Disposable PVDF ultrasonic transducers for nondestructive testing applications

Disposable ultrasonic contact transducers have been constructed with inexpensive PVDF films for nondestructive testing (NDT) applications. This paper reports the temperature-dependent ultrasonic performance of commercial polyvinylidene fluoride (PVDF) films and PVDF sensors. PVDF film was evaluated for its material properties of interest for ultrasonic transducer performance including the relative dielectric constant epsilon(r), dielectric loss tangent tan delta(e), electromechanical coupling constant k(t), mechanical quality factor Q(m), and acoustic impedance Z. Disposable PVDF transducers were then coupled to both low and high acoustic impedance test panels and their ultrasonic insertion loss/sensitivity and frequency bandwidth measured as a function of temperature in both the pulse-echo and pitch-catch modes. The results yielded center frequencies in the 3.5-16 MHz range, with -6-dB fractional bandwidths for some of the transducers exceeding 100%. The temperature dependence of the transducers' performance is reported for the -40 to 80 degrees C range and shows an approximate linear decrease in center frequency and increase in fractional bandwidth with increasing temperature. These measured results are compared with predictions from computer simulations based on Mason's model. Each of the contact transducers was also mounted on a holder which permitted use as an immersion transducer, and its pulse-echo acoustic performance was measured in water. The results varied widely, from excessive ringing and poor performance to very broad-band short pulsewidth (1.2 lambda) performance. Finally, since the same transducers can also be used as acoustic emission contact transducers, their response to a standard pencil lead break test, on both high and low acoustic impedance structural materials, was also evaluated. The response of the transducers showed exceptionally high sensitivity and bandwidth for in-plane vibrations when mounted on low impedance (i.e., acrylic, fiber composite) test panels, and lower sensitivity on high impedance test panels (i.e., aluminum, glass). These tests showed that the same disposable transducers can be used for both ultrasonic and acoustic emission sensing NDT applications, This paper summarizes the quantitative ultrasonic and vibration sensing performance for all of the disposable PVDF film transducers which were tested.