Mapping out fiber directions in composite laminates using ultrasound

Abstract

A method for ultrasonically mapping out the fiber orientation of the plies in a composite laminate is presented. The method employs normal-incidence longitudinal ultrasound but exploits the presence of small structural imperfections that are always present at the interfaces between plies. C-scan images of ply interfaces contain streaking features that contain fiber orientation information; however, such images can often be quite noisy and the fiber orientations were difficult to discern visually. To overcome such difficulties, 2-D FFT was applied to the C-scan image of an interface, and the resultant amplitude-versus-angle plot was used to determine the fiber orientation. Since the ply thickness was an unknown and the interface echoes were usually not distinct, the placement of time windows was impractical. Therefore, the entirety of full-waveform B-scan data were used. The 2-D FFT was performed on C-scan slices at small time (depth) increments regardless of the actual location of the ply interfaces. This method was more robust and worked successfully on a large variety of composite laminates. [Work supported by NSF IUCRC.]