensitivity coefficients. When al l nine elastic constants are reconstructed from velocity data in nonsy mmetry planes, the inversion is highly dependent on the initial guesse s and susceptible to random data scatter. When seven elastic constants are found from velocity data in planes of symmetry, the remaining two (C-12 and C-66) can be found from nonsymmetry-plane data independentl y of initial guesses and scatter levels. This is especially important since the elastic constants C-12 and C-66 cannot be determined from im mersion velocity measurements in two accessible planes of symmetry.