Analysis of interference patterns in acoustic photomicrographs

Abstract

Images from the Scanning Acoustic Microscope (SAM) have been used previously to study fiber orientations in opaque matrices. SAM images are superficially similar to reflected light images. They also frequently exhibit interference patterns which can be used for quantitative analysis. These patterns arise at discontinuities near or intersecting the specimen surface, and result from interference between acoustic waves reflecting from subsurface boundaries and from the specimen surface. For the case of fibers intersecting a planar surface, the position of the interference fringes gives the orientation of the fiber with respect to the surface, and the shape of the fringes indicates any surface irregularities. Thus, if the fiber shape and orientation are known and if the waves are determined to be longitudinal, then fringe spacings can be used to calculate the velocity and attenuation of the longitudinal acoustic wave in the matrix surrounding the fiber.This technique was applied to graphite-epoxy composites, imaged in the SAM at a frequency of 1.2 GHz. First it was determined that other waves, such as shear waves, surface-skimming longitudinal waves, and surface acoustic waves (Rayleigh waves) were not present in the composite. Their addition to the interference pattern would have disrupted the interpretation of the object’s shape and surface features from the longitudinal fringes and complicated the solutions for wave reflections.