| This paper describes the use of a large aperture PVDF receiver in angle beam through-transmission method of velocity measurement in fiber reinforced composites. This technique avoids the beam diffraction effect that occurs when using limited size ultrasonic transducers. This effect increases as the frequency decreases for the same size of transducer. On the other hand, the velocity dispersion effect, present in composite materials, increases with frequency. Analyzing the diffraction effect, it was observed that the longitudinal velocity in an aluminum plate increases more than 1 % when using a pair of 1 MHz transducers. That effect disappears when using the large aperture receiver and it is negligible when using a pair of 10 MHz transducers. On the other hand, it was observed that in the acrylic plate the longitudinal velocity increases 0.7 %, and increases more than 0.8 % in a 2.115 mm thick CFRP (carbon fiber reinforced plastic) plate, from 1 to 10 MHz. As a compromise between axial resolution and velocity dispersion, the elastic constants of the unidirectional CFRP plate were determined in the frequency of 2.25 MHz, showing good agreement with tensile test results. |