INVESTIGATION OF THE ANISOTROPIC NATURE OF LASER-GENERATED ULTRASOUNDIN ZINC AND UNIDIRECTIONAL CARBON-EPOXY COMPOSITES

Laser generation of ultrasound in carbon epoxy composites provides an ultrasonic signature which is difficult to interpret owing to the elas tic anisotropic and inhomogeneous nature of these materials. In the pr esent work, a line source representation of laser-generated ultrasound in materials exhibiting transverse isotropy is presented. The boundin g plane of the half-space is assumed to be the plane of isotropy. Negl ecting thermal diffusion, it is shown in the limit of strong optical a bsorption, the buried line source is equivalent to applying a shear st ress dipole at the bounding surface. A formal solution is found using double (Fourier-Laplace) transforms. The Cagniard-de Hoop technique is used to analytically invert the transform for the epicentral case as well as the surface wave case. A solution for a sub-surface source is obtained numerically. Experimental validation of the theory is perform ed using single crystal zinc and a unidirectional carbon epoxy sample with the plane of isotropy perpendicular to the fiber direction. For z inc, the experimentally obtained epicentral and surface wave displacem ents agree well with theoretical predictions. The carbon epoxy sample exhibits homogeneous behavior when the wave vector is perpendicular to the fiber direction. When the wave vector is aligned with the fiber d irection, the wave form appears to be influenced by the inhomogeneous nature of the composite. (C) 1998 Elsevier Science B.V.