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Detection of Fiber Direction in Composites by Means of a High-Frequency Wide-Bounded Ultrasonic Beam and Schlieren Photography

ABSTRACT This work describes an experimental method, based on the combination of schlieren photography and a 2.5-cm-wide-bounded ultrasonic beam of 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber-reinforced composites. The method exploits the spatially dependent reflection coefficient because of discontinuities in the fiber density. The typical discontinuities that reflect the fiber direction are detectable if 10-MHz ultrasound is applied. Lower frequencies are not susceptible to these typical discontinuities, whereas larger frequencies would make the use of schlieren photography more difficult.