Measuring in-plane mechanical properties of plate-like samples using phonographic pickups

Abstract

An affordable acoustic method to estimate mechanical film characteristics with in-plane shear and longitudinal waves was developed. A 23 kHz (24% FBW, −3 dB) signal was excited into thin plate-like samples with a piezoceramic pickup (Ronette ST105/Tonar) and received with an inductive pickup (Shure M92E). Signal-to-noise ratios of 12–27 dB were obtained corresponding to a two-way insertion loss of −105.5, −94.5 dB (shear, long). From the time-of-flight the phase velocity of the wave was estimated. This actuation–detection scheme can excite in-plane acoustic waves in the sample. An in-plane mode purity of −45 dB was recorded. Laser Doppler vibrometry showed that the waves were truly in-plane. The method showed good repeatability with a standard deviation of 2 m s−1 (0.17%) for shear and 5 m s−1 (0.22%) for longitudinal waves. Elastic and shear moduli as well as the Poisson ratio were determined for 174 ± 2 µm thick Mylar® and 184 ± 2 µm thick (795 ± 10 kg m−3) coated paper samples (24 ± 0.5 °C and 40 ± 5% RH). The estimated in-plane moduli for the Mylar® showed a 30% offset, but exhibited the same ratio, compared to the sample manufacturer values (ASTM D 882 test). The effect of heating the sample (140 ± 1 °C) was studied and changes in the dynamic mechanical moduli of the samples on the order of 3% were detected.