A Large Aperture Ultrasonic Receiver for Through- Transmission Determination of Elastic Constants of Composite Materials Julio C. Adamowski , Marco A. B. Andrade, Nicolas Perez, Flávio Buiochi Department of Mechatronics Engineering Escola Politécnica da Universidade de São Paulo São Paulo, Brazil jcadamow@usp.br Abstract—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. Keywords: composite materials, elastic constants, PVDF receiver, diffraction I. INTRODUCTION The determination of elastic constants of anisotropic materials by measurement of the density and ultrasonic velocities has been studied by several researchers in the last four decades [1], [2]. An anisotropic material is described by 21 independent elastic constants. The number of elastic constants is reduced to nine when the material has orthotropic symmetry. This number may be further reduced when there is more symmetry in the material. A unidirectional carbon-fiber/epoxy laminate can be considered as transversely isotropic and the number of independent elastic constants is reduced to five [2]. The determination of elastic constants from a set of bulk ultrasonic wave phase velocities in an arbitrary direction of a measured sample of composite material is based on the Christoffel’s equation [3]. The accuracy of elastic constants is highly dependent on the precision of the velocity measurement. The immersion through-transmission method is based on the measurement of a time delay between the time spent by the wave traveling in the absence of the sample material and the time with the sample material. This time delay is used to calculate the phase velocity and the refraction angle in the sample [4]. There are several factors which introduce errors when measuring velocities, such as: parallelism of the sample surfaces, temperature gradients, velocity dispersion, acoustic diffraction, mechanical precision of the measurement device, etc [5]. It can be shown that an infinite-plane receiver with uniform sensitivity yields a plane wave-only measurement [6]. In practice this receiver is obtained by using a piezoelectric PVDF (Polyvinylidene Fluoride) thin-film receiver, sufficiently large to intercept the entire propagating wave and electroded throughout its entire extent [7]. 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. In this work a large aperture PVDF receiver is used together with a piezoelectric ceramic emitter in through- transmission method of ultrasonic velocity measurement in solid material plates immersed in water. It is analyzed the diffraction effect in longitudinal velocity measurement when using conventional non-destructive testing (NDT) ultrasonic transducers in the range of 1 to 10 MHz and the velocity dispersion in acrylic and CFRP plates. The elastic constants of a unidirectional CFRP plate are determined using phase velocities measured with an angle beam through-transmission assembly using the diffraction-free PVDF receiver. II. DIFFRACTION AND DISPERSION EFFECTS ON VELOCITY MEASUREMENT The experiments have been made in a goniometer device immersed in distilled water. The water inside the goniometer was kept at 21.50 ± 0.05 o C with the aid of a thermostatic bath. The goniometer device allows changing the emitter and the We would like to thank the following Brazilian sponsor agencies: CAPES, CNPq and ANP; and PETROBRAS for financial support. 1524 978-1-4244-2480-1/08/$25.00 ©2008 IEEE 2008 IEEE International Ultrasonics Symposium Proceedings Digital Object Identifier: 10.1109/ULTSYM.2008.0371