Ultrasonic plate waves in three-dimensional braided composites

Abstract

This paper reports on a study of ultrasonic wave propagation in three-dimensional braided composites; an extensive experimental survey is compared to the results of an approximate analytical model. The model describes the average elastic behavior of composites having continuous fiber weaving in three spatial dimensions. The approximation rests upon the treatment of the composite as a homogeneous medium that possesses the average directional stiffnesses implied by the fiber placement and braid angles. The model is compared to the results of experimental ultrasonic guided wave studies in 3-D woven AS4/PEEK. This composite has the structure of a three-dimensional braided fabric infiltrated with thermoplastic polyether ether ketone matrix. Its unit-cell size dimensions are approximately 8 by 2.5 by 1.2 mm. Some 90% of the fibers are braided in the ±20° directions, with the balance being laid-in fibers in the warp (0°) direction. It is found that the approximate model yields an acceptably good description of the measurements. The observed disparities likely arise from the geometrical structuring of the 3-D braid, which is generally larger than the acoustic wavelengths of interest, and not yet included in the model. Comparisons between measurements at low and intermediate acoustic frequencies using relatively wide beams and the approximate calculations are presented.