Nonlinear Elastic Effects On The Energy Flux Deviation Of Ultrasonic Waves In Gr/Ep Composites

In highly anisotropic media such as composite materials, the direction of the energy flux of an ultrasonic wave does not coincide with the wave normal except along symmetry directions. The energy flux deviation angle is a function of the elastic coefficients of the material. Due to nonlinear elastic effects, the elastic coefficients and thus the deviation angle is a function of stress. The shift in deviation angle has been calculated in unidirectional T300/5208 graphite/epoxy using the previously measured complete set of second and third order elastic coefficients. Calculated were the effects of uniaxial compressive and tensile stresses along the fiber direction (x3) and along the laminate stacking direction (x1) for waves propagating at various angles in the x1x3 plane. These calculations indicate that the shift in the deviation angle is greatest for the quasi-transverse wave with a shift as large as three degrees for a stress equal to the ultimate tensile strength applied along the fibers. This nonlinear effect on the energy flux deviation offers the potential for a novel new method of nondestructively evaluating stress in composites or for characterizing their nonlinear properties.