Shear Horizontal Wave Propagation Speed in Mylar Sheet and Coated Paper

Soft plate‐like membranes find application e.g. as pill or paper coatings, bio‐filter membranes, and gas seals in food products. For these applications the integrity and the mechanical properties of the membrane are important. Mechanical properties of these products can be determined by stretching or bending tests, but such methods can damage these fragile products. We propose a rapid nondestructive acoustic method to estimate mechanical film characteristics with shear horizontal (in‐plane shear) waves. A 23 kHz, 1‐cycle square signal was excited into a thin foil with a piezoceramic pickup and received with an inductive pickup. The SNR (power) was 20 dB in 1 kHz –50 kHz bandwidth. This actuation‐detection scheme can be used to excite in‐plane longitudinal, shear and even elliptic waves in a thin foil. The method was validated by measuring in‐plane shear wave and longitudinal wave time‐of‐flight TOF at different actuator‐receiver separations and calculating the corresponding longitudinal and shear modulus. The samples were Mylar® sheet and coated paper. The anisotropy of MOE for Mylar sheet was close to the manufacturer specifications. For coated paper a maximum shear modulus anisotropy of 5% and a shear modulus dependence on temperature of 0.7 MPa/°C were found. Laser doppler vibrometry showed that the excited waves were confined in‐plane.