Fiber direction determination in composites by means of Schlieren photography

After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on Lamb wave phenomena, which can be complicated for interpretation by NDT engineers, but rather is based on relatively simple principles, i.e. inhomogeneity and symmetry of composites.