Ultrasonic characterization of aramid-epoxy/aluminum laminates from pulse-echo data

The objectives of this study are to determine the through-thickness elastic coefficient of fiber-epoxy/metal laminates with ultrasonic measurements and to determine the elastic coefficients of the constituent layers. For the measurements, a broadband, piezopolymer, transmitting transducer is excited with the leading edge of a square wave. A specimen is aligned perpendicular to the transducer in an immersion tank. Signals reflected from the specimen are captured by the piezopolymer transducer and Fourier transformed. Signals transmitted through the specimen are captured by a hydrophone, and the first arrival of the signals is used to determine the average velocity through the laminate. For this study we consider four regimes. There is a static regime and a second regime where the wavelength of the ultrasound is much greater than the thickness of the laminate. In the third, intermediate wavelength regime the velocities and densities of the individual layers are determined using "stop bands" in the spectrum. Short-duration pulse determinations of the elastic stiffness coefficients are valid in the fourth, short wavelength regime. Elastic coefficients determined from pulse measurements compare favorably with predictions based on the assumption that velocities of the layers act in series as the sound travels through the laminate.